1 | Libpng-manual.txt - A description on how to use and modify libpng |
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2 | |
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3 | libpng version 1.5.12 - July 11, 2012 |
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4 | Updated and distributed by Glenn Randers-Pehrson |
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5 | <glennrp at users.sourceforge.net> |
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6 | Copyright (c) 1998-2011 Glenn Randers-Pehrson |
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7 | |
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8 | This document is released under the libpng license. |
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9 | For conditions of distribution and use, see the disclaimer |
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10 | and license in png.h |
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11 | |
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12 | Based on: |
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13 | |
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14 | libpng versions 0.97, January 1998, through 1.5.12 - July 11, 2012 |
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15 | Updated and distributed by Glenn Randers-Pehrson |
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16 | Copyright (c) 1998-2011 Glenn Randers-Pehrson |
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17 | |
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18 | libpng 1.0 beta 6 version 0.96 May 28, 1997 |
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19 | Updated and distributed by Andreas Dilger |
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20 | Copyright (c) 1996, 1997 Andreas Dilger |
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21 | |
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22 | libpng 1.0 beta 2 - version 0.88 January 26, 1996 |
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23 | For conditions of distribution and use, see copyright |
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24 | notice in png.h. Copyright (c) 1995, 1996 Guy Eric |
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25 | Schalnat, Group 42, Inc. |
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26 | |
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27 | Updated/rewritten per request in the libpng FAQ |
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28 | Copyright (c) 1995, 1996 Frank J. T. Wojcik |
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29 | December 18, 1995 & January 20, 1996 |
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30 | |
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31 | I. Introduction |
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32 | |
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33 | This file describes how to use and modify the PNG reference library |
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34 | (known as libpng) for your own use. There are five sections to this |
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35 | file: introduction, structures, reading, writing, and modification and |
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36 | configuration notes for various special platforms. In addition to this |
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37 | file, example.c is a good starting point for using the library, as |
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38 | it is heavily commented and should include everything most people |
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39 | will need. We assume that libpng is already installed; see the |
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40 | INSTALL file for instructions on how to install libpng. |
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41 | |
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42 | For examples of libpng usage, see the files "example.c", "pngtest.c", |
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43 | and the files in the "contrib" directory, all of which are included in |
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44 | the libpng distribution. |
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45 | |
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46 | Libpng was written as a companion to the PNG specification, as a way |
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47 | of reducing the amount of time and effort it takes to support the PNG |
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48 | file format in application programs. |
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49 | |
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50 | The PNG specification (second edition), November 2003, is available as |
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51 | a W3C Recommendation and as an ISO Standard (ISO/IEC 15948:2003 (E)) at |
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52 | <http://www.w3.org/TR/2003/REC-PNG-20031110/ |
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53 | The W3C and ISO documents have identical technical content. |
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54 | |
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55 | The PNG-1.2 specification is available at |
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56 | <http://www.libpng.org/pub/png/documents/>. It is technically equivalent |
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57 | to the PNG specification (second edition) but has some additional material. |
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58 | |
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59 | The PNG-1.0 specification is available |
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60 | as RFC 2083 <http://www.libpng.org/pub/png/documents/> and as a |
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61 | W3C Recommendation <http://www.w3.org/TR/REC.png.html>. |
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62 | |
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63 | Some additional chunks are described in the special-purpose public chunks |
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64 | documents at <http://www.libpng.org/pub/png/documents/>. |
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65 | |
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66 | Other information |
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67 | about PNG, and the latest version of libpng, can be found at the PNG home |
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68 | page, <http://www.libpng.org/pub/png/>. |
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69 | |
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70 | Most users will not have to modify the library significantly; advanced |
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71 | users may want to modify it more. All attempts were made to make it as |
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72 | complete as possible, while keeping the code easy to understand. |
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73 | Currently, this library only supports C. Support for other languages |
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74 | is being considered. |
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75 | |
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76 | Libpng has been designed to handle multiple sessions at one time, |
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77 | to be easily modifiable, to be portable to the vast majority of |
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78 | machines (ANSI, K&R, 16-, 32-, and 64-bit) available, and to be easy |
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79 | to use. The ultimate goal of libpng is to promote the acceptance of |
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80 | the PNG file format in whatever way possible. While there is still |
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81 | work to be done (see the TODO file), libpng should cover the |
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82 | majority of the needs of its users. |
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83 | |
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84 | Libpng uses zlib for its compression and decompression of PNG files. |
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85 | Further information about zlib, and the latest version of zlib, can |
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86 | be found at the zlib home page, <http://www.info-zip.org/pub/infozip/zlib/>. |
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87 | The zlib compression utility is a general purpose utility that is |
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88 | useful for more than PNG files, and can be used without libpng. |
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89 | See the documentation delivered with zlib for more details. |
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90 | You can usually find the source files for the zlib utility wherever you |
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91 | find the libpng source files. |
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92 | |
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93 | Libpng is thread safe, provided the threads are using different |
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94 | instances of the structures. Each thread should have its own |
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95 | png_struct and png_info instances, and thus its own image. |
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96 | Libpng does not protect itself against two threads using the |
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97 | same instance of a structure. |
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98 | |
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99 | II. Structures |
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100 | |
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101 | There are two main structures that are important to libpng, png_struct |
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102 | and png_info. Both are internal structures that are no longer exposed |
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103 | in the libpng interface (as of libpng 1.5.0). |
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104 | |
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105 | The png_info structure is designed to provide information about the |
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106 | PNG file. At one time, the fields of png_info were intended to be |
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107 | directly accessible to the user. However, this tended to cause problems |
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108 | with applications using dynamically loaded libraries, and as a result |
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109 | a set of interface functions for png_info (the png_get_*() and png_set_*() |
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110 | functions) was developed, and direct access to the png_info fields was |
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111 | deprecated.. |
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112 | |
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113 | The png_struct structure is the object used by the library to decode a |
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114 | single image. As of 1.5.0 this structure is also not exposed. |
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115 | |
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116 | Almost all libpng APIs require a pointer to a png_struct as the first argument. |
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117 | Many (in particular the png_set and png_get APIs) also require a pointer |
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118 | to png_info as the second argument. Some application visible macros |
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119 | defined in png.h designed for basic data access (reading and writing |
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120 | integers in the PNG format) don't take a png_info pointer, but it's almost |
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121 | always safe to assume that a (png_struct*) has to be passed to call an API |
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122 | function. |
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123 | |
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124 | You can have more than one png_info structure associated with an image, |
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125 | as illustrated in pngtest.c, one for information valid prior to the |
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126 | IDAT chunks and another (called "end_info" below) for things after them. |
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127 | |
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128 | The png.h header file is an invaluable reference for programming with libpng. |
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129 | And while I'm on the topic, make sure you include the libpng header file: |
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130 | |
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131 | #include <png.h> |
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132 | |
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133 | and also (as of libpng-1.5.0) the zlib header file, if you need it: |
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134 | |
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135 | #include <zlib.h> |
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136 | |
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137 | Types |
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138 | |
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139 | The png.h header file defines a number of integral types used by the |
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140 | APIs. Most of these are fairly obvious; for example types corresponding |
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141 | to integers of particular sizes and types for passing color values. |
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142 | |
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143 | One exception is how non-integral numbers are handled. For application |
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144 | convenience most APIs that take such numbers have C (double) arguments; |
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145 | however, internally PNG, and libpng, use 32 bit signed integers and encode |
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146 | the value by multiplying by 100,000. As of libpng 1.5.0 a convenience |
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147 | macro PNG_FP_1 is defined in png.h along with a type (png_fixed_point) |
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148 | which is simply (png_int_32). |
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149 | |
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150 | All APIs that take (double) arguments also have a matching API that |
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151 | takes the corresponding fixed point integer arguments. The fixed point |
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152 | API has the same name as the floating point one with "_fixed" appended. |
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153 | The actual range of values permitted in the APIs is frequently less than |
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154 | the full range of (png_fixed_point) (-21474 to +21474). When APIs require |
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155 | a non-negative argument the type is recorded as png_uint_32 above. Consult |
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156 | the header file and the text below for more information. |
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157 | |
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158 | Special care must be take with sCAL chunk handling because the chunk itself |
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159 | uses non-integral values encoded as strings containing decimal floating point |
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160 | numbers. See the comments in the header file. |
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161 | |
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162 | Configuration |
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163 | |
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164 | The main header file function declarations are frequently protected by C |
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165 | preprocessing directives of the form: |
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166 | |
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167 | #ifdef PNG_feature_SUPPORTED |
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168 | declare-function |
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169 | #endif |
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170 | ... |
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171 | #ifdef PNG_feature_SUPPORTED |
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172 | use-function |
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173 | #endif |
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174 | |
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175 | The library can be built without support for these APIs, although a |
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176 | standard build will have all implemented APIs. Application programs |
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177 | should check the feature macros before using an API for maximum |
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178 | portability. From libpng 1.5.0 the feature macros set during the build |
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179 | of libpng are recorded in the header file "pnglibconf.h" and this file |
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180 | is always included by png.h. |
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181 | |
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182 | If you don't need to change the library configuration from the default, skip to |
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183 | the next section ("Reading"). |
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184 | |
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185 | Notice that some of the makefiles in the 'scripts' directory and (in 1.5.0) all |
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186 | of the build project files in the 'projects' directory simply copy |
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187 | scripts/pnglibconf.h.prebuilt to pnglibconf.h. This means that these build |
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188 | systems do not permit easy auto-configuration of the library - they only |
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189 | support the default configuration. |
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190 | |
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191 | The easiest way to make minor changes to the libpng configuration when |
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192 | auto-configuration is supported is to add definitions to the command line |
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193 | using (typically) CPPFLAGS. For example: |
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194 | |
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195 | CPPFLAGS=-DPNG_NO_FLOATING_ARITHMETIC |
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196 | |
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197 | will change the internal libpng math implementation for gamma correction and |
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198 | other arithmetic calculations to fixed point, avoiding the need for fast |
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199 | floating point support. The result can be seen in the generated pnglibconf.h - |
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200 | make sure it contains the changed feature macro setting. |
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201 | |
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202 | If you need to make more extensive configuration changes - more than one or two |
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203 | feature macro settings - you can either add -DPNG_USER_CONFIG to the build |
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204 | command line and put a list of feature macro settings in pngusr.h or you can set |
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205 | DFA_XTRA (a makefile variable) to a file containing the same information in the |
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206 | form of 'option' settings. |
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207 | |
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208 | A. Changing pnglibconf.h |
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209 | |
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210 | A variety of methods exist to build libpng. Not all of these support |
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211 | reconfiguration of pnglibconf.h. To reconfigure pnglibconf.h it must either be |
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212 | rebuilt from scripts/pnglibconf.dfa using awk or it must be edited by hand. |
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213 | |
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214 | Hand editing is achieved by copying scripts/pnglibconf.h.prebuilt to |
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215 | pnglibconf.h and changing the lines defining the supported features, paying |
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216 | very close attention to the 'option' information in scripts/pnglibconf.dfa |
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217 | that describes those features and their requirements. This is easy to get |
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218 | wrong. |
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219 | |
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220 | B. Configuration using DFA_XTRA |
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221 | |
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222 | Rebuilding from pnglibconf.dfa is easy if a functioning 'awk', or a later |
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223 | variant such as 'nawk' or 'gawk', is available. The configure build will |
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224 | automatically find an appropriate awk and build pnglibconf.h. |
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225 | The scripts/pnglibconf.mak file contains a set of make rules for doing the |
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226 | same thing if configure is not used, and many of the makefiles in the scripts |
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227 | directory use this approach. |
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228 | |
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229 | When rebuilding simply write a new file containing changed options and set |
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230 | DFA_XTRA to the name of this file. This causes the build to append the new file |
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231 | to the end of scripts/pnglibconf.dfa. The pngusr.dfa file should contain lines |
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232 | of the following forms: |
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233 | |
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234 | everything = off |
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235 | |
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236 | This turns all optional features off. Include it at the start of pngusr.dfa to |
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237 | make it easier to build a minimal configuration. You will need to turn at least |
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238 | some features on afterward to enable either reading or writing code, or both. |
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239 | |
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240 | option feature on |
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241 | option feature off |
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242 | |
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243 | Enable or disable a single feature. This will automatically enable other |
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244 | features required by a feature that is turned on or disable other features that |
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245 | require a feature which is turned off. Conflicting settings will cause an error |
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246 | message to be emitted by awk. |
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247 | |
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248 | setting feature default value |
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249 | |
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250 | Changes the default value of setting 'feature' to 'value'. There are a small |
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251 | number of settings listed at the top of pnglibconf.h, they are documented in the |
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252 | source code. Most of these values have performance implications for the library |
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253 | but most of them have no visible effect on the API. Some can also be overridden |
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254 | from the API. |
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255 | |
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256 | This method of building a customized pnglibconf.h is illustrated in |
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257 | contrib/pngminim/*. See the "$(PNGCONF):" target in the makefile and |
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258 | pngusr.dfa in these directories. |
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259 | |
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260 | C. Configuration using PNG_USR_CONFIG |
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261 | |
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262 | If -DPNG_USR_CONFIG is added to the CFLAGS when pnglibconf.h is built the file |
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263 | pngusr.h will automatically be included before the options in |
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264 | scripts/pnglibconf.dfa are processed. Your pngusr.h file should contain only |
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265 | macro definitions turning features on or off or setting settings. |
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266 | |
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267 | Apart from the global setting "everything = off" all the options listed above |
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268 | can be set using macros in pngusr.h: |
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269 | |
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270 | #define PNG_feature_SUPPORTED |
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271 | |
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272 | is equivalent to: |
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273 | |
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274 | option feature on |
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275 | |
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276 | #define PNG_NO_feature |
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277 | |
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278 | is equivalent to: |
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279 | |
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280 | option feature off |
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281 | |
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282 | #define PNG_feature value |
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283 | |
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284 | is equivalent to: |
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285 | |
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286 | setting feature default value |
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287 | |
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288 | Notice that in both cases, pngusr.dfa and pngusr.h, the contents of the |
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289 | pngusr file you supply override the contents of scripts/pnglibconf.dfa |
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290 | |
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291 | If confusing or incomprehensible behavior results it is possible to |
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292 | examine the intermediate file pnglibconf.dfn to find the full set of |
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293 | dependency information for each setting and option. Simply locate the |
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294 | feature in the file and read the C comments that precede it. |
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295 | |
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296 | This method is also illustrated in the contrib/pngminim/* makefiles and |
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297 | pngusr.h. |
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298 | |
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299 | III. Reading |
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300 | |
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301 | We'll now walk you through the possible functions to call when reading |
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302 | in a PNG file sequentially, briefly explaining the syntax and purpose |
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303 | of each one. See example.c and png.h for more detail. While |
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304 | progressive reading is covered in the next section, you will still |
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305 | need some of the functions discussed in this section to read a PNG |
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306 | file. |
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307 | |
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308 | Setup |
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309 | |
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310 | You will want to do the I/O initialization(*) before you get into libpng, |
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311 | so if it doesn't work, you don't have much to undo. Of course, you |
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312 | will also want to insure that you are, in fact, dealing with a PNG |
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313 | file. Libpng provides a simple check to see if a file is a PNG file. |
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314 | To use it, pass in the first 1 to 8 bytes of the file to the function |
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315 | png_sig_cmp(), and it will return 0 (false) if the bytes match the |
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316 | corresponding bytes of the PNG signature, or nonzero (true) otherwise. |
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317 | Of course, the more bytes you pass in, the greater the accuracy of the |
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318 | prediction. |
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319 | |
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320 | If you are intending to keep the file pointer open for use in libpng, |
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321 | you must ensure you don't read more than 8 bytes from the beginning |
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322 | of the file, and you also have to make a call to png_set_sig_bytes_read() |
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323 | with the number of bytes you read from the beginning. Libpng will |
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324 | then only check the bytes (if any) that your program didn't read. |
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325 | |
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326 | (*): If you are not using the standard I/O functions, you will need |
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327 | to replace them with custom functions. See the discussion under |
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328 | Customizing libpng. |
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329 | |
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330 | |
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331 | FILE *fp = fopen(file_name, "rb"); |
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332 | if (!fp) |
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333 | { |
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334 | return (ERROR); |
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335 | } |
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336 | |
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337 | fread(header, 1, number, fp); |
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338 | is_png = !png_sig_cmp(header, 0, number); |
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339 | |
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340 | if (!is_png) |
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341 | { |
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342 | return (NOT_PNG); |
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343 | } |
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344 | |
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345 | |
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346 | Next, png_struct and png_info need to be allocated and initialized. In |
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347 | order to ensure that the size of these structures is correct even with a |
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348 | dynamically linked libpng, there are functions to initialize and |
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349 | allocate the structures. We also pass the library version, optional |
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350 | pointers to error handling functions, and a pointer to a data struct for |
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351 | use by the error functions, if necessary (the pointer and functions can |
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352 | be NULL if the default error handlers are to be used). See the section |
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353 | on Changes to Libpng below regarding the old initialization functions. |
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354 | The structure allocation functions quietly return NULL if they fail to |
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355 | create the structure, so your application should check for that. |
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356 | |
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357 | png_structp png_ptr = png_create_read_struct |
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358 | (PNG_LIBPNG_VER_STRING, (png_voidp)user_error_ptr, |
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359 | user_error_fn, user_warning_fn); |
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360 | |
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361 | if (!png_ptr) |
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362 | return (ERROR); |
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363 | |
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364 | png_infop info_ptr = png_create_info_struct(png_ptr); |
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365 | |
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366 | if (!info_ptr) |
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367 | { |
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368 | png_destroy_read_struct(&png_ptr, |
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369 | (png_infopp)NULL, (png_infopp)NULL); |
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370 | return (ERROR); |
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371 | } |
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372 | |
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373 | If you want to use your own memory allocation routines, |
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374 | use a libpng that was built with PNG_USER_MEM_SUPPORTED defined, and use |
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375 | png_create_read_struct_2() instead of png_create_read_struct(): |
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376 | |
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377 | png_structp png_ptr = png_create_read_struct_2 |
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378 | (PNG_LIBPNG_VER_STRING, (png_voidp)user_error_ptr, |
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379 | user_error_fn, user_warning_fn, (png_voidp) |
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380 | user_mem_ptr, user_malloc_fn, user_free_fn); |
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381 | |
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382 | The error handling routines passed to png_create_read_struct() |
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383 | and the memory alloc/free routines passed to png_create_struct_2() |
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384 | are only necessary if you are not using the libpng supplied error |
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385 | handling and memory alloc/free functions. |
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386 | |
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387 | When libpng encounters an error, it expects to longjmp back |
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388 | to your routine. Therefore, you will need to call setjmp and pass |
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389 | your png_jmpbuf(png_ptr). If you read the file from different |
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390 | routines, you will need to update the longjmp buffer every time you enter |
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391 | a new routine that will call a png_*() function. |
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392 | |
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393 | See your documentation of setjmp/longjmp for your compiler for more |
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394 | information on setjmp/longjmp. See the discussion on libpng error |
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395 | handling in the Customizing Libpng section below for more information |
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396 | on the libpng error handling. If an error occurs, and libpng longjmp's |
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397 | back to your setjmp, you will want to call png_destroy_read_struct() to |
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398 | free any memory. |
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399 | |
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400 | if (setjmp(png_jmpbuf(png_ptr))) |
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401 | { |
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402 | png_destroy_read_struct(&png_ptr, &info_ptr, |
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403 | &end_info); |
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404 | fclose(fp); |
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405 | return (ERROR); |
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406 | } |
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407 | |
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408 | Pass (png_infopp)NULL instead of &end_info if you didn't create |
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409 | an end_info structure. |
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410 | |
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411 | If you would rather avoid the complexity of setjmp/longjmp issues, |
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412 | you can compile libpng with PNG_NO_SETJMP, in which case |
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413 | errors will result in a call to PNG_ABORT() which defaults to abort(). |
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414 | |
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415 | You can #define PNG_ABORT() to a function that does something |
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416 | more useful than abort(), as long as your function does not |
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417 | return. |
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418 | |
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419 | Now you need to set up the input code. The default for libpng is to |
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420 | use the C function fread(). If you use this, you will need to pass a |
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421 | valid FILE * in the function png_init_io(). Be sure that the file is |
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422 | opened in binary mode. If you wish to handle reading data in another |
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423 | way, you need not call the png_init_io() function, but you must then |
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424 | implement the libpng I/O methods discussed in the Customizing Libpng |
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425 | section below. |
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426 | |
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427 | png_init_io(png_ptr, fp); |
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428 | |
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429 | If you had previously opened the file and read any of the signature from |
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430 | the beginning in order to see if this was a PNG file, you need to let |
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431 | libpng know that there are some bytes missing from the start of the file. |
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432 | |
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433 | png_set_sig_bytes(png_ptr, number); |
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434 | |
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435 | You can change the zlib compression buffer size to be used while |
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436 | reading compressed data with |
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437 | |
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438 | png_set_compression_buffer_size(png_ptr, buffer_size); |
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439 | |
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440 | where the default size is 8192 bytes. Note that the buffer size |
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441 | is changed immediately and the buffer is reallocated immediately, |
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442 | instead of setting a flag to be acted upon later. |
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443 | |
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444 | If you want CRC errors to be handled in a different manner than |
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445 | the default, use |
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446 | |
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447 | png_set_crc_action(png_ptr, crit_action, ancil_action); |
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448 | |
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449 | The values for png_set_crc_action() say how libpng is to handle CRC errors in |
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450 | ancillary and critical chunks, and whether to use the data contained |
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451 | therein. Note that it is impossible to "discard" data in a critical |
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452 | chunk. |
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453 | |
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454 | Choices for (int) crit_action are |
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455 | PNG_CRC_DEFAULT 0 error/quit |
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456 | PNG_CRC_ERROR_QUIT 1 error/quit |
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457 | PNG_CRC_WARN_USE 3 warn/use data |
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458 | PNG_CRC_QUIET_USE 4 quiet/use data |
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459 | PNG_CRC_NO_CHANGE 5 use the current value |
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460 | |
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461 | Choices for (int) ancil_action are |
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462 | PNG_CRC_DEFAULT 0 error/quit |
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463 | PNG_CRC_ERROR_QUIT 1 error/quit |
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464 | PNG_CRC_WARN_DISCARD 2 warn/discard data |
---|
465 | PNG_CRC_WARN_USE 3 warn/use data |
---|
466 | PNG_CRC_QUIET_USE 4 quiet/use data |
---|
467 | PNG_CRC_NO_CHANGE 5 use the current value |
---|
468 | |
---|
469 | Setting up callback code |
---|
470 | |
---|
471 | You can set up a callback function to handle any unknown chunks in the |
---|
472 | input stream. You must supply the function |
---|
473 | |
---|
474 | read_chunk_callback(png_structp png_ptr, |
---|
475 | png_unknown_chunkp chunk); |
---|
476 | { |
---|
477 | /* The unknown chunk structure contains your |
---|
478 | chunk data, along with similar data for any other |
---|
479 | unknown chunks: */ |
---|
480 | |
---|
481 | png_byte name[5]; |
---|
482 | png_byte *data; |
---|
483 | png_size_t size; |
---|
484 | |
---|
485 | /* Note that libpng has already taken care of |
---|
486 | the CRC handling */ |
---|
487 | |
---|
488 | /* put your code here. Search for your chunk in the |
---|
489 | unknown chunk structure, process it, and return one |
---|
490 | of the following: */ |
---|
491 | |
---|
492 | return (-n); /* chunk had an error */ |
---|
493 | return (0); /* did not recognize */ |
---|
494 | return (n); /* success */ |
---|
495 | } |
---|
496 | |
---|
497 | (You can give your function another name that you like instead of |
---|
498 | "read_chunk_callback") |
---|
499 | |
---|
500 | To inform libpng about your function, use |
---|
501 | |
---|
502 | png_set_read_user_chunk_fn(png_ptr, user_chunk_ptr, |
---|
503 | read_chunk_callback); |
---|
504 | |
---|
505 | This names not only the callback function, but also a user pointer that |
---|
506 | you can retrieve with |
---|
507 | |
---|
508 | png_get_user_chunk_ptr(png_ptr); |
---|
509 | |
---|
510 | If you call the png_set_read_user_chunk_fn() function, then all unknown |
---|
511 | chunks will be saved when read, in case your callback function will need |
---|
512 | one or more of them. This behavior can be changed with the |
---|
513 | png_set_keep_unknown_chunks() function, described below. |
---|
514 | |
---|
515 | At this point, you can set up a callback function that will be |
---|
516 | called after each row has been read, which you can use to control |
---|
517 | a progress meter or the like. It's demonstrated in pngtest.c. |
---|
518 | You must supply a function |
---|
519 | |
---|
520 | void read_row_callback(png_structp png_ptr, |
---|
521 | png_uint_32 row, int pass); |
---|
522 | { |
---|
523 | /* put your code here */ |
---|
524 | } |
---|
525 | |
---|
526 | (You can give it another name that you like instead of "read_row_callback") |
---|
527 | |
---|
528 | To inform libpng about your function, use |
---|
529 | |
---|
530 | png_set_read_status_fn(png_ptr, read_row_callback); |
---|
531 | |
---|
532 | When this function is called the row has already been completely processed and |
---|
533 | the 'row' and 'pass' refer to the next row to be handled. For the |
---|
534 | non-interlaced case the row that was just handled is simply one less than the |
---|
535 | passed in row number, and pass will always be 0. For the interlaced case the |
---|
536 | same applies unless the row value is 0, in which case the row just handled was |
---|
537 | the last one from one of the preceding passes. Because interlacing may skip a |
---|
538 | pass you cannot be sure that the preceding pass is just 'pass-1', if you really |
---|
539 | need to know what the last pass is record (row,pass) from the callback and use |
---|
540 | the last recorded value each time. |
---|
541 | |
---|
542 | As with the user transform you can find the output row using the |
---|
543 | PNG_ROW_FROM_PASS_ROW macro. |
---|
544 | |
---|
545 | Unknown-chunk handling |
---|
546 | |
---|
547 | Now you get to set the way the library processes unknown chunks in the |
---|
548 | input PNG stream. Both known and unknown chunks will be read. Normal |
---|
549 | behavior is that known chunks will be parsed into information in |
---|
550 | various info_ptr members while unknown chunks will be discarded. This |
---|
551 | behavior can be wasteful if your application will never use some known |
---|
552 | chunk types. To change this, you can call: |
---|
553 | |
---|
554 | png_set_keep_unknown_chunks(png_ptr, keep, |
---|
555 | chunk_list, num_chunks); |
---|
556 | keep - 0: default unknown chunk handling |
---|
557 | 1: ignore; do not keep |
---|
558 | 2: keep only if safe-to-copy |
---|
559 | 3: keep even if unsafe-to-copy |
---|
560 | |
---|
561 | You can use these definitions: |
---|
562 | PNG_HANDLE_CHUNK_AS_DEFAULT 0 |
---|
563 | PNG_HANDLE_CHUNK_NEVER 1 |
---|
564 | PNG_HANDLE_CHUNK_IF_SAFE 2 |
---|
565 | PNG_HANDLE_CHUNK_ALWAYS 3 |
---|
566 | |
---|
567 | chunk_list - list of chunks affected (a byte string, |
---|
568 | five bytes per chunk, NULL or '\0' if |
---|
569 | num_chunks is 0) |
---|
570 | |
---|
571 | num_chunks - number of chunks affected; if 0, all |
---|
572 | unknown chunks are affected. If nonzero, |
---|
573 | only the chunks in the list are affected |
---|
574 | |
---|
575 | Unknown chunks declared in this way will be saved as raw data onto a |
---|
576 | list of png_unknown_chunk structures. If a chunk that is normally |
---|
577 | known to libpng is named in the list, it will be handled as unknown, |
---|
578 | according to the "keep" directive. If a chunk is named in successive |
---|
579 | instances of png_set_keep_unknown_chunks(), the final instance will |
---|
580 | take precedence. The IHDR and IEND chunks should not be named in |
---|
581 | chunk_list; if they are, libpng will process them normally anyway. |
---|
582 | If you know that your application will never make use of some particular |
---|
583 | chunks, use PNG_HANDLE_CHUNK_NEVER (or 1) as demonstrated below. |
---|
584 | |
---|
585 | Here is an example of the usage of png_set_keep_unknown_chunks(), |
---|
586 | where the private "vpAg" chunk will later be processed by a user chunk |
---|
587 | callback function: |
---|
588 | |
---|
589 | png_byte vpAg[5]={118, 112, 65, 103, (png_byte) '\0'}; |
---|
590 | |
---|
591 | #if defined(PNG_UNKNOWN_CHUNKS_SUPPORTED) |
---|
592 | png_byte unused_chunks[]= |
---|
593 | { |
---|
594 | 104, 73, 83, 84, (png_byte) '\0', /* hIST */ |
---|
595 | 105, 84, 88, 116, (png_byte) '\0', /* iTXt */ |
---|
596 | 112, 67, 65, 76, (png_byte) '\0', /* pCAL */ |
---|
597 | 115, 67, 65, 76, (png_byte) '\0', /* sCAL */ |
---|
598 | 115, 80, 76, 84, (png_byte) '\0', /* sPLT */ |
---|
599 | 116, 73, 77, 69, (png_byte) '\0', /* tIME */ |
---|
600 | }; |
---|
601 | #endif |
---|
602 | |
---|
603 | ... |
---|
604 | |
---|
605 | #if defined(PNG_UNKNOWN_CHUNKS_SUPPORTED) |
---|
606 | /* ignore all unknown chunks: */ |
---|
607 | png_set_keep_unknown_chunks(read_ptr, 1, NULL, 0); |
---|
608 | |
---|
609 | /* except for vpAg: */ |
---|
610 | png_set_keep_unknown_chunks(read_ptr, 2, vpAg, 1); |
---|
611 | |
---|
612 | /* also ignore unused known chunks: */ |
---|
613 | png_set_keep_unknown_chunks(read_ptr, 1, unused_chunks, |
---|
614 | (int)sizeof(unused_chunks)/5); |
---|
615 | #endif |
---|
616 | |
---|
617 | User limits |
---|
618 | |
---|
619 | The PNG specification allows the width and height of an image to be as |
---|
620 | large as 2^31-1 (0x7fffffff), or about 2.147 billion rows and columns. |
---|
621 | Since very few applications really need to process such large images, |
---|
622 | we have imposed an arbitrary 1-million limit on rows and columns. |
---|
623 | Larger images will be rejected immediately with a png_error() call. If |
---|
624 | you wish to change this limit, you can use |
---|
625 | |
---|
626 | png_set_user_limits(png_ptr, width_max, height_max); |
---|
627 | |
---|
628 | to set your own limits, or use width_max = height_max = 0x7fffffffL |
---|
629 | to allow all valid dimensions (libpng may reject some very large images |
---|
630 | anyway because of potential buffer overflow conditions). |
---|
631 | |
---|
632 | You should put this statement after you create the PNG structure and |
---|
633 | before calling png_read_info(), png_read_png(), or png_process_data(). |
---|
634 | |
---|
635 | When writing a PNG datastream, put this statement before calling |
---|
636 | png_write_info() or png_write_png(). |
---|
637 | |
---|
638 | If you need to retrieve the limits that are being applied, use |
---|
639 | |
---|
640 | width_max = png_get_user_width_max(png_ptr); |
---|
641 | height_max = png_get_user_height_max(png_ptr); |
---|
642 | |
---|
643 | The PNG specification sets no limit on the number of ancillary chunks |
---|
644 | allowed in a PNG datastream. You can impose a limit on the total number |
---|
645 | of sPLT, tEXt, iTXt, zTXt, and unknown chunks that will be stored, with |
---|
646 | |
---|
647 | png_set_chunk_cache_max(png_ptr, user_chunk_cache_max); |
---|
648 | |
---|
649 | where 0x7fffffffL means unlimited. You can retrieve this limit with |
---|
650 | |
---|
651 | chunk_cache_max = png_get_chunk_cache_max(png_ptr); |
---|
652 | |
---|
653 | This limit also applies to the number of buffers that can be allocated |
---|
654 | by png_decompress_chunk() while decompressing iTXt, zTXt, and iCCP chunks. |
---|
655 | |
---|
656 | You can also set a limit on the amount of memory that a compressed chunk |
---|
657 | other than IDAT can occupy, with |
---|
658 | |
---|
659 | png_set_chunk_malloc_max(png_ptr, user_chunk_malloc_max); |
---|
660 | |
---|
661 | and you can retrieve the limit with |
---|
662 | |
---|
663 | chunk_malloc_max = png_get_chunk_malloc_max(png_ptr); |
---|
664 | |
---|
665 | Any chunks that would cause either of these limits to be exceeded will |
---|
666 | be ignored. |
---|
667 | |
---|
668 | Information about your system |
---|
669 | |
---|
670 | If you intend to display the PNG or to incorporate it in other image data you |
---|
671 | need to tell libpng information about your display or drawing surface so that |
---|
672 | libpng can convert the values in the image to match the display. |
---|
673 | |
---|
674 | From libpng-1.5.4 this information can be set before reading the PNG file |
---|
675 | header. In earlier versions png_set_gamma() existed but behaved incorrectly if |
---|
676 | called before the PNG file header had been read and png_set_alpha_mode() did not |
---|
677 | exist. |
---|
678 | |
---|
679 | If you need to support versions prior to libpng-1.5.4 test the version number |
---|
680 | as illustrated below using "PNG_LIBPNG_VER >= 10504" and follow the procedures |
---|
681 | described in the appropriate manual page. |
---|
682 | |
---|
683 | You give libpng the encoding expected by your system expressed as a 'gamma' |
---|
684 | value. You can also specify a default encoding for the PNG file in |
---|
685 | case the required information is missing from the file. By default libpng |
---|
686 | assumes that the PNG data matches your system, to keep this default call: |
---|
687 | |
---|
688 | png_set_gamma(png_ptr, screen_gamma, 1/screen_gamma/*file gamma*/); |
---|
689 | |
---|
690 | or you can use the fixed point equivalent: |
---|
691 | |
---|
692 | png_set_gamma_fixed(png_ptr, PNG_FP_1*screen_gamma, PNG_FP_1/screen_gamma); |
---|
693 | |
---|
694 | If you don't know the gamma for your system it is probably 2.2 - a good |
---|
695 | approximation to the IEC standard for display systems (sRGB). If images are |
---|
696 | too contrasty or washed out you got the value wrong - check your system |
---|
697 | documentation! |
---|
698 | |
---|
699 | Many systems permit the system gamma to be changed via a lookup table in the |
---|
700 | display driver, a few systems, including older Macs, change the response by |
---|
701 | default. As of 1.5.4 three special values are available to handle common |
---|
702 | situations: |
---|
703 | |
---|
704 | PNG_DEFAULT_sRGB: Indicates that the system conforms to the IEC 61966-2-1 |
---|
705 | standard. This matches almost all systems. |
---|
706 | PNG_GAMMA_MAC_18: Indicates that the system is an older (pre Mac OS 10.6) |
---|
707 | Apple Macintosh system with the default settings. |
---|
708 | PNG_GAMMA_LINEAR: Just the fixed point value for 1.0 - indicates that the |
---|
709 | system expects data with no gamma encoding. |
---|
710 | |
---|
711 | You would use the linear (unencoded) value if you need to process the pixel |
---|
712 | values further because this avoids the need to decode and reencode each |
---|
713 | component value whenever arithmetic is performed. A lot of graphics software |
---|
714 | uses linear values for this reason, often with higher precision component values |
---|
715 | to preserve overall accuracy. |
---|
716 | |
---|
717 | The second thing you may need to tell libpng about is how your system handles |
---|
718 | alpha channel information. Some, but not all, PNG files contain an alpha |
---|
719 | channel. To display these files correctly you need to compose the data onto a |
---|
720 | suitable background, as described in the PNG specification. |
---|
721 | |
---|
722 | Libpng only supports composing onto a single color (using png_set_background; |
---|
723 | see below). Otherwise you must do the composition yourself and, in this case, |
---|
724 | you may need to call png_set_alpha_mode: |
---|
725 | |
---|
726 | #if PNG_LIBPNG_VER >= 10504 |
---|
727 | png_set_alpha_mode(png_ptr, mode, screen_gamma); |
---|
728 | #else |
---|
729 | png_set_gamma(png_ptr, screen_gamma, 1.0/screen_gamma); |
---|
730 | #endif |
---|
731 | |
---|
732 | The screen_gamma value is the same as the argument to png_set_gamma; however, |
---|
733 | how it affects the output depends on the mode. png_set_alpha_mode() sets the |
---|
734 | file gamma default to 1/screen_gamma, so normally you don't need to call |
---|
735 | png_set_gamma. If you need different defaults call png_set_gamma() before |
---|
736 | png_set_alpha_mode() - if you call it after it will override the settings made |
---|
737 | by png_set_alpha_mode(). |
---|
738 | |
---|
739 | The mode is as follows: |
---|
740 | |
---|
741 | PNG_ALPHA_PNG: The data is encoded according to the PNG specification. Red, |
---|
742 | green and blue, or gray, components are gamma encoded color |
---|
743 | values and are not premultiplied by the alpha value. The |
---|
744 | alpha value is a linear measure of the contribution of the |
---|
745 | pixel to the corresponding final output pixel. |
---|
746 | |
---|
747 | You should normally use this format if you intend to perform |
---|
748 | color correction on the color values; most, maybe all, color |
---|
749 | correction software has no handling for the alpha channel and, |
---|
750 | anyway, the math to handle pre-multiplied component values is |
---|
751 | unnecessarily complex. |
---|
752 | |
---|
753 | Before you do any arithmetic on the component values you need |
---|
754 | to remove the gamma encoding and multiply out the alpha |
---|
755 | channel. See the PNG specification for more detail. It is |
---|
756 | important to note that when an image with an alpha channel is |
---|
757 | scaled, linear encoded, pre-multiplied component values must |
---|
758 | be used! |
---|
759 | |
---|
760 | The remaining modes assume you don't need to do any further color correction or |
---|
761 | that if you do, your color correction software knows all about alpha (it |
---|
762 | probably doesn't!) |
---|
763 | |
---|
764 | PNG_ALPHA_STANDARD: The data libpng produces |
---|
765 | is encoded in the standard way |
---|
766 | assumed by most correctly written graphics software. |
---|
767 | The gamma encoding will be removed by libpng and the |
---|
768 | linear component values will be pre-multiplied by the |
---|
769 | alpha channel. |
---|
770 | |
---|
771 | With this format the final image must be re-encoded to |
---|
772 | match the display gamma before the image is displayed. |
---|
773 | If your system doesn't do that, yet still seems to |
---|
774 | perform arithmetic on the pixels without decoding them, |
---|
775 | it is broken - check out the modes below. |
---|
776 | |
---|
777 | With PNG_ALPHA_STANDARD libpng always produces linear |
---|
778 | component values, whatever screen_gamma you supply. The |
---|
779 | screen_gamma value is, however, used as a default for |
---|
780 | the file gamma if the PNG file has no gamma information. |
---|
781 | |
---|
782 | If you call png_set_gamma() after png_set_alpha_mode() you |
---|
783 | will override the linear encoding. Instead the |
---|
784 | pre-multiplied pixel values will be gamma encoded but |
---|
785 | the alpha channel will still be linear. This may |
---|
786 | actually match the requirements of some broken software, |
---|
787 | but it is unlikely. |
---|
788 | |
---|
789 | While linear 8-bit data is often used it has |
---|
790 | insufficient precision for any image with a reasonable |
---|
791 | dynamic range. To avoid problems, and if your software |
---|
792 | supports it, use png_set_expand_16() to force all |
---|
793 | components to 16 bits. |
---|
794 | |
---|
795 | PNG_ALPHA_OPTIMIZED: This mode is the same |
---|
796 | as PNG_ALPHA_STANDARD except that |
---|
797 | completely opaque pixels are gamma encoded according to |
---|
798 | the screen_gamma value. Pixels with alpha less than 1.0 |
---|
799 | will still have linear components. |
---|
800 | |
---|
801 | Use this format if you have control over your |
---|
802 | compositing software and so don't do other arithmetic |
---|
803 | (such as scaling) on the data you get from libpng. Your |
---|
804 | compositing software can simply copy opaque pixels to |
---|
805 | the output but still has linear values for the |
---|
806 | non-opaque pixels. |
---|
807 | |
---|
808 | In normal compositing, where the alpha channel encodes |
---|
809 | partial pixel coverage (as opposed to broad area |
---|
810 | translucency), the inaccuracies of the 8-bit |
---|
811 | representation of non-opaque pixels are irrelevant. |
---|
812 | |
---|
813 | You can also try this format if your software is broken; |
---|
814 | it might look better. |
---|
815 | |
---|
816 | PNG_ALPHA_BROKEN: This is PNG_ALPHA_STANDARD; |
---|
817 | however, all component values, |
---|
818 | including the alpha channel are gamma encoded. This is |
---|
819 | an appropriate format to try if your software, or more |
---|
820 | likely hardware, is totally broken, i.e., if it performs |
---|
821 | linear arithmetic directly on gamma encoded values. |
---|
822 | |
---|
823 | In most cases of broken software or hardware the bug in the final display |
---|
824 | manifests as a subtle halo around composited parts of the image. You may not |
---|
825 | even perceive this as a halo; the composited part of the image may simply appear |
---|
826 | separate from the background, as though it had been cut out of paper and pasted |
---|
827 | on afterward. |
---|
828 | |
---|
829 | If you don't have to deal with bugs in software or hardware, or if you can fix |
---|
830 | them, there are three recommended ways of using png_set_alpha_mode(): |
---|
831 | |
---|
832 | png_set_alpha_mode(png_ptr, PNG_ALPHA_PNG, |
---|
833 | screen_gamma); |
---|
834 | |
---|
835 | You can do color correction on the result (libpng does not currently |
---|
836 | support color correction internally). When you handle the alpha channel |
---|
837 | you need to undo the gamma encoding and multiply out the alpha. |
---|
838 | |
---|
839 | png_set_alpha_mode(png_ptr, PNG_ALPHA_STANDARD, |
---|
840 | screen_gamma); |
---|
841 | png_set_expand_16(png_ptr); |
---|
842 | |
---|
843 | If you are using the high level interface, don't call png_set_expand_16(); |
---|
844 | instead pass PNG_TRANSFORM_EXPAND_16 to the interface. |
---|
845 | |
---|
846 | With this mode you can't do color correction, but you can do arithmetic, |
---|
847 | including composition and scaling, on the data without further processing. |
---|
848 | |
---|
849 | png_set_alpha_mode(png_ptr, PNG_ALPHA_OPTIMIZED, |
---|
850 | screen_gamma); |
---|
851 | |
---|
852 | You can avoid the expansion to 16-bit components with this mode, but you |
---|
853 | lose the ability to scale the image or perform other linear arithmetic. |
---|
854 | All you can do is compose the result onto a matching output. Since this |
---|
855 | mode is libpng-specific you also need to write your own composition |
---|
856 | software. |
---|
857 | |
---|
858 | If you don't need, or can't handle, the alpha channel you can call |
---|
859 | png_set_background() to remove it by compositing against a fixed color. Don't |
---|
860 | call png_set_strip_alpha() to do this - it will leave spurious pixel values in |
---|
861 | transparent parts of this image. |
---|
862 | |
---|
863 | png_set_background(png_ptr, &background_color, |
---|
864 | PNG_BACKGROUND_GAMMA_SCREEN, 0, 1); |
---|
865 | |
---|
866 | The background_color is an RGB or grayscale value according to the data format |
---|
867 | libpng will produce for you. Because you don't yet know the format of the PNG |
---|
868 | file, if you call png_set_background at this point you must arrange for the |
---|
869 | format produced by libpng to always have 8-bit or 16-bit components and then |
---|
870 | store the color as an 8-bit or 16-bit color as appropriate. The color contains |
---|
871 | separate gray and RGB component values, so you can let libpng produce gray or |
---|
872 | RGB output according to the input format, but low bit depth grayscale images |
---|
873 | must always be converted to at least 8-bit format. (Even though low bit depth |
---|
874 | grayscale images can't have an alpha channel they can have a transparent |
---|
875 | color!) |
---|
876 | |
---|
877 | You set the transforms you need later, either as flags to the high level |
---|
878 | interface or libpng API calls for the low level interface. For reference the |
---|
879 | settings and API calls required are: |
---|
880 | |
---|
881 | 8-bit values: |
---|
882 | PNG_TRANSFORM_SCALE_16 | PNG_EXPAND |
---|
883 | png_set_expand(png_ptr); png_set_scale_16(png_ptr); |
---|
884 | |
---|
885 | If you must get exactly the same inaccurate results |
---|
886 | produced by default in versions prior to libpng-1.5.4, |
---|
887 | use PNG_TRANSFORM_STRIP_16 and png_set_strip_16(png_ptr) |
---|
888 | instead. |
---|
889 | |
---|
890 | 16-bit values: |
---|
891 | PNG_TRANSFORM_EXPAND_16 |
---|
892 | png_set_expand_16(png_ptr); |
---|
893 | |
---|
894 | In either case palette image data will be expanded to RGB. If you just want |
---|
895 | color data you can add PNG_TRANSFORM_GRAY_TO_RGB or png_set_gray_to_rgb(png_ptr) |
---|
896 | to the list. |
---|
897 | |
---|
898 | Calling png_set_background before the PNG file header is read will not work |
---|
899 | prior to libpng-1.5.4. Because the failure may result in unexpected warnings or |
---|
900 | errors it is therefore much safer to call png_set_background after the head has |
---|
901 | been read. Unfortunately this means that prior to libpng-1.5.4 it cannot be |
---|
902 | used with the high level interface. |
---|
903 | |
---|
904 | The high-level read interface |
---|
905 | |
---|
906 | At this point there are two ways to proceed; through the high-level |
---|
907 | read interface, or through a sequence of low-level read operations. |
---|
908 | You can use the high-level interface if (a) you are willing to read |
---|
909 | the entire image into memory, and (b) the input transformations |
---|
910 | you want to do are limited to the following set: |
---|
911 | |
---|
912 | PNG_TRANSFORM_IDENTITY No transformation |
---|
913 | PNG_TRANSFORM_SCALE_16 Strip 16-bit samples to |
---|
914 | 8-bit accurately |
---|
915 | PNG_TRANSFORM_STRIP_16 Chop 16-bit samples to |
---|
916 | 8-bit less accurately |
---|
917 | PNG_TRANSFORM_STRIP_ALPHA Discard the alpha channel |
---|
918 | PNG_TRANSFORM_PACKING Expand 1, 2 and 4-bit |
---|
919 | samples to bytes |
---|
920 | PNG_TRANSFORM_PACKSWAP Change order of packed |
---|
921 | pixels to LSB first |
---|
922 | PNG_TRANSFORM_EXPAND Perform set_expand() |
---|
923 | PNG_TRANSFORM_INVERT_MONO Invert monochrome images |
---|
924 | PNG_TRANSFORM_SHIFT Normalize pixels to the |
---|
925 | sBIT depth |
---|
926 | PNG_TRANSFORM_BGR Flip RGB to BGR, RGBA |
---|
927 | to BGRA |
---|
928 | PNG_TRANSFORM_SWAP_ALPHA Flip RGBA to ARGB or GA |
---|
929 | to AG |
---|
930 | PNG_TRANSFORM_INVERT_ALPHA Change alpha from opacity |
---|
931 | to transparency |
---|
932 | PNG_TRANSFORM_SWAP_ENDIAN Byte-swap 16-bit samples |
---|
933 | PNG_TRANSFORM_GRAY_TO_RGB Expand grayscale samples |
---|
934 | to RGB (or GA to RGBA) |
---|
935 | PNG_TRANSFORM_EXPAND_16 Expand samples to 16 bits |
---|
936 | |
---|
937 | (This excludes setting a background color, doing gamma transformation, |
---|
938 | quantizing, and setting filler.) If this is the case, simply do this: |
---|
939 | |
---|
940 | png_read_png(png_ptr, info_ptr, png_transforms, NULL) |
---|
941 | |
---|
942 | where png_transforms is an integer containing the bitwise OR of some |
---|
943 | set of transformation flags. This call is equivalent to png_read_info(), |
---|
944 | followed the set of transformations indicated by the transform mask, |
---|
945 | then png_read_image(), and finally png_read_end(). |
---|
946 | |
---|
947 | (The final parameter of this call is not yet used. Someday it might point |
---|
948 | to transformation parameters required by some future input transform.) |
---|
949 | |
---|
950 | You must use png_transforms and not call any png_set_transform() functions |
---|
951 | when you use png_read_png(). |
---|
952 | |
---|
953 | After you have called png_read_png(), you can retrieve the image data |
---|
954 | with |
---|
955 | |
---|
956 | row_pointers = png_get_rows(png_ptr, info_ptr); |
---|
957 | |
---|
958 | where row_pointers is an array of pointers to the pixel data for each row: |
---|
959 | |
---|
960 | png_bytep row_pointers[height]; |
---|
961 | |
---|
962 | If you know your image size and pixel size ahead of time, you can allocate |
---|
963 | row_pointers prior to calling png_read_png() with |
---|
964 | |
---|
965 | if (height > PNG_UINT_32_MAX/png_sizeof(png_byte)) |
---|
966 | png_error (png_ptr, |
---|
967 | "Image is too tall to process in memory"); |
---|
968 | |
---|
969 | if (width > PNG_UINT_32_MAX/pixel_size) |
---|
970 | png_error (png_ptr, |
---|
971 | "Image is too wide to process in memory"); |
---|
972 | |
---|
973 | row_pointers = png_malloc(png_ptr, |
---|
974 | height*png_sizeof(png_bytep)); |
---|
975 | |
---|
976 | for (int i=0; i<height, i++) |
---|
977 | row_pointers[i]=NULL; /* security precaution */ |
---|
978 | |
---|
979 | for (int i=0; i<height, i++) |
---|
980 | row_pointers[i]=png_malloc(png_ptr, |
---|
981 | width*pixel_size); |
---|
982 | |
---|
983 | png_set_rows(png_ptr, info_ptr, &row_pointers); |
---|
984 | |
---|
985 | Alternatively you could allocate your image in one big block and define |
---|
986 | row_pointers[i] to point into the proper places in your block. |
---|
987 | |
---|
988 | If you use png_set_rows(), the application is responsible for freeing |
---|
989 | row_pointers (and row_pointers[i], if they were separately allocated). |
---|
990 | |
---|
991 | If you don't allocate row_pointers ahead of time, png_read_png() will |
---|
992 | do it, and it'll be free'ed by libpng when you call png_destroy_*(). |
---|
993 | |
---|
994 | The low-level read interface |
---|
995 | |
---|
996 | If you are going the low-level route, you are now ready to read all |
---|
997 | the file information up to the actual image data. You do this with a |
---|
998 | call to png_read_info(). |
---|
999 | |
---|
1000 | png_read_info(png_ptr, info_ptr); |
---|
1001 | |
---|
1002 | This will process all chunks up to but not including the image data. |
---|
1003 | |
---|
1004 | This also copies some of the data from the PNG file into the decode structure |
---|
1005 | for use in later transformations. Important information copied in is: |
---|
1006 | |
---|
1007 | 1) The PNG file gamma from the gAMA chunk. This overwrites the default value |
---|
1008 | provided by an earlier call to png_set_gamma or png_set_alpha_mode. |
---|
1009 | |
---|
1010 | 2) Prior to libpng-1.5.4 the background color from a bKGd chunk. This |
---|
1011 | damages the information provided by an earlier call to png_set_background |
---|
1012 | resulting in unexpected behavior. Libpng-1.5.4 no longer does this. |
---|
1013 | |
---|
1014 | 3) The number of significant bits in each component value. Libpng uses this to |
---|
1015 | optimize gamma handling by reducing the internal lookup table sizes. |
---|
1016 | |
---|
1017 | 4) The transparent color information from a tRNS chunk. This can be modified by |
---|
1018 | a later call to png_set_tRNS. |
---|
1019 | |
---|
1020 | Querying the info structure |
---|
1021 | |
---|
1022 | Functions are used to get the information from the info_ptr once it |
---|
1023 | has been read. Note that these fields may not be completely filled |
---|
1024 | in until png_read_end() has read the chunk data following the image. |
---|
1025 | |
---|
1026 | png_get_IHDR(png_ptr, info_ptr, &width, &height, |
---|
1027 | &bit_depth, &color_type, &interlace_type, |
---|
1028 | &compression_type, &filter_method); |
---|
1029 | |
---|
1030 | width - holds the width of the image |
---|
1031 | in pixels (up to 2^31). |
---|
1032 | |
---|
1033 | height - holds the height of the image |
---|
1034 | in pixels (up to 2^31). |
---|
1035 | |
---|
1036 | bit_depth - holds the bit depth of one of the |
---|
1037 | image channels. (valid values are |
---|
1038 | 1, 2, 4, 8, 16 and depend also on |
---|
1039 | the color_type. See also |
---|
1040 | significant bits (sBIT) below). |
---|
1041 | |
---|
1042 | color_type - describes which color/alpha channels |
---|
1043 | are present. |
---|
1044 | PNG_COLOR_TYPE_GRAY |
---|
1045 | (bit depths 1, 2, 4, 8, 16) |
---|
1046 | PNG_COLOR_TYPE_GRAY_ALPHA |
---|
1047 | (bit depths 8, 16) |
---|
1048 | PNG_COLOR_TYPE_PALETTE |
---|
1049 | (bit depths 1, 2, 4, 8) |
---|
1050 | PNG_COLOR_TYPE_RGB |
---|
1051 | (bit_depths 8, 16) |
---|
1052 | PNG_COLOR_TYPE_RGB_ALPHA |
---|
1053 | (bit_depths 8, 16) |
---|
1054 | |
---|
1055 | PNG_COLOR_MASK_PALETTE |
---|
1056 | PNG_COLOR_MASK_COLOR |
---|
1057 | PNG_COLOR_MASK_ALPHA |
---|
1058 | |
---|
1059 | interlace_type - (PNG_INTERLACE_NONE or |
---|
1060 | PNG_INTERLACE_ADAM7) |
---|
1061 | |
---|
1062 | compression_type - (must be PNG_COMPRESSION_TYPE_BASE |
---|
1063 | for PNG 1.0) |
---|
1064 | |
---|
1065 | filter_method - (must be PNG_FILTER_TYPE_BASE |
---|
1066 | for PNG 1.0, and can also be |
---|
1067 | PNG_INTRAPIXEL_DIFFERENCING if |
---|
1068 | the PNG datastream is embedded in |
---|
1069 | a MNG-1.0 datastream) |
---|
1070 | |
---|
1071 | Any or all of interlace_type, compression_type, or |
---|
1072 | filter_method can be NULL if you are |
---|
1073 | not interested in their values. |
---|
1074 | |
---|
1075 | Note that png_get_IHDR() returns 32-bit data into |
---|
1076 | the application's width and height variables. |
---|
1077 | This is an unsafe situation if these are 16-bit |
---|
1078 | variables. In such situations, the |
---|
1079 | png_get_image_width() and png_get_image_height() |
---|
1080 | functions described below are safer. |
---|
1081 | |
---|
1082 | width = png_get_image_width(png_ptr, |
---|
1083 | info_ptr); |
---|
1084 | |
---|
1085 | height = png_get_image_height(png_ptr, |
---|
1086 | info_ptr); |
---|
1087 | |
---|
1088 | bit_depth = png_get_bit_depth(png_ptr, |
---|
1089 | info_ptr); |
---|
1090 | |
---|
1091 | color_type = png_get_color_type(png_ptr, |
---|
1092 | info_ptr); |
---|
1093 | |
---|
1094 | interlace_type = png_get_interlace_type(png_ptr, |
---|
1095 | info_ptr); |
---|
1096 | |
---|
1097 | compression_type = png_get_compression_type(png_ptr, |
---|
1098 | info_ptr); |
---|
1099 | |
---|
1100 | filter_method = png_get_filter_type(png_ptr, |
---|
1101 | info_ptr); |
---|
1102 | |
---|
1103 | channels = png_get_channels(png_ptr, info_ptr); |
---|
1104 | |
---|
1105 | channels - number of channels of info for the |
---|
1106 | color type (valid values are 1 (GRAY, |
---|
1107 | PALETTE), 2 (GRAY_ALPHA), 3 (RGB), |
---|
1108 | 4 (RGB_ALPHA or RGB + filler byte)) |
---|
1109 | |
---|
1110 | rowbytes = png_get_rowbytes(png_ptr, info_ptr); |
---|
1111 | |
---|
1112 | rowbytes - number of bytes needed to hold a row |
---|
1113 | |
---|
1114 | signature = png_get_signature(png_ptr, info_ptr); |
---|
1115 | |
---|
1116 | signature - holds the signature read from the |
---|
1117 | file (if any). The data is kept in |
---|
1118 | the same offset it would be if the |
---|
1119 | whole signature were read (i.e. if an |
---|
1120 | application had already read in 4 |
---|
1121 | bytes of signature before starting |
---|
1122 | libpng, the remaining 4 bytes would |
---|
1123 | be in signature[4] through signature[7] |
---|
1124 | (see png_set_sig_bytes())). |
---|
1125 | |
---|
1126 | These are also important, but their validity depends on whether the chunk |
---|
1127 | has been read. The png_get_valid(png_ptr, info_ptr, PNG_INFO_<chunk>) and |
---|
1128 | png_get_<chunk>(png_ptr, info_ptr, ...) functions return non-zero if the |
---|
1129 | data has been read, or zero if it is missing. The parameters to the |
---|
1130 | png_get_<chunk> are set directly if they are simple data types, or a |
---|
1131 | pointer into the info_ptr is returned for any complex types. |
---|
1132 | |
---|
1133 | The colorspace data from gAMA, cHRM, sRGB, iCCP, and sBIT chunks |
---|
1134 | is simply returned to give the application information about how the |
---|
1135 | image was encoded. Libpng itself only does transformations using the file |
---|
1136 | gamma when combining semitransparent pixels with the background color. |
---|
1137 | |
---|
1138 | png_get_PLTE(png_ptr, info_ptr, &palette, |
---|
1139 | &num_palette); |
---|
1140 | |
---|
1141 | palette - the palette for the file |
---|
1142 | (array of png_color) |
---|
1143 | |
---|
1144 | num_palette - number of entries in the palette |
---|
1145 | |
---|
1146 | png_get_gAMA(png_ptr, info_ptr, &file_gamma); |
---|
1147 | png_get_gAMA_fixed(png_ptr, info_ptr, &int_file_gamma); |
---|
1148 | |
---|
1149 | file_gamma - the gamma at which the file was |
---|
1150 | written (PNG_INFO_gAMA) |
---|
1151 | |
---|
1152 | int_file_gamma - 100,000 times the gamma at which the |
---|
1153 | file is written |
---|
1154 | |
---|
1155 | png_get_cHRM(png_ptr, info_ptr, &white_x, &white_y, &red_x, &red_y, |
---|
1156 | &green_x, &green_y, &blue_x, &blue_y) |
---|
1157 | png_get_cHRM_XYZ(png_ptr, info_ptr, &red_X, &red_Y, &red_Z, &green_X, |
---|
1158 | &green_Y, &green_Z, &blue_X, &blue_Y, &blue_Z) |
---|
1159 | png_get_cHRM_fixed(png_ptr, info_ptr, &int_white_x, &int_white_y, |
---|
1160 | &int_red_x, &int_red_y, &int_green_x, &int_green_y, |
---|
1161 | &int_blue_x, &int_blue_y) |
---|
1162 | png_get_cHRM_XYZ_fixed(png_ptr, info_ptr, &int_red_X, &int_red_Y, |
---|
1163 | &int_red_Z, &int_green_X, &int_green_Y, &int_green_Z, |
---|
1164 | &int_blue_X, &int_blue_Y, &int_blue_Z) |
---|
1165 | |
---|
1166 | {white,red,green,blue}_{x,y} |
---|
1167 | A color space encoding specified using the chromaticities |
---|
1168 | of the end points and the white point. (PNG_INFO_cHRM) |
---|
1169 | |
---|
1170 | {red,green,blue}_{X,Y,Z} |
---|
1171 | A color space encoding specified using the encoding end |
---|
1172 | points - the CIE tristimulus specification of the intended |
---|
1173 | color of the red, green and blue channels in the PNG RGB |
---|
1174 | data. The white point is simply the sum of the three end |
---|
1175 | points. (PNG_INFO_cHRM) |
---|
1176 | |
---|
1177 | png_get_sRGB(png_ptr, info_ptr, &srgb_intent); |
---|
1178 | |
---|
1179 | file_srgb_intent - the rendering intent (PNG_INFO_sRGB) |
---|
1180 | The presence of the sRGB chunk |
---|
1181 | means that the pixel data is in the |
---|
1182 | sRGB color space. This chunk also |
---|
1183 | implies specific values of gAMA and |
---|
1184 | cHRM. |
---|
1185 | |
---|
1186 | png_get_iCCP(png_ptr, info_ptr, &name, |
---|
1187 | &compression_type, &profile, &proflen); |
---|
1188 | |
---|
1189 | name - The profile name. |
---|
1190 | |
---|
1191 | compression_type - The compression type; always |
---|
1192 | PNG_COMPRESSION_TYPE_BASE for PNG 1.0. |
---|
1193 | You may give NULL to this argument to |
---|
1194 | ignore it. |
---|
1195 | |
---|
1196 | profile - International Color Consortium color |
---|
1197 | profile data. May contain NULs. |
---|
1198 | |
---|
1199 | proflen - length of profile data in bytes. |
---|
1200 | |
---|
1201 | png_get_sBIT(png_ptr, info_ptr, &sig_bit); |
---|
1202 | |
---|
1203 | sig_bit - the number of significant bits for |
---|
1204 | (PNG_INFO_sBIT) each of the gray, |
---|
1205 | red, green, and blue channels, |
---|
1206 | whichever are appropriate for the |
---|
1207 | given color type (png_color_16) |
---|
1208 | |
---|
1209 | png_get_tRNS(png_ptr, info_ptr, &trans_alpha, |
---|
1210 | &num_trans, &trans_color); |
---|
1211 | |
---|
1212 | trans_alpha - array of alpha (transparency) |
---|
1213 | entries for palette (PNG_INFO_tRNS) |
---|
1214 | |
---|
1215 | num_trans - number of transparent entries |
---|
1216 | (PNG_INFO_tRNS) |
---|
1217 | |
---|
1218 | trans_color - graylevel or color sample values of |
---|
1219 | the single transparent color for |
---|
1220 | non-paletted images (PNG_INFO_tRNS) |
---|
1221 | |
---|
1222 | png_get_hIST(png_ptr, info_ptr, &hist); |
---|
1223 | (PNG_INFO_hIST) |
---|
1224 | |
---|
1225 | hist - histogram of palette (array of |
---|
1226 | png_uint_16) |
---|
1227 | |
---|
1228 | png_get_tIME(png_ptr, info_ptr, &mod_time); |
---|
1229 | |
---|
1230 | mod_time - time image was last modified |
---|
1231 | (PNG_VALID_tIME) |
---|
1232 | |
---|
1233 | png_get_bKGD(png_ptr, info_ptr, &background); |
---|
1234 | |
---|
1235 | background - background color (of type |
---|
1236 | png_color_16p) (PNG_VALID_bKGD) |
---|
1237 | valid 16-bit red, green and blue |
---|
1238 | values, regardless of color_type |
---|
1239 | |
---|
1240 | num_comments = png_get_text(png_ptr, info_ptr, |
---|
1241 | &text_ptr, &num_text); |
---|
1242 | |
---|
1243 | num_comments - number of comments |
---|
1244 | |
---|
1245 | text_ptr - array of png_text holding image |
---|
1246 | comments |
---|
1247 | |
---|
1248 | text_ptr[i].compression - type of compression used |
---|
1249 | on "text" PNG_TEXT_COMPRESSION_NONE |
---|
1250 | PNG_TEXT_COMPRESSION_zTXt |
---|
1251 | PNG_ITXT_COMPRESSION_NONE |
---|
1252 | PNG_ITXT_COMPRESSION_zTXt |
---|
1253 | |
---|
1254 | text_ptr[i].key - keyword for comment. Must contain |
---|
1255 | 1-79 characters. |
---|
1256 | |
---|
1257 | text_ptr[i].text - text comments for current |
---|
1258 | keyword. Can be empty. |
---|
1259 | |
---|
1260 | text_ptr[i].text_length - length of text string, |
---|
1261 | after decompression, 0 for iTXt |
---|
1262 | |
---|
1263 | text_ptr[i].itxt_length - length of itxt string, |
---|
1264 | after decompression, 0 for tEXt/zTXt |
---|
1265 | |
---|
1266 | text_ptr[i].lang - language of comment (empty |
---|
1267 | string for unknown). |
---|
1268 | |
---|
1269 | text_ptr[i].lang_key - keyword in UTF-8 |
---|
1270 | (empty string for unknown). |
---|
1271 | |
---|
1272 | Note that the itxt_length, lang, and lang_key |
---|
1273 | members of the text_ptr structure only exist when the |
---|
1274 | library is built with iTXt chunk support. Prior to |
---|
1275 | libpng-1.4.0 the library was built by default without |
---|
1276 | iTXt support. Also note that when iTXt is supported, |
---|
1277 | they contain NULL pointers when the "compression" |
---|
1278 | field contains PNG_TEXT_COMPRESSION_NONE or |
---|
1279 | PNG_TEXT_COMPRESSION_zTXt. |
---|
1280 | |
---|
1281 | num_text - number of comments (same as |
---|
1282 | num_comments; you can put NULL here |
---|
1283 | to avoid the duplication) |
---|
1284 | |
---|
1285 | Note while png_set_text() will accept text, language, |
---|
1286 | and translated keywords that can be NULL pointers, the |
---|
1287 | structure returned by png_get_text will always contain |
---|
1288 | regular zero-terminated C strings. They might be |
---|
1289 | empty strings but they will never be NULL pointers. |
---|
1290 | |
---|
1291 | num_spalettes = png_get_sPLT(png_ptr, info_ptr, |
---|
1292 | &palette_ptr); |
---|
1293 | |
---|
1294 | num_spalettes - number of sPLT chunks read. |
---|
1295 | |
---|
1296 | palette_ptr - array of palette structures holding |
---|
1297 | contents of one or more sPLT chunks |
---|
1298 | read. |
---|
1299 | |
---|
1300 | png_get_oFFs(png_ptr, info_ptr, &offset_x, &offset_y, |
---|
1301 | &unit_type); |
---|
1302 | |
---|
1303 | offset_x - positive offset from the left edge |
---|
1304 | of the screen (can be negative) |
---|
1305 | |
---|
1306 | offset_y - positive offset from the top edge |
---|
1307 | of the screen (can be negative) |
---|
1308 | |
---|
1309 | unit_type - PNG_OFFSET_PIXEL, PNG_OFFSET_MICROMETER |
---|
1310 | |
---|
1311 | png_get_pHYs(png_ptr, info_ptr, &res_x, &res_y, |
---|
1312 | &unit_type); |
---|
1313 | |
---|
1314 | res_x - pixels/unit physical resolution in |
---|
1315 | x direction |
---|
1316 | |
---|
1317 | res_y - pixels/unit physical resolution in |
---|
1318 | x direction |
---|
1319 | |
---|
1320 | unit_type - PNG_RESOLUTION_UNKNOWN, |
---|
1321 | PNG_RESOLUTION_METER |
---|
1322 | |
---|
1323 | png_get_sCAL(png_ptr, info_ptr, &unit, &width, |
---|
1324 | &height) |
---|
1325 | |
---|
1326 | unit - physical scale units (an integer) |
---|
1327 | |
---|
1328 | width - width of a pixel in physical scale units |
---|
1329 | |
---|
1330 | height - height of a pixel in physical scale units |
---|
1331 | (width and height are doubles) |
---|
1332 | |
---|
1333 | png_get_sCAL_s(png_ptr, info_ptr, &unit, &width, |
---|
1334 | &height) |
---|
1335 | |
---|
1336 | unit - physical scale units (an integer) |
---|
1337 | |
---|
1338 | width - width of a pixel in physical scale units |
---|
1339 | (expressed as a string) |
---|
1340 | |
---|
1341 | height - height of a pixel in physical scale units |
---|
1342 | (width and height are strings like "2.54") |
---|
1343 | |
---|
1344 | num_unknown_chunks = png_get_unknown_chunks(png_ptr, |
---|
1345 | info_ptr, &unknowns) |
---|
1346 | |
---|
1347 | unknowns - array of png_unknown_chunk |
---|
1348 | structures holding unknown chunks |
---|
1349 | |
---|
1350 | unknowns[i].name - name of unknown chunk |
---|
1351 | |
---|
1352 | unknowns[i].data - data of unknown chunk |
---|
1353 | |
---|
1354 | unknowns[i].size - size of unknown chunk's data |
---|
1355 | |
---|
1356 | unknowns[i].location - position of chunk in file |
---|
1357 | |
---|
1358 | The value of "i" corresponds to the order in which the |
---|
1359 | chunks were read from the PNG file or inserted with the |
---|
1360 | png_set_unknown_chunks() function. |
---|
1361 | |
---|
1362 | The value of "location" is a bitwise "or" of |
---|
1363 | |
---|
1364 | PNG_HAVE_IHDR (0x01) |
---|
1365 | PNG_HAVE_PLTE (0x02) |
---|
1366 | PNG_AFTER_IDAT (0x08) |
---|
1367 | |
---|
1368 | The data from the pHYs chunk can be retrieved in several convenient |
---|
1369 | forms: |
---|
1370 | |
---|
1371 | res_x = png_get_x_pixels_per_meter(png_ptr, |
---|
1372 | info_ptr) |
---|
1373 | |
---|
1374 | res_y = png_get_y_pixels_per_meter(png_ptr, |
---|
1375 | info_ptr) |
---|
1376 | |
---|
1377 | res_x_and_y = png_get_pixels_per_meter(png_ptr, |
---|
1378 | info_ptr) |
---|
1379 | |
---|
1380 | res_x = png_get_x_pixels_per_inch(png_ptr, |
---|
1381 | info_ptr) |
---|
1382 | |
---|
1383 | res_y = png_get_y_pixels_per_inch(png_ptr, |
---|
1384 | info_ptr) |
---|
1385 | |
---|
1386 | res_x_and_y = png_get_pixels_per_inch(png_ptr, |
---|
1387 | info_ptr) |
---|
1388 | |
---|
1389 | aspect_ratio = png_get_pixel_aspect_ratio(png_ptr, |
---|
1390 | info_ptr) |
---|
1391 | |
---|
1392 | Each of these returns 0 [signifying "unknown"] if |
---|
1393 | the data is not present or if res_x is 0; |
---|
1394 | res_x_and_y is 0 if res_x != res_y |
---|
1395 | |
---|
1396 | Note that because of the way the resolutions are |
---|
1397 | stored internally, the inch conversions won't |
---|
1398 | come out to exactly even number. For example, |
---|
1399 | 72 dpi is stored as 0.28346 pixels/meter, and |
---|
1400 | when this is retrieved it is 71.9988 dpi, so |
---|
1401 | be sure to round the returned value appropriately |
---|
1402 | if you want to display a reasonable-looking result. |
---|
1403 | |
---|
1404 | The data from the oFFs chunk can be retrieved in several convenient |
---|
1405 | forms: |
---|
1406 | |
---|
1407 | x_offset = png_get_x_offset_microns(png_ptr, info_ptr); |
---|
1408 | |
---|
1409 | y_offset = png_get_y_offset_microns(png_ptr, info_ptr); |
---|
1410 | |
---|
1411 | x_offset = png_get_x_offset_inches(png_ptr, info_ptr); |
---|
1412 | |
---|
1413 | y_offset = png_get_y_offset_inches(png_ptr, info_ptr); |
---|
1414 | |
---|
1415 | Each of these returns 0 [signifying "unknown" if both |
---|
1416 | x and y are 0] if the data is not present or if the |
---|
1417 | chunk is present but the unit is the pixel. The |
---|
1418 | remark about inexact inch conversions applies here |
---|
1419 | as well, because a value in inches can't always be |
---|
1420 | converted to microns and back without some loss |
---|
1421 | of precision. |
---|
1422 | |
---|
1423 | For more information, see the |
---|
1424 | PNG specification for chunk contents. Be careful with trusting |
---|
1425 | rowbytes, as some of the transformations could increase the space |
---|
1426 | needed to hold a row (expand, filler, gray_to_rgb, etc.). |
---|
1427 | See png_read_update_info(), below. |
---|
1428 | |
---|
1429 | A quick word about text_ptr and num_text. PNG stores comments in |
---|
1430 | keyword/text pairs, one pair per chunk, with no limit on the number |
---|
1431 | of text chunks, and a 2^31 byte limit on their size. While there are |
---|
1432 | suggested keywords, there is no requirement to restrict the use to these |
---|
1433 | strings. It is strongly suggested that keywords and text be sensible |
---|
1434 | to humans (that's the point), so don't use abbreviations. Non-printing |
---|
1435 | symbols are not allowed. See the PNG specification for more details. |
---|
1436 | There is also no requirement to have text after the keyword. |
---|
1437 | |
---|
1438 | Keywords should be limited to 79 Latin-1 characters without leading or |
---|
1439 | trailing spaces, but non-consecutive spaces are allowed within the |
---|
1440 | keyword. It is possible to have the same keyword any number of times. |
---|
1441 | The text_ptr is an array of png_text structures, each holding a |
---|
1442 | pointer to a language string, a pointer to a keyword and a pointer to |
---|
1443 | a text string. The text string, language code, and translated |
---|
1444 | keyword may be empty or NULL pointers. The keyword/text |
---|
1445 | pairs are put into the array in the order that they are received. |
---|
1446 | However, some or all of the text chunks may be after the image, so, to |
---|
1447 | make sure you have read all the text chunks, don't mess with these |
---|
1448 | until after you read the stuff after the image. This will be |
---|
1449 | mentioned again below in the discussion that goes with png_read_end(). |
---|
1450 | |
---|
1451 | Input transformations |
---|
1452 | |
---|
1453 | After you've read the header information, you can set up the library |
---|
1454 | to handle any special transformations of the image data. The various |
---|
1455 | ways to transform the data will be described in the order that they |
---|
1456 | should occur. This is important, as some of these change the color |
---|
1457 | type and/or bit depth of the data, and some others only work on |
---|
1458 | certain color types and bit depths. |
---|
1459 | |
---|
1460 | Transformations you request are ignored if they don't have any meaning for a |
---|
1461 | particular input data format. However some transformations can have an effect |
---|
1462 | as a result of a previous transformation. If you specify a contradictory set of |
---|
1463 | transformations, for example both adding and removing the alpha channel, you |
---|
1464 | cannot predict the final result. |
---|
1465 | |
---|
1466 | The color used for the transparency values should be supplied in the same |
---|
1467 | format/depth as the current image data. It is stored in the same format/depth |
---|
1468 | as the image data in a tRNS chunk, so this is what libpng expects for this data. |
---|
1469 | |
---|
1470 | The color used for the background value depends on the need_expand argument as |
---|
1471 | described below. |
---|
1472 | |
---|
1473 | Data will be decoded into the supplied row buffers packed into bytes |
---|
1474 | unless the library has been told to transform it into another format. |
---|
1475 | For example, 4 bit/pixel paletted or grayscale data will be returned |
---|
1476 | 2 pixels/byte with the leftmost pixel in the high-order bits of the |
---|
1477 | byte, unless png_set_packing() is called. 8-bit RGB data will be stored |
---|
1478 | in RGB RGB RGB format unless png_set_filler() or png_set_add_alpha() |
---|
1479 | is called to insert filler bytes, either before or after each RGB triplet. |
---|
1480 | 16-bit RGB data will be returned RRGGBB RRGGBB, with the most significant |
---|
1481 | byte of the color value first, unless png_set_scale_16() is called to |
---|
1482 | transform it to regular RGB RGB triplets, or png_set_filler() or |
---|
1483 | png_set_add alpha() is called to insert filler bytes, either before or |
---|
1484 | after each RRGGBB triplet. Similarly, 8-bit or 16-bit grayscale data can |
---|
1485 | be modified with png_set_filler(), png_set_add_alpha(), png_set_strip_16(), |
---|
1486 | or png_set_scale_16(). |
---|
1487 | |
---|
1488 | The following code transforms grayscale images of less than 8 to 8 bits, |
---|
1489 | changes paletted images to RGB, and adds a full alpha channel if there is |
---|
1490 | transparency information in a tRNS chunk. This is most useful on |
---|
1491 | grayscale images with bit depths of 2 or 4 or if there is a multiple-image |
---|
1492 | viewing application that wishes to treat all images in the same way. |
---|
1493 | |
---|
1494 | if (color_type == PNG_COLOR_TYPE_PALETTE) |
---|
1495 | png_set_palette_to_rgb(png_ptr); |
---|
1496 | |
---|
1497 | if (png_get_valid(png_ptr, info_ptr, |
---|
1498 | PNG_INFO_tRNS)) png_set_tRNS_to_alpha(png_ptr); |
---|
1499 | |
---|
1500 | if (color_type == PNG_COLOR_TYPE_GRAY && |
---|
1501 | bit_depth < 8) png_set_expand_gray_1_2_4_to_8(png_ptr); |
---|
1502 | |
---|
1503 | The first two functions are actually aliases for png_set_expand(), added |
---|
1504 | in libpng version 1.0.4, with the function names expanded to improve code |
---|
1505 | readability. In some future version they may actually do different |
---|
1506 | things. |
---|
1507 | |
---|
1508 | As of libpng version 1.2.9, png_set_expand_gray_1_2_4_to_8() was |
---|
1509 | added. It expands the sample depth without changing tRNS to alpha. |
---|
1510 | |
---|
1511 | As of libpng version 1.5.2, png_set_expand_16() was added. It behaves as |
---|
1512 | png_set_expand(); however, the resultant channels have 16 bits rather than 8. |
---|
1513 | Use this when the output color or gray channels are made linear to avoid fairly |
---|
1514 | severe accuracy loss. |
---|
1515 | |
---|
1516 | if (bit_depth < 16) |
---|
1517 | png_set_expand_16(png_ptr); |
---|
1518 | |
---|
1519 | PNG can have files with 16 bits per channel. If you only can handle |
---|
1520 | 8 bits per channel, this will strip the pixels down to 8-bit. |
---|
1521 | |
---|
1522 | if (bit_depth == 16) |
---|
1523 | #if PNG_LIBPNG_VER >= 10504 |
---|
1524 | png_set_scale_16(png_ptr); |
---|
1525 | #else |
---|
1526 | png_set_strip_16(png_ptr); |
---|
1527 | #endif |
---|
1528 | |
---|
1529 | (The more accurate "png_set_scale_16()" API became available in libpng version |
---|
1530 | 1.5.4). |
---|
1531 | |
---|
1532 | If you need to process the alpha channel on the image separately from the image |
---|
1533 | data (for example if you convert it to a bitmap mask) it is possible to have |
---|
1534 | libpng strip the channel leaving just RGB or gray data: |
---|
1535 | |
---|
1536 | if (color_type & PNG_COLOR_MASK_ALPHA) |
---|
1537 | png_set_strip_alpha(png_ptr); |
---|
1538 | |
---|
1539 | If you strip the alpha channel you need to find some other way of dealing with |
---|
1540 | the information. If, instead, you want to convert the image to an opaque |
---|
1541 | version with no alpha channel use png_set_background; see below. |
---|
1542 | |
---|
1543 | As of libpng version 1.5.2, almost all useful expansions are supported, the |
---|
1544 | major ommissions are conversion of grayscale to indexed images (which can be |
---|
1545 | done trivially in the application) and conversion of indexed to grayscale (which |
---|
1546 | can be done by a trivial manipulation of the palette.) |
---|
1547 | |
---|
1548 | In the following table, the 01 means grayscale with depth<8, 31 means |
---|
1549 | indexed with depth<8, other numerals represent the color type, "T" means |
---|
1550 | the tRNS chunk is present, A means an alpha channel is present, and O |
---|
1551 | means tRNS or alpha is present but all pixels in the image are opaque. |
---|
1552 | |
---|
1553 | FROM 01 31 0 0T 0O 2 2T 2O 3 3T 3O 4A 4O 6A 6O |
---|
1554 | TO |
---|
1555 | 01 - [G] - - - - - - - - - - - - - |
---|
1556 | 31 [Q] Q [Q] [Q] [Q] Q Q Q Q Q Q [Q] [Q] Q Q |
---|
1557 | 0 1 G + . . G G G G G G B B GB GB |
---|
1558 | 0T lt Gt t + . Gt G G Gt G G Bt Bt GBt GBt |
---|
1559 | 0O lt Gt t . + Gt Gt G Gt Gt G Bt Bt GBt GBt |
---|
1560 | 2 C P C C C + . . C - - CB CB B B |
---|
1561 | 2T Ct - Ct C C t + t - - - CBt CBt Bt Bt |
---|
1562 | 2O Ct - Ct C C t t + - - - CBt CBt Bt Bt |
---|
1563 | 3 [Q] p [Q] [Q] [Q] Q Q Q + . . [Q] [Q] Q Q |
---|
1564 | 3T [Qt] p [Qt][Q] [Q] Qt Qt Qt t + t [Qt][Qt] Qt Qt |
---|
1565 | 3O [Qt] p [Qt][Q] [Q] Qt Qt Qt t t + [Qt][Qt] Qt Qt |
---|
1566 | 4A lA G A T T GA GT GT GA GT GT + BA G GBA |
---|
1567 | 4O lA GBA A T T GA GT GT GA GT GT BA + GBA G |
---|
1568 | 6A CA PA CA C C A T tT PA P P C CBA + BA |
---|
1569 | 6O CA PBA CA C C A tT T PA P P CBA C BA + |
---|
1570 | |
---|
1571 | Within the matrix, |
---|
1572 | "+" identifies entries where 'from' and 'to' are the same. |
---|
1573 | "-" means the transformation is not supported. |
---|
1574 | "." means nothing is necessary (a tRNS chunk can just be ignored). |
---|
1575 | "t" means the transformation is obtained by png_set_tRNS. |
---|
1576 | "A" means the transformation is obtained by png_set_add_alpha(). |
---|
1577 | "X" means the transformation is obtained by png_set_expand(). |
---|
1578 | "1" means the transformation is obtained by |
---|
1579 | png_set_expand_gray_1_2_4_to_8() (and by png_set_expand() if there |
---|
1580 | is no transparency in the original or the final format). |
---|
1581 | "C" means the transformation is obtained by png_set_gray_to_rgb(). |
---|
1582 | "G" means the transformation is obtained by png_set_rgb_to_gray(). |
---|
1583 | "P" means the transformation is obtained by |
---|
1584 | png_set_expand_palette_to_rgb(). |
---|
1585 | "p" means the transformation is obtained by png_set_packing(). |
---|
1586 | "Q" means the transformation is obtained by png_set_quantize(). |
---|
1587 | "T" means the transformation is obtained by png_set_tRNS_to_alpha(). |
---|
1588 | "B" means the transformation is obtained by png_set_background(), or |
---|
1589 | png_strip_alpha(). |
---|
1590 | |
---|
1591 | When an entry has multiple transforms listed all are required to cause the |
---|
1592 | right overall transformation. When two transforms are separated by a comma |
---|
1593 | either will do the job. When transforms are enclosed in [] the transform should |
---|
1594 | do the job but this is currently unimplemented - a different format will result |
---|
1595 | if the suggested transformations are used. |
---|
1596 | |
---|
1597 | In PNG files, the alpha channel in an image |
---|
1598 | is the level of opacity. If you need the alpha channel in an image to |
---|
1599 | be the level of transparency instead of opacity, you can invert the |
---|
1600 | alpha channel (or the tRNS chunk data) after it's read, so that 0 is |
---|
1601 | fully opaque and 255 (in 8-bit or paletted images) or 65535 (in 16-bit |
---|
1602 | images) is fully transparent, with |
---|
1603 | |
---|
1604 | png_set_invert_alpha(png_ptr); |
---|
1605 | |
---|
1606 | PNG files pack pixels of bit depths 1, 2, and 4 into bytes as small as |
---|
1607 | they can, resulting in, for example, 8 pixels per byte for 1 bit |
---|
1608 | files. This code expands to 1 pixel per byte without changing the |
---|
1609 | values of the pixels: |
---|
1610 | |
---|
1611 | if (bit_depth < 8) |
---|
1612 | png_set_packing(png_ptr); |
---|
1613 | |
---|
1614 | PNG files have possible bit depths of 1, 2, 4, 8, and 16. All pixels |
---|
1615 | stored in a PNG image have been "scaled" or "shifted" up to the next |
---|
1616 | higher possible bit depth (e.g. from 5 bits/sample in the range [0,31] |
---|
1617 | to 8 bits/sample in the range [0, 255]). However, it is also possible |
---|
1618 | to convert the PNG pixel data back to the original bit depth of the |
---|
1619 | image. This call reduces the pixels back down to the original bit depth: |
---|
1620 | |
---|
1621 | png_color_8p sig_bit; |
---|
1622 | |
---|
1623 | if (png_get_sBIT(png_ptr, info_ptr, &sig_bit)) |
---|
1624 | png_set_shift(png_ptr, sig_bit); |
---|
1625 | |
---|
1626 | PNG files store 3-color pixels in red, green, blue order. This code |
---|
1627 | changes the storage of the pixels to blue, green, red: |
---|
1628 | |
---|
1629 | if (color_type == PNG_COLOR_TYPE_RGB || |
---|
1630 | color_type == PNG_COLOR_TYPE_RGB_ALPHA) |
---|
1631 | png_set_bgr(png_ptr); |
---|
1632 | |
---|
1633 | PNG files store RGB pixels packed into 3 or 6 bytes. This code expands them |
---|
1634 | into 4 or 8 bytes for windowing systems that need them in this format: |
---|
1635 | |
---|
1636 | if (color_type == PNG_COLOR_TYPE_RGB) |
---|
1637 | png_set_filler(png_ptr, filler, PNG_FILLER_BEFORE); |
---|
1638 | |
---|
1639 | where "filler" is the 8 or 16-bit number to fill with, and the location is |
---|
1640 | either PNG_FILLER_BEFORE or PNG_FILLER_AFTER, depending upon whether |
---|
1641 | you want the filler before the RGB or after. This transformation |
---|
1642 | does not affect images that already have full alpha channels. To add an |
---|
1643 | opaque alpha channel, use filler=0xff or 0xffff and PNG_FILLER_AFTER which |
---|
1644 | will generate RGBA pixels. |
---|
1645 | |
---|
1646 | Note that png_set_filler() does not change the color type. If you want |
---|
1647 | to do that, you can add a true alpha channel with |
---|
1648 | |
---|
1649 | if (color_type == PNG_COLOR_TYPE_RGB || |
---|
1650 | color_type == PNG_COLOR_TYPE_GRAY) |
---|
1651 | png_set_add_alpha(png_ptr, filler, PNG_FILLER_AFTER); |
---|
1652 | |
---|
1653 | where "filler" contains the alpha value to assign to each pixel. |
---|
1654 | This function was added in libpng-1.2.7. |
---|
1655 | |
---|
1656 | If you are reading an image with an alpha channel, and you need the |
---|
1657 | data as ARGB instead of the normal PNG format RGBA: |
---|
1658 | |
---|
1659 | if (color_type == PNG_COLOR_TYPE_RGB_ALPHA) |
---|
1660 | png_set_swap_alpha(png_ptr); |
---|
1661 | |
---|
1662 | For some uses, you may want a grayscale image to be represented as |
---|
1663 | RGB. This code will do that conversion: |
---|
1664 | |
---|
1665 | if (color_type == PNG_COLOR_TYPE_GRAY || |
---|
1666 | color_type == PNG_COLOR_TYPE_GRAY_ALPHA) |
---|
1667 | png_set_gray_to_rgb(png_ptr); |
---|
1668 | |
---|
1669 | Conversely, you can convert an RGB or RGBA image to grayscale or grayscale |
---|
1670 | with alpha. |
---|
1671 | |
---|
1672 | if (color_type == PNG_COLOR_TYPE_RGB || |
---|
1673 | color_type == PNG_COLOR_TYPE_RGB_ALPHA) |
---|
1674 | png_set_rgb_to_gray(png_ptr, error_action, double red_weight, |
---|
1675 | double green_weight); |
---|
1676 | |
---|
1677 | error_action = 1: silently do the conversion |
---|
1678 | |
---|
1679 | error_action = 2: issue a warning if the original |
---|
1680 | image has any pixel where |
---|
1681 | red != green or red != blue |
---|
1682 | |
---|
1683 | error_action = 3: issue an error and abort the |
---|
1684 | conversion if the original |
---|
1685 | image has any pixel where |
---|
1686 | red != green or red != blue |
---|
1687 | |
---|
1688 | red_weight: weight of red component |
---|
1689 | |
---|
1690 | green_weight: weight of green component |
---|
1691 | If either weight is negative, default |
---|
1692 | weights are used. |
---|
1693 | |
---|
1694 | In the corresponding fixed point API the red_weight and green_weight values are |
---|
1695 | simply scaled by 100,000: |
---|
1696 | |
---|
1697 | png_set_rgb_to_gray(png_ptr, error_action, png_fixed_point red_weight, |
---|
1698 | png_fixed_point green_weight); |
---|
1699 | |
---|
1700 | If you have set error_action = 1 or 2, you can |
---|
1701 | later check whether the image really was gray, after processing |
---|
1702 | the image rows, with the png_get_rgb_to_gray_status(png_ptr) function. |
---|
1703 | It will return a png_byte that is zero if the image was gray or |
---|
1704 | 1 if there were any non-gray pixels. Background and sBIT data |
---|
1705 | will be silently converted to grayscale, using the green channel |
---|
1706 | data for sBIT, regardless of the error_action setting. |
---|
1707 | |
---|
1708 | The default values come from the PNG file cHRM chunk if present; otherwise, the |
---|
1709 | defaults correspond to the ITU-R recommendation 709, and also the sRGB color |
---|
1710 | space, as recommended in the Charles Poynton's Colour FAQ, |
---|
1711 | <http://www.poynton.com/>, in section 9: |
---|
1712 | |
---|
1713 | <http://www.poynton.com/notes/colour_and_gamma/ColorFAQ.html#RTFToC9> |
---|
1714 | |
---|
1715 | Y = 0.2126 * R + 0.7152 * G + 0.0722 * B |
---|
1716 | |
---|
1717 | Previous versions of this document, 1998 through 2002, recommended a slightly |
---|
1718 | different formula: |
---|
1719 | |
---|
1720 | Y = 0.212671 * R + 0.715160 * G + 0.072169 * B |
---|
1721 | |
---|
1722 | Libpng uses an integer approximation: |
---|
1723 | |
---|
1724 | Y = (6968 * R + 23434 * G + 2366 * B)/32768 |
---|
1725 | |
---|
1726 | The calculation is done in a linear colorspace, if the image gamma |
---|
1727 | can be determined. |
---|
1728 | |
---|
1729 | The png_set_background() function has been described already; it tells libpng to |
---|
1730 | composite images with alpha or simple transparency against the supplied |
---|
1731 | background color. For compatibility with versions of libpng earlier than |
---|
1732 | libpng-1.5.4 it is recommended that you call the function after reading the file |
---|
1733 | header, even if you don't want to use the color in a bKGD chunk, if one exists. |
---|
1734 | |
---|
1735 | If the PNG file contains a bKGD chunk (PNG_INFO_bKGD valid), |
---|
1736 | you may use this color, or supply another color more suitable for |
---|
1737 | the current display (e.g., the background color from a web page). You |
---|
1738 | need to tell libpng how the color is represented, both the format of the |
---|
1739 | component values in the color (the number of bits) and the gamma encoding of the |
---|
1740 | color. The function takes two arguments, background_gamma_mode and need_expand |
---|
1741 | to convey this information; however, only two combinations are likely to be |
---|
1742 | useful: |
---|
1743 | |
---|
1744 | png_color_16 my_background; |
---|
1745 | png_color_16p image_background; |
---|
1746 | |
---|
1747 | if (png_get_bKGD(png_ptr, info_ptr, &image_background)) |
---|
1748 | png_set_background(png_ptr, image_background, |
---|
1749 | PNG_BACKGROUND_GAMMA_FILE, 1/*needs to be expanded*/, 1); |
---|
1750 | else |
---|
1751 | png_set_background(png_ptr, &my_background, |
---|
1752 | PNG_BACKGROUND_GAMMA_SCREEN, 0/*do not expand*/, 1); |
---|
1753 | |
---|
1754 | The second call was described above - my_background is in the format of the |
---|
1755 | final, display, output produced by libpng. Because you now know the format of |
---|
1756 | the PNG it is possible to avoid the need to choose either 8-bit or 16-bit |
---|
1757 | output and to retain palette images (the palette colors will be modified |
---|
1758 | appropriately and the tRNS chunk removed.) However, if you are doing this, |
---|
1759 | take great care not to ask for transformations without checking first that |
---|
1760 | they apply! |
---|
1761 | |
---|
1762 | In the first call the background color has the original bit depth and color type |
---|
1763 | of the PNG file. So, for palette images the color is supplied as a palette |
---|
1764 | index and for low bit greyscale images the color is a reduced bit value in |
---|
1765 | image_background->gray. |
---|
1766 | |
---|
1767 | If you didn't call png_set_gamma() before reading the file header, for example |
---|
1768 | if you need your code to remain compatible with older versions of libpng prior |
---|
1769 | to libpng-1.5.4, this is the place to call it. |
---|
1770 | |
---|
1771 | Do not call it if you called png_set_alpha_mode(); doing so will damage the |
---|
1772 | settings put in place by png_set_alpha_mode(). (If png_set_alpha_mode() is |
---|
1773 | supported then you can certainly do png_set_gamma() before reading the PNG |
---|
1774 | header.) |
---|
1775 | |
---|
1776 | This API unconditionally sets the screen and file gamma values, so it will |
---|
1777 | override the value in the PNG file unless it is called before the PNG file |
---|
1778 | reading starts. For this reason you must always call it with the PNG file |
---|
1779 | value when you call it in this position: |
---|
1780 | |
---|
1781 | if (png_get_gAMA(png_ptr, info_ptr, &file_gamma)) |
---|
1782 | png_set_gamma(png_ptr, screen_gamma, file_gamma); |
---|
1783 | |
---|
1784 | else |
---|
1785 | png_set_gamma(png_ptr, screen_gamma, 0.45455); |
---|
1786 | |
---|
1787 | If you need to reduce an RGB file to a paletted file, or if a paletted |
---|
1788 | file has more entries then will fit on your screen, png_set_quantize() |
---|
1789 | will do that. Note that this is a simple match quantization that merely |
---|
1790 | finds the closest color available. This should work fairly well with |
---|
1791 | optimized palettes, but fairly badly with linear color cubes. If you |
---|
1792 | pass a palette that is larger than maximum_colors, the file will |
---|
1793 | reduce the number of colors in the palette so it will fit into |
---|
1794 | maximum_colors. If there is a histogram, libpng will use it to make |
---|
1795 | more intelligent choices when reducing the palette. If there is no |
---|
1796 | histogram, it may not do as good a job. |
---|
1797 | |
---|
1798 | if (color_type & PNG_COLOR_MASK_COLOR) |
---|
1799 | { |
---|
1800 | if (png_get_valid(png_ptr, info_ptr, |
---|
1801 | PNG_INFO_PLTE)) |
---|
1802 | { |
---|
1803 | png_uint_16p histogram = NULL; |
---|
1804 | |
---|
1805 | png_get_hIST(png_ptr, info_ptr, |
---|
1806 | &histogram); |
---|
1807 | png_set_quantize(png_ptr, palette, num_palette, |
---|
1808 | max_screen_colors, histogram, 1); |
---|
1809 | } |
---|
1810 | |
---|
1811 | else |
---|
1812 | { |
---|
1813 | png_color std_color_cube[MAX_SCREEN_COLORS] = |
---|
1814 | { ... colors ... }; |
---|
1815 | |
---|
1816 | png_set_quantize(png_ptr, std_color_cube, |
---|
1817 | MAX_SCREEN_COLORS, MAX_SCREEN_COLORS, |
---|
1818 | NULL,0); |
---|
1819 | } |
---|
1820 | } |
---|
1821 | |
---|
1822 | PNG files describe monochrome as black being zero and white being one. |
---|
1823 | The following code will reverse this (make black be one and white be |
---|
1824 | zero): |
---|
1825 | |
---|
1826 | if (bit_depth == 1 && color_type == PNG_COLOR_TYPE_GRAY) |
---|
1827 | png_set_invert_mono(png_ptr); |
---|
1828 | |
---|
1829 | This function can also be used to invert grayscale and gray-alpha images: |
---|
1830 | |
---|
1831 | if (color_type == PNG_COLOR_TYPE_GRAY || |
---|
1832 | color_type == PNG_COLOR_TYPE_GRAY_ALPHA) |
---|
1833 | png_set_invert_mono(png_ptr); |
---|
1834 | |
---|
1835 | PNG files store 16-bit pixels in network byte order (big-endian, |
---|
1836 | ie. most significant bits first). This code changes the storage to the |
---|
1837 | other way (little-endian, i.e. least significant bits first, the |
---|
1838 | way PCs store them): |
---|
1839 | |
---|
1840 | if (bit_depth == 16) |
---|
1841 | png_set_swap(png_ptr); |
---|
1842 | |
---|
1843 | If you are using packed-pixel images (1, 2, or 4 bits/pixel), and you |
---|
1844 | need to change the order the pixels are packed into bytes, you can use: |
---|
1845 | |
---|
1846 | if (bit_depth < 8) |
---|
1847 | png_set_packswap(png_ptr); |
---|
1848 | |
---|
1849 | Finally, you can write your own transformation function if none of |
---|
1850 | the existing ones meets your needs. This is done by setting a callback |
---|
1851 | with |
---|
1852 | |
---|
1853 | png_set_read_user_transform_fn(png_ptr, |
---|
1854 | read_transform_fn); |
---|
1855 | |
---|
1856 | You must supply the function |
---|
1857 | |
---|
1858 | void read_transform_fn(png_structp png_ptr, png_row_infop |
---|
1859 | row_info, png_bytep data) |
---|
1860 | |
---|
1861 | See pngtest.c for a working example. Your function will be called |
---|
1862 | after all of the other transformations have been processed. Take care with |
---|
1863 | interlaced images if you do the interlace yourself - the width of the row is the |
---|
1864 | width in 'row_info', not the overall image width. |
---|
1865 | |
---|
1866 | If supported, libpng provides two information routines that you can use to find |
---|
1867 | where you are in processing the image: |
---|
1868 | |
---|
1869 | png_get_current_pass_number(png_structp png_ptr); |
---|
1870 | png_get_current_row_number(png_structp png_ptr); |
---|
1871 | |
---|
1872 | Don't try using these outside a transform callback - firstly they are only |
---|
1873 | supported if user transforms are supported, secondly they may well return |
---|
1874 | unexpected results unless the row is actually being processed at the moment they |
---|
1875 | are called. |
---|
1876 | |
---|
1877 | With interlaced |
---|
1878 | images the value returned is the row in the input sub-image image. Use |
---|
1879 | PNG_ROW_FROM_PASS_ROW(row, pass) and PNG_COL_FROM_PASS_COL(col, pass) to |
---|
1880 | find the output pixel (x,y) given an interlaced sub-image pixel (row,col,pass). |
---|
1881 | |
---|
1882 | The discussion of interlace handling above contains more information on how to |
---|
1883 | use these values. |
---|
1884 | |
---|
1885 | You can also set up a pointer to a user structure for use by your |
---|
1886 | callback function, and you can inform libpng that your transform |
---|
1887 | function will change the number of channels or bit depth with the |
---|
1888 | function |
---|
1889 | |
---|
1890 | png_set_user_transform_info(png_ptr, user_ptr, |
---|
1891 | user_depth, user_channels); |
---|
1892 | |
---|
1893 | The user's application, not libpng, is responsible for allocating and |
---|
1894 | freeing any memory required for the user structure. |
---|
1895 | |
---|
1896 | You can retrieve the pointer via the function |
---|
1897 | png_get_user_transform_ptr(). For example: |
---|
1898 | |
---|
1899 | voidp read_user_transform_ptr = |
---|
1900 | png_get_user_transform_ptr(png_ptr); |
---|
1901 | |
---|
1902 | The last thing to handle is interlacing; this is covered in detail below, |
---|
1903 | but you must call the function here if you want libpng to handle expansion |
---|
1904 | of the interlaced image. |
---|
1905 | |
---|
1906 | number_of_passes = png_set_interlace_handling(png_ptr); |
---|
1907 | |
---|
1908 | After setting the transformations, libpng can update your png_info |
---|
1909 | structure to reflect any transformations you've requested with this |
---|
1910 | call. |
---|
1911 | |
---|
1912 | png_read_update_info(png_ptr, info_ptr); |
---|
1913 | |
---|
1914 | This is most useful to update the info structure's rowbytes |
---|
1915 | field so you can use it to allocate your image memory. This function |
---|
1916 | will also update your palette with the correct screen_gamma and |
---|
1917 | background if these have been given with the calls above. You may |
---|
1918 | only call png_read_update_info() once with a particular info_ptr. |
---|
1919 | |
---|
1920 | After you call png_read_update_info(), you can allocate any |
---|
1921 | memory you need to hold the image. The row data is simply |
---|
1922 | raw byte data for all forms of images. As the actual allocation |
---|
1923 | varies among applications, no example will be given. If you |
---|
1924 | are allocating one large chunk, you will need to build an |
---|
1925 | array of pointers to each row, as it will be needed for some |
---|
1926 | of the functions below. |
---|
1927 | |
---|
1928 | Remember: Before you call png_read_update_info(), the png_get_*() |
---|
1929 | functions return the values corresponding to the original PNG image. |
---|
1930 | After you call png_read_update_info the values refer to the image |
---|
1931 | that libpng will output. Consequently you must call all the png_set_ |
---|
1932 | functions before you call png_read_update_info(). This is particularly |
---|
1933 | important for png_set_interlace_handling() - if you are going to call |
---|
1934 | png_read_update_info() you must call png_set_interlace_handling() before |
---|
1935 | it unless you want to receive interlaced output. |
---|
1936 | |
---|
1937 | Reading image data |
---|
1938 | |
---|
1939 | After you've allocated memory, you can read the image data. |
---|
1940 | The simplest way to do this is in one function call. If you are |
---|
1941 | allocating enough memory to hold the whole image, you can just |
---|
1942 | call png_read_image() and libpng will read in all the image data |
---|
1943 | and put it in the memory area supplied. You will need to pass in |
---|
1944 | an array of pointers to each row. |
---|
1945 | |
---|
1946 | This function automatically handles interlacing, so you don't |
---|
1947 | need to call png_set_interlace_handling() (unless you call |
---|
1948 | png_read_update_info()) or call this function multiple times, or any |
---|
1949 | of that other stuff necessary with png_read_rows(). |
---|
1950 | |
---|
1951 | png_read_image(png_ptr, row_pointers); |
---|
1952 | |
---|
1953 | where row_pointers is: |
---|
1954 | |
---|
1955 | png_bytep row_pointers[height]; |
---|
1956 | |
---|
1957 | You can point to void or char or whatever you use for pixels. |
---|
1958 | |
---|
1959 | If you don't want to read in the whole image at once, you can |
---|
1960 | use png_read_rows() instead. If there is no interlacing (check |
---|
1961 | interlace_type == PNG_INTERLACE_NONE), this is simple: |
---|
1962 | |
---|
1963 | png_read_rows(png_ptr, row_pointers, NULL, |
---|
1964 | number_of_rows); |
---|
1965 | |
---|
1966 | where row_pointers is the same as in the png_read_image() call. |
---|
1967 | |
---|
1968 | If you are doing this just one row at a time, you can do this with |
---|
1969 | a single row_pointer instead of an array of row_pointers: |
---|
1970 | |
---|
1971 | png_bytep row_pointer = row; |
---|
1972 | png_read_row(png_ptr, row_pointer, NULL); |
---|
1973 | |
---|
1974 | If the file is interlaced (interlace_type != 0 in the IHDR chunk), things |
---|
1975 | get somewhat harder. The only current (PNG Specification version 1.2) |
---|
1976 | interlacing type for PNG is (interlace_type == PNG_INTERLACE_ADAM7); |
---|
1977 | a somewhat complicated 2D interlace scheme, known as Adam7, that |
---|
1978 | breaks down an image into seven smaller images of varying size, based |
---|
1979 | on an 8x8 grid. This number is defined (from libpng 1.5) as |
---|
1980 | PNG_INTERLACE_ADAM7_PASSES in png.h |
---|
1981 | |
---|
1982 | libpng can fill out those images or it can give them to you "as is". |
---|
1983 | It is almost always better to have libpng handle the interlacing for you. |
---|
1984 | If you want the images filled out, there are two ways to do that. The one |
---|
1985 | mentioned in the PNG specification is to expand each pixel to cover |
---|
1986 | those pixels that have not been read yet (the "rectangle" method). |
---|
1987 | This results in a blocky image for the first pass, which gradually |
---|
1988 | smooths out as more pixels are read. The other method is the "sparkle" |
---|
1989 | method, where pixels are drawn only in their final locations, with the |
---|
1990 | rest of the image remaining whatever colors they were initialized to |
---|
1991 | before the start of the read. The first method usually looks better, |
---|
1992 | but tends to be slower, as there are more pixels to put in the rows. |
---|
1993 | |
---|
1994 | If, as is likely, you want libpng to expand the images, call this before |
---|
1995 | calling png_start_read_image() or png_read_update_info(): |
---|
1996 | |
---|
1997 | if (interlace_type == PNG_INTERLACE_ADAM7) |
---|
1998 | number_of_passes |
---|
1999 | = png_set_interlace_handling(png_ptr); |
---|
2000 | |
---|
2001 | This will return the number of passes needed. Currently, this is seven, |
---|
2002 | but may change if another interlace type is added. This function can be |
---|
2003 | called even if the file is not interlaced, where it will return one pass. |
---|
2004 | You then need to read the whole image 'number_of_passes' times. Each time |
---|
2005 | will distribute the pixels from the current pass to the correct place in |
---|
2006 | the output image, so you need to supply the same rows to png_read_rows in |
---|
2007 | each pass. |
---|
2008 | |
---|
2009 | If you are not going to display the image after each pass, but are |
---|
2010 | going to wait until the entire image is read in, use the sparkle |
---|
2011 | effect. This effect is faster and the end result of either method |
---|
2012 | is exactly the same. If you are planning on displaying the image |
---|
2013 | after each pass, the "rectangle" effect is generally considered the |
---|
2014 | better looking one. |
---|
2015 | |
---|
2016 | If you only want the "sparkle" effect, just call png_read_rows() as |
---|
2017 | normal, with the third parameter NULL. Make sure you make pass over |
---|
2018 | the image number_of_passes times, and you don't change the data in the |
---|
2019 | rows between calls. You can change the locations of the data, just |
---|
2020 | not the data. Each pass only writes the pixels appropriate for that |
---|
2021 | pass, and assumes the data from previous passes is still valid. |
---|
2022 | |
---|
2023 | png_read_rows(png_ptr, row_pointers, NULL, |
---|
2024 | number_of_rows); |
---|
2025 | |
---|
2026 | If you only want the first effect (the rectangles), do the same as |
---|
2027 | before except pass the row buffer in the third parameter, and leave |
---|
2028 | the second parameter NULL. |
---|
2029 | |
---|
2030 | png_read_rows(png_ptr, NULL, row_pointers, |
---|
2031 | number_of_rows); |
---|
2032 | |
---|
2033 | If you don't want libpng to handle the interlacing details, just call |
---|
2034 | png_read_rows() PNG_INTERLACE_ADAM7_PASSES times to read in all the images. |
---|
2035 | Each of the images is a valid image by itself; however, you will almost |
---|
2036 | certainly need to distribute the pixels from each sub-image to the |
---|
2037 | correct place. This is where everything gets very tricky. |
---|
2038 | |
---|
2039 | If you want to retrieve the separate images you must pass the correct |
---|
2040 | number of rows to each successive call of png_read_rows(). The calculation |
---|
2041 | gets pretty complicated for small images, where some sub-images may |
---|
2042 | not even exist because either their width or height ends up zero. |
---|
2043 | libpng provides two macros to help you in 1.5 and later versions: |
---|
2044 | |
---|
2045 | png_uint_32 width = PNG_PASS_COLS(image_width, pass_number); |
---|
2046 | png_uint_32 height = PNG_PASS_ROWS(image_height, pass_number); |
---|
2047 | |
---|
2048 | Respectively these tell you the width and height of the sub-image |
---|
2049 | corresponding to the numbered pass. 'pass' is in in the range 0 to 6 - |
---|
2050 | this can be confusing because the specification refers to the same passes |
---|
2051 | as 1 to 7! Be careful, you must check both the width and height before |
---|
2052 | calling png_read_rows() and not call it for that pass if either is zero. |
---|
2053 | |
---|
2054 | You can, of course, read each sub-image row by row. If you want to |
---|
2055 | produce optimal code to make a pixel-by-pixel transformation of an |
---|
2056 | interlaced image this is the best approach; read each row of each pass, |
---|
2057 | transform it, and write it out to a new interlaced image. |
---|
2058 | |
---|
2059 | If you want to de-interlace the image yourself libpng provides further |
---|
2060 | macros to help that tell you where to place the pixels in the output image. |
---|
2061 | Because the interlacing scheme is rectangular - sub-image pixels are always |
---|
2062 | arranged on a rectangular grid - all you need to know for each pass is the |
---|
2063 | starting column and row in the output image of the first pixel plus the |
---|
2064 | spacing between each pixel. As of libpng 1.5 there are four macros to |
---|
2065 | retrieve this information: |
---|
2066 | |
---|
2067 | png_uint_32 x = PNG_PASS_START_COL(pass); |
---|
2068 | png_uint_32 y = PNG_PASS_START_ROW(pass); |
---|
2069 | png_uint_32 xStep = 1U << PNG_PASS_COL_SHIFT(pass); |
---|
2070 | png_uint_32 yStep = 1U << PNG_PASS_ROW_SHIFT(pass); |
---|
2071 | |
---|
2072 | These allow you to write the obvious loop: |
---|
2073 | |
---|
2074 | png_uint_32 input_y = 0; |
---|
2075 | png_uint_32 output_y = PNG_PASS_START_ROW(pass); |
---|
2076 | |
---|
2077 | while (output_y < output_image_height) |
---|
2078 | { |
---|
2079 | png_uint_32 input_x = 0; |
---|
2080 | png_uint_32 output_x = PNG_PASS_START_COL(pass); |
---|
2081 | |
---|
2082 | while (output_x < output_image_width) |
---|
2083 | { |
---|
2084 | image[output_y][output_x] = |
---|
2085 | subimage[pass][input_y][input_x++]; |
---|
2086 | |
---|
2087 | output_x += xStep; |
---|
2088 | } |
---|
2089 | |
---|
2090 | ++input_y; |
---|
2091 | output_y += yStep; |
---|
2092 | } |
---|
2093 | |
---|
2094 | Notice that the steps between successive output rows and columns are |
---|
2095 | returned as shifts. This is possible because the pixels in the subimages |
---|
2096 | are always a power of 2 apart - 1, 2, 4 or 8 pixels - in the original |
---|
2097 | image. In practice you may need to directly calculate the output coordinate |
---|
2098 | given an input coordinate. libpng provides two further macros for this |
---|
2099 | purpose: |
---|
2100 | |
---|
2101 | png_uint_32 output_x = PNG_COL_FROM_PASS_COL(input_x, pass); |
---|
2102 | png_uint_32 output_y = PNG_ROW_FROM_PASS_ROW(input_y, pass); |
---|
2103 | |
---|
2104 | Finally a pair of macros are provided to tell you if a particular image |
---|
2105 | row or column appears in a given pass: |
---|
2106 | |
---|
2107 | int col_in_pass = PNG_COL_IN_INTERLACE_PASS(output_x, pass); |
---|
2108 | int row_in_pass = PNG_ROW_IN_INTERLACE_PASS(output_y, pass); |
---|
2109 | |
---|
2110 | Bear in mind that you will probably also need to check the width and height |
---|
2111 | of the pass in addition to the above to be sure the pass even exists! |
---|
2112 | |
---|
2113 | With any luck you are convinced by now that you don't want to do your own |
---|
2114 | interlace handling. In reality normally the only good reason for doing this |
---|
2115 | is if you are processing PNG files on a pixel-by-pixel basis and don't want |
---|
2116 | to load the whole file into memory when it is interlaced. |
---|
2117 | |
---|
2118 | libpng includes a test program, pngvalid, that illustrates reading and |
---|
2119 | writing of interlaced images. If you can't get interlacing to work in your |
---|
2120 | code and don't want to leave it to libpng (the recommended approach), see |
---|
2121 | how pngvalid.c does it. |
---|
2122 | |
---|
2123 | Finishing a sequential read |
---|
2124 | |
---|
2125 | After you are finished reading the image through the |
---|
2126 | low-level interface, you can finish reading the file. If you are |
---|
2127 | interested in comments or time, which may be stored either before or |
---|
2128 | after the image data, you should pass the separate png_info struct if |
---|
2129 | you want to keep the comments from before and after the image |
---|
2130 | separate. |
---|
2131 | |
---|
2132 | png_infop end_info = png_create_info_struct(png_ptr); |
---|
2133 | |
---|
2134 | if (!end_info) |
---|
2135 | { |
---|
2136 | png_destroy_read_struct(&png_ptr, &info_ptr, |
---|
2137 | (png_infopp)NULL); |
---|
2138 | return (ERROR); |
---|
2139 | } |
---|
2140 | |
---|
2141 | png_read_end(png_ptr, end_info); |
---|
2142 | |
---|
2143 | If you are not interested, you should still call png_read_end() |
---|
2144 | but you can pass NULL, avoiding the need to create an end_info structure. |
---|
2145 | |
---|
2146 | png_read_end(png_ptr, (png_infop)NULL); |
---|
2147 | |
---|
2148 | If you don't call png_read_end(), then your file pointer will be |
---|
2149 | left pointing to the first chunk after the last IDAT, which is probably |
---|
2150 | not what you want if you expect to read something beyond the end of |
---|
2151 | the PNG datastream. |
---|
2152 | |
---|
2153 | When you are done, you can free all memory allocated by libpng like this: |
---|
2154 | |
---|
2155 | png_destroy_read_struct(&png_ptr, &info_ptr, |
---|
2156 | &end_info); |
---|
2157 | |
---|
2158 | or, if you didn't create an end_info structure, |
---|
2159 | |
---|
2160 | png_destroy_read_struct(&png_ptr, &info_ptr, |
---|
2161 | (png_infopp)NULL); |
---|
2162 | |
---|
2163 | It is also possible to individually free the info_ptr members that |
---|
2164 | point to libpng-allocated storage with the following function: |
---|
2165 | |
---|
2166 | png_free_data(png_ptr, info_ptr, mask, seq) |
---|
2167 | |
---|
2168 | mask - identifies data to be freed, a mask |
---|
2169 | containing the bitwise OR of one or |
---|
2170 | more of |
---|
2171 | PNG_FREE_PLTE, PNG_FREE_TRNS, |
---|
2172 | PNG_FREE_HIST, PNG_FREE_ICCP, |
---|
2173 | PNG_FREE_PCAL, PNG_FREE_ROWS, |
---|
2174 | PNG_FREE_SCAL, PNG_FREE_SPLT, |
---|
2175 | PNG_FREE_TEXT, PNG_FREE_UNKN, |
---|
2176 | or simply PNG_FREE_ALL |
---|
2177 | |
---|
2178 | seq - sequence number of item to be freed |
---|
2179 | (-1 for all items) |
---|
2180 | |
---|
2181 | This function may be safely called when the relevant storage has |
---|
2182 | already been freed, or has not yet been allocated, or was allocated |
---|
2183 | by the user and not by libpng, and will in those cases do nothing. |
---|
2184 | The "seq" parameter is ignored if only one item of the selected data |
---|
2185 | type, such as PLTE, is allowed. If "seq" is not -1, and multiple items |
---|
2186 | are allowed for the data type identified in the mask, such as text or |
---|
2187 | sPLT, only the n'th item in the structure is freed, where n is "seq". |
---|
2188 | |
---|
2189 | The default behavior is only to free data that was allocated internally |
---|
2190 | by libpng. This can be changed, so that libpng will not free the data, |
---|
2191 | or so that it will free data that was allocated by the user with png_malloc() |
---|
2192 | or png_zalloc() and passed in via a png_set_*() function, with |
---|
2193 | |
---|
2194 | png_data_freer(png_ptr, info_ptr, freer, mask) |
---|
2195 | |
---|
2196 | freer - one of |
---|
2197 | PNG_DESTROY_WILL_FREE_DATA |
---|
2198 | PNG_SET_WILL_FREE_DATA |
---|
2199 | PNG_USER_WILL_FREE_DATA |
---|
2200 | |
---|
2201 | mask - which data elements are affected |
---|
2202 | same choices as in png_free_data() |
---|
2203 | |
---|
2204 | This function only affects data that has already been allocated. |
---|
2205 | You can call this function after reading the PNG data but before calling |
---|
2206 | any png_set_*() functions, to control whether the user or the png_set_*() |
---|
2207 | function is responsible for freeing any existing data that might be present, |
---|
2208 | and again after the png_set_*() functions to control whether the user |
---|
2209 | or png_destroy_*() is supposed to free the data. When the user assumes |
---|
2210 | responsibility for libpng-allocated data, the application must use |
---|
2211 | png_free() to free it, and when the user transfers responsibility to libpng |
---|
2212 | for data that the user has allocated, the user must have used png_malloc() |
---|
2213 | or png_zalloc() to allocate it. |
---|
2214 | |
---|
2215 | If you allocated your row_pointers in a single block, as suggested above in |
---|
2216 | the description of the high level read interface, you must not transfer |
---|
2217 | responsibility for freeing it to the png_set_rows or png_read_destroy function, |
---|
2218 | because they would also try to free the individual row_pointers[i]. |
---|
2219 | |
---|
2220 | If you allocated text_ptr.text, text_ptr.lang, and text_ptr.translated_keyword |
---|
2221 | separately, do not transfer responsibility for freeing text_ptr to libpng, |
---|
2222 | because when libpng fills a png_text structure it combines these members with |
---|
2223 | the key member, and png_free_data() will free only text_ptr.key. Similarly, |
---|
2224 | if you transfer responsibility for free'ing text_ptr from libpng to your |
---|
2225 | application, your application must not separately free those members. |
---|
2226 | |
---|
2227 | The png_free_data() function will turn off the "valid" flag for anything |
---|
2228 | it frees. If you need to turn the flag off for a chunk that was freed by |
---|
2229 | your application instead of by libpng, you can use |
---|
2230 | |
---|
2231 | png_set_invalid(png_ptr, info_ptr, mask); |
---|
2232 | |
---|
2233 | mask - identifies the chunks to be made invalid, |
---|
2234 | containing the bitwise OR of one or |
---|
2235 | more of |
---|
2236 | PNG_INFO_gAMA, PNG_INFO_sBIT, |
---|
2237 | PNG_INFO_cHRM, PNG_INFO_PLTE, |
---|
2238 | PNG_INFO_tRNS, PNG_INFO_bKGD, |
---|
2239 | PNG_INFO_hIST, PNG_INFO_pHYs, |
---|
2240 | PNG_INFO_oFFs, PNG_INFO_tIME, |
---|
2241 | PNG_INFO_pCAL, PNG_INFO_sRGB, |
---|
2242 | PNG_INFO_iCCP, PNG_INFO_sPLT, |
---|
2243 | PNG_INFO_sCAL, PNG_INFO_IDAT |
---|
2244 | |
---|
2245 | For a more compact example of reading a PNG image, see the file example.c. |
---|
2246 | |
---|
2247 | Reading PNG files progressively |
---|
2248 | |
---|
2249 | The progressive reader is slightly different then the non-progressive |
---|
2250 | reader. Instead of calling png_read_info(), png_read_rows(), and |
---|
2251 | png_read_end(), you make one call to png_process_data(), which calls |
---|
2252 | callbacks when it has the info, a row, or the end of the image. You |
---|
2253 | set up these callbacks with png_set_progressive_read_fn(). You don't |
---|
2254 | have to worry about the input/output functions of libpng, as you are |
---|
2255 | giving the library the data directly in png_process_data(). I will |
---|
2256 | assume that you have read the section on reading PNG files above, |
---|
2257 | so I will only highlight the differences (although I will show |
---|
2258 | all of the code). |
---|
2259 | |
---|
2260 | png_structp png_ptr; |
---|
2261 | png_infop info_ptr; |
---|
2262 | |
---|
2263 | /* An example code fragment of how you would |
---|
2264 | initialize the progressive reader in your |
---|
2265 | application. */ |
---|
2266 | int |
---|
2267 | initialize_png_reader() |
---|
2268 | { |
---|
2269 | png_ptr = png_create_read_struct |
---|
2270 | (PNG_LIBPNG_VER_STRING, (png_voidp)user_error_ptr, |
---|
2271 | user_error_fn, user_warning_fn); |
---|
2272 | |
---|
2273 | if (!png_ptr) |
---|
2274 | return (ERROR); |
---|
2275 | |
---|
2276 | info_ptr = png_create_info_struct(png_ptr); |
---|
2277 | |
---|
2278 | if (!info_ptr) |
---|
2279 | { |
---|
2280 | png_destroy_read_struct(&png_ptr, |
---|
2281 | (png_infopp)NULL, (png_infopp)NULL); |
---|
2282 | return (ERROR); |
---|
2283 | } |
---|
2284 | |
---|
2285 | if (setjmp(png_jmpbuf(png_ptr))) |
---|
2286 | { |
---|
2287 | png_destroy_read_struct(&png_ptr, &info_ptr, |
---|
2288 | (png_infopp)NULL); |
---|
2289 | return (ERROR); |
---|
2290 | } |
---|
2291 | |
---|
2292 | /* This one's new. You can provide functions |
---|
2293 | to be called when the header info is valid, |
---|
2294 | when each row is completed, and when the image |
---|
2295 | is finished. If you aren't using all functions, |
---|
2296 | you can specify NULL parameters. Even when all |
---|
2297 | three functions are NULL, you need to call |
---|
2298 | png_set_progressive_read_fn(). You can use |
---|
2299 | any struct as the user_ptr (cast to a void pointer |
---|
2300 | for the function call), and retrieve the pointer |
---|
2301 | from inside the callbacks using the function |
---|
2302 | |
---|
2303 | png_get_progressive_ptr(png_ptr); |
---|
2304 | |
---|
2305 | which will return a void pointer, which you have |
---|
2306 | to cast appropriately. |
---|
2307 | */ |
---|
2308 | png_set_progressive_read_fn(png_ptr, (void *)user_ptr, |
---|
2309 | info_callback, row_callback, end_callback); |
---|
2310 | |
---|
2311 | return 0; |
---|
2312 | } |
---|
2313 | |
---|
2314 | /* A code fragment that you call as you receive blocks |
---|
2315 | of data */ |
---|
2316 | int |
---|
2317 | process_data(png_bytep buffer, png_uint_32 length) |
---|
2318 | { |
---|
2319 | if (setjmp(png_jmpbuf(png_ptr))) |
---|
2320 | { |
---|
2321 | png_destroy_read_struct(&png_ptr, &info_ptr, |
---|
2322 | (png_infopp)NULL); |
---|
2323 | return (ERROR); |
---|
2324 | } |
---|
2325 | |
---|
2326 | /* This one's new also. Simply give it a chunk |
---|
2327 | of data from the file stream (in order, of |
---|
2328 | course). On machines with segmented memory |
---|
2329 | models machines, don't give it any more than |
---|
2330 | 64K. The library seems to run fine with sizes |
---|
2331 | of 4K. Although you can give it much less if |
---|
2332 | necessary (I assume you can give it chunks of |
---|
2333 | 1 byte, I haven't tried less then 256 bytes |
---|
2334 | yet). When this function returns, you may |
---|
2335 | want to display any rows that were generated |
---|
2336 | in the row callback if you don't already do |
---|
2337 | so there. |
---|
2338 | */ |
---|
2339 | png_process_data(png_ptr, info_ptr, buffer, length); |
---|
2340 | |
---|
2341 | /* At this point you can call png_process_data_skip if |
---|
2342 | you want to handle data the library will skip yourself; |
---|
2343 | it simply returns the number of bytes to skip (and stops |
---|
2344 | libpng skipping that number of bytes on the next |
---|
2345 | png_process_data call). |
---|
2346 | return 0; |
---|
2347 | } |
---|
2348 | |
---|
2349 | /* This function is called (as set by |
---|
2350 | png_set_progressive_read_fn() above) when enough data |
---|
2351 | has been supplied so all of the header has been |
---|
2352 | read. |
---|
2353 | */ |
---|
2354 | void |
---|
2355 | info_callback(png_structp png_ptr, png_infop info) |
---|
2356 | { |
---|
2357 | /* Do any setup here, including setting any of |
---|
2358 | the transformations mentioned in the Reading |
---|
2359 | PNG files section. For now, you _must_ call |
---|
2360 | either png_start_read_image() or |
---|
2361 | png_read_update_info() after all the |
---|
2362 | transformations are set (even if you don't set |
---|
2363 | any). You may start getting rows before |
---|
2364 | png_process_data() returns, so this is your |
---|
2365 | last chance to prepare for that. |
---|
2366 | |
---|
2367 | This is where you turn on interlace handling, |
---|
2368 | assuming you don't want to do it yourself. |
---|
2369 | |
---|
2370 | If you need to you can stop the processing of |
---|
2371 | your original input data at this point by calling |
---|
2372 | png_process_data_pause. This returns the number |
---|
2373 | of unprocessed bytes from the last png_process_data |
---|
2374 | call - it is up to you to ensure that the next call |
---|
2375 | sees these bytes again. If you don't want to bother |
---|
2376 | with this you can get libpng to cache the unread |
---|
2377 | bytes by setting the 'save' parameter (see png.h) but |
---|
2378 | then libpng will have to copy the data internally. |
---|
2379 | */ |
---|
2380 | } |
---|
2381 | |
---|
2382 | /* This function is called when each row of image |
---|
2383 | data is complete */ |
---|
2384 | void |
---|
2385 | row_callback(png_structp png_ptr, png_bytep new_row, |
---|
2386 | png_uint_32 row_num, int pass) |
---|
2387 | { |
---|
2388 | /* If the image is interlaced, and you turned |
---|
2389 | on the interlace handler, this function will |
---|
2390 | be called for every row in every pass. Some |
---|
2391 | of these rows will not be changed from the |
---|
2392 | previous pass. When the row is not changed, |
---|
2393 | the new_row variable will be NULL. The rows |
---|
2394 | and passes are called in order, so you don't |
---|
2395 | really need the row_num and pass, but I'm |
---|
2396 | supplying them because it may make your life |
---|
2397 | easier. |
---|
2398 | |
---|
2399 | If you did not turn on interlace handling then |
---|
2400 | the callback is called for each row of each |
---|
2401 | sub-image when the image is interlaced. In this |
---|
2402 | case 'row_num' is the row in the sub-image, not |
---|
2403 | the row in the output image as it is in all other |
---|
2404 | cases. |
---|
2405 | |
---|
2406 | For the non-NULL rows of interlaced images when |
---|
2407 | you have switched on libpng interlace handling, |
---|
2408 | you must call png_progressive_combine_row() |
---|
2409 | passing in the row and the old row. You can |
---|
2410 | call this function for NULL rows (it will just |
---|
2411 | return) and for non-interlaced images (it just |
---|
2412 | does the memcpy for you) if it will make the |
---|
2413 | code easier. Thus, you can just do this for |
---|
2414 | all cases if you switch on interlace handling; |
---|
2415 | */ |
---|
2416 | |
---|
2417 | png_progressive_combine_row(png_ptr, old_row, |
---|
2418 | new_row); |
---|
2419 | |
---|
2420 | /* where old_row is what was displayed for |
---|
2421 | previously for the row. Note that the first |
---|
2422 | pass (pass == 0, really) will completely cover |
---|
2423 | the old row, so the rows do not have to be |
---|
2424 | initialized. After the first pass (and only |
---|
2425 | for interlaced images), you will have to pass |
---|
2426 | the current row, and the function will combine |
---|
2427 | the old row and the new row. |
---|
2428 | |
---|
2429 | You can also call png_process_data_pause in this |
---|
2430 | callback - see above. |
---|
2431 | */ |
---|
2432 | } |
---|
2433 | |
---|
2434 | void |
---|
2435 | end_callback(png_structp png_ptr, png_infop info) |
---|
2436 | { |
---|
2437 | /* This function is called after the whole image |
---|
2438 | has been read, including any chunks after the |
---|
2439 | image (up to and including the IEND). You |
---|
2440 | will usually have the same info chunk as you |
---|
2441 | had in the header, although some data may have |
---|
2442 | been added to the comments and time fields. |
---|
2443 | |
---|
2444 | Most people won't do much here, perhaps setting |
---|
2445 | a flag that marks the image as finished. |
---|
2446 | */ |
---|
2447 | } |
---|
2448 | |
---|
2449 | |
---|
2450 | |
---|
2451 | IV. Writing |
---|
2452 | |
---|
2453 | Much of this is very similar to reading. However, everything of |
---|
2454 | importance is repeated here, so you won't have to constantly look |
---|
2455 | back up in the reading section to understand writing. |
---|
2456 | |
---|
2457 | Setup |
---|
2458 | |
---|
2459 | You will want to do the I/O initialization before you get into libpng, |
---|
2460 | so if it doesn't work, you don't have anything to undo. If you are not |
---|
2461 | using the standard I/O functions, you will need to replace them with |
---|
2462 | custom writing functions. See the discussion under Customizing libpng. |
---|
2463 | |
---|
2464 | FILE *fp = fopen(file_name, "wb"); |
---|
2465 | |
---|
2466 | if (!fp) |
---|
2467 | return (ERROR); |
---|
2468 | |
---|
2469 | Next, png_struct and png_info need to be allocated and initialized. |
---|
2470 | As these can be both relatively large, you may not want to store these |
---|
2471 | on the stack, unless you have stack space to spare. Of course, you |
---|
2472 | will want to check if they return NULL. If you are also reading, |
---|
2473 | you won't want to name your read structure and your write structure |
---|
2474 | both "png_ptr"; you can call them anything you like, such as |
---|
2475 | "read_ptr" and "write_ptr". Look at pngtest.c, for example. |
---|
2476 | |
---|
2477 | png_structp png_ptr = png_create_write_struct |
---|
2478 | (PNG_LIBPNG_VER_STRING, (png_voidp)user_error_ptr, |
---|
2479 | user_error_fn, user_warning_fn); |
---|
2480 | |
---|
2481 | if (!png_ptr) |
---|
2482 | return (ERROR); |
---|
2483 | |
---|
2484 | png_infop info_ptr = png_create_info_struct(png_ptr); |
---|
2485 | if (!info_ptr) |
---|
2486 | { |
---|
2487 | png_destroy_write_struct(&png_ptr, |
---|
2488 | (png_infopp)NULL); |
---|
2489 | return (ERROR); |
---|
2490 | } |
---|
2491 | |
---|
2492 | If you want to use your own memory allocation routines, |
---|
2493 | define PNG_USER_MEM_SUPPORTED and use |
---|
2494 | png_create_write_struct_2() instead of png_create_write_struct(): |
---|
2495 | |
---|
2496 | png_structp png_ptr = png_create_write_struct_2 |
---|
2497 | (PNG_LIBPNG_VER_STRING, (png_voidp)user_error_ptr, |
---|
2498 | user_error_fn, user_warning_fn, (png_voidp) |
---|
2499 | user_mem_ptr, user_malloc_fn, user_free_fn); |
---|
2500 | |
---|
2501 | After you have these structures, you will need to set up the |
---|
2502 | error handling. When libpng encounters an error, it expects to |
---|
2503 | longjmp() back to your routine. Therefore, you will need to call |
---|
2504 | setjmp() and pass the png_jmpbuf(png_ptr). If you |
---|
2505 | write the file from different routines, you will need to update |
---|
2506 | the png_jmpbuf(png_ptr) every time you enter a new routine that will |
---|
2507 | call a png_*() function. See your documentation of setjmp/longjmp |
---|
2508 | for your compiler for more information on setjmp/longjmp. See |
---|
2509 | the discussion on libpng error handling in the Customizing Libpng |
---|
2510 | section below for more information on the libpng error handling. |
---|
2511 | |
---|
2512 | if (setjmp(png_jmpbuf(png_ptr))) |
---|
2513 | { |
---|
2514 | png_destroy_write_struct(&png_ptr, &info_ptr); |
---|
2515 | fclose(fp); |
---|
2516 | return (ERROR); |
---|
2517 | } |
---|
2518 | ... |
---|
2519 | return; |
---|
2520 | |
---|
2521 | If you would rather avoid the complexity of setjmp/longjmp issues, |
---|
2522 | you can compile libpng with PNG_NO_SETJMP, in which case |
---|
2523 | errors will result in a call to PNG_ABORT() which defaults to abort(). |
---|
2524 | |
---|
2525 | You can #define PNG_ABORT() to a function that does something |
---|
2526 | more useful than abort(), as long as your function does not |
---|
2527 | return. |
---|
2528 | |
---|
2529 | Now you need to set up the output code. The default for libpng is to |
---|
2530 | use the C function fwrite(). If you use this, you will need to pass a |
---|
2531 | valid FILE * in the function png_init_io(). Be sure that the file is |
---|
2532 | opened in binary mode. Again, if you wish to handle writing data in |
---|
2533 | another way, see the discussion on libpng I/O handling in the Customizing |
---|
2534 | Libpng section below. |
---|
2535 | |
---|
2536 | png_init_io(png_ptr, fp); |
---|
2537 | |
---|
2538 | If you are embedding your PNG into a datastream such as MNG, and don't |
---|
2539 | want libpng to write the 8-byte signature, or if you have already |
---|
2540 | written the signature in your application, use |
---|
2541 | |
---|
2542 | png_set_sig_bytes(png_ptr, 8); |
---|
2543 | |
---|
2544 | to inform libpng that it should not write a signature. |
---|
2545 | |
---|
2546 | Write callbacks |
---|
2547 | |
---|
2548 | At this point, you can set up a callback function that will be |
---|
2549 | called after each row has been written, which you can use to control |
---|
2550 | a progress meter or the like. It's demonstrated in pngtest.c. |
---|
2551 | You must supply a function |
---|
2552 | |
---|
2553 | void write_row_callback(png_structp png_ptr, png_uint_32 row, |
---|
2554 | int pass); |
---|
2555 | { |
---|
2556 | /* put your code here */ |
---|
2557 | } |
---|
2558 | |
---|
2559 | (You can give it another name that you like instead of "write_row_callback") |
---|
2560 | |
---|
2561 | To inform libpng about your function, use |
---|
2562 | |
---|
2563 | png_set_write_status_fn(png_ptr, write_row_callback); |
---|
2564 | |
---|
2565 | When this function is called the row has already been completely processed and |
---|
2566 | it has also been written out. The 'row' and 'pass' refer to the next row to be |
---|
2567 | handled. For the |
---|
2568 | non-interlaced case the row that was just handled is simply one less than the |
---|
2569 | passed in row number, and pass will always be 0. For the interlaced case the |
---|
2570 | same applies unless the row value is 0, in which case the row just handled was |
---|
2571 | the last one from one of the preceding passes. Because interlacing may skip a |
---|
2572 | pass you cannot be sure that the preceding pass is just 'pass-1', if you really |
---|
2573 | need to know what the last pass is record (row,pass) from the callback and use |
---|
2574 | the last recorded value each time. |
---|
2575 | |
---|
2576 | As with the user transform you can find the output row using the |
---|
2577 | PNG_ROW_FROM_PASS_ROW macro. |
---|
2578 | |
---|
2579 | You now have the option of modifying how the compression library will |
---|
2580 | run. The following functions are mainly for testing, but may be useful |
---|
2581 | in some cases, like if you need to write PNG files extremely fast and |
---|
2582 | are willing to give up some compression, or if you want to get the |
---|
2583 | maximum possible compression at the expense of slower writing. If you |
---|
2584 | have no special needs in this area, let the library do what it wants by |
---|
2585 | not calling this function at all, as it has been tuned to deliver a good |
---|
2586 | speed/compression ratio. The second parameter to png_set_filter() is |
---|
2587 | the filter method, for which the only valid values are 0 (as of the |
---|
2588 | July 1999 PNG specification, version 1.2) or 64 (if you are writing |
---|
2589 | a PNG datastream that is to be embedded in a MNG datastream). The third |
---|
2590 | parameter is a flag that indicates which filter type(s) are to be tested |
---|
2591 | for each scanline. See the PNG specification for details on the specific |
---|
2592 | filter types. |
---|
2593 | |
---|
2594 | |
---|
2595 | /* turn on or off filtering, and/or choose |
---|
2596 | specific filters. You can use either a single |
---|
2597 | PNG_FILTER_VALUE_NAME or the bitwise OR of one |
---|
2598 | or more PNG_FILTER_NAME masks. |
---|
2599 | */ |
---|
2600 | png_set_filter(png_ptr, 0, |
---|
2601 | PNG_FILTER_NONE | PNG_FILTER_VALUE_NONE | |
---|
2602 | PNG_FILTER_SUB | PNG_FILTER_VALUE_SUB | |
---|
2603 | PNG_FILTER_UP | PNG_FILTER_VALUE_UP | |
---|
2604 | PNG_FILTER_AVG | PNG_FILTER_VALUE_AVG | |
---|
2605 | PNG_FILTER_PAETH | PNG_FILTER_VALUE_PAETH| |
---|
2606 | PNG_ALL_FILTERS); |
---|
2607 | |
---|
2608 | If an application wants to start and stop using particular filters during |
---|
2609 | compression, it should start out with all of the filters (to ensure that |
---|
2610 | the previous row of pixels will be stored in case it's needed later), |
---|
2611 | and then add and remove them after the start of compression. |
---|
2612 | |
---|
2613 | If you are writing a PNG datastream that is to be embedded in a MNG |
---|
2614 | datastream, the second parameter can be either 0 or 64. |
---|
2615 | |
---|
2616 | The png_set_compression_*() functions interface to the zlib compression |
---|
2617 | library, and should mostly be ignored unless you really know what you are |
---|
2618 | doing. The only generally useful call is png_set_compression_level() |
---|
2619 | which changes how much time zlib spends on trying to compress the image |
---|
2620 | data. See the Compression Library (zlib.h and algorithm.txt, distributed |
---|
2621 | with zlib) for details on the compression levels. |
---|
2622 | |
---|
2623 | #include zlib.h |
---|
2624 | |
---|
2625 | /* Set the zlib compression level */ |
---|
2626 | png_set_compression_level(png_ptr, |
---|
2627 | Z_BEST_COMPRESSION); |
---|
2628 | |
---|
2629 | /* Set other zlib parameters for compressing IDAT */ |
---|
2630 | png_set_compression_mem_level(png_ptr, 8); |
---|
2631 | png_set_compression_strategy(png_ptr, |
---|
2632 | Z_DEFAULT_STRATEGY); |
---|
2633 | png_set_compression_window_bits(png_ptr, 15); |
---|
2634 | png_set_compression_method(png_ptr, 8); |
---|
2635 | png_set_compression_buffer_size(png_ptr, 8192) |
---|
2636 | |
---|
2637 | /* Set zlib parameters for text compression |
---|
2638 | * If you don't call these, the parameters |
---|
2639 | * fall back on those defined for IDAT chunks |
---|
2640 | */ |
---|
2641 | png_set_text_compression_mem_level(png_ptr, 8); |
---|
2642 | png_set_text_compression_strategy(png_ptr, |
---|
2643 | Z_DEFAULT_STRATEGY); |
---|
2644 | png_set_text_compression_window_bits(png_ptr, 15); |
---|
2645 | png_set_text_compression_method(png_ptr, 8); |
---|
2646 | |
---|
2647 | Setting the contents of info for output |
---|
2648 | |
---|
2649 | You now need to fill in the png_info structure with all the data you |
---|
2650 | wish to write before the actual image. Note that the only thing you |
---|
2651 | are allowed to write after the image is the text chunks and the time |
---|
2652 | chunk (as of PNG Specification 1.2, anyway). See png_write_end() and |
---|
2653 | the latest PNG specification for more information on that. If you |
---|
2654 | wish to write them before the image, fill them in now, and flag that |
---|
2655 | data as being valid. If you want to wait until after the data, don't |
---|
2656 | fill them until png_write_end(). For all the fields in png_info and |
---|
2657 | their data types, see png.h. For explanations of what the fields |
---|
2658 | contain, see the PNG specification. |
---|
2659 | |
---|
2660 | Some of the more important parts of the png_info are: |
---|
2661 | |
---|
2662 | png_set_IHDR(png_ptr, info_ptr, width, height, |
---|
2663 | bit_depth, color_type, interlace_type, |
---|
2664 | compression_type, filter_method) |
---|
2665 | |
---|
2666 | width - holds the width of the image |
---|
2667 | in pixels (up to 2^31). |
---|
2668 | |
---|
2669 | height - holds the height of the image |
---|
2670 | in pixels (up to 2^31). |
---|
2671 | |
---|
2672 | bit_depth - holds the bit depth of one of the |
---|
2673 | image channels. |
---|
2674 | (valid values are 1, 2, 4, 8, 16 |
---|
2675 | and depend also on the |
---|
2676 | color_type. See also significant |
---|
2677 | bits (sBIT) below). |
---|
2678 | |
---|
2679 | color_type - describes which color/alpha |
---|
2680 | channels are present. |
---|
2681 | PNG_COLOR_TYPE_GRAY |
---|
2682 | (bit depths 1, 2, 4, 8, 16) |
---|
2683 | PNG_COLOR_TYPE_GRAY_ALPHA |
---|
2684 | (bit depths 8, 16) |
---|
2685 | PNG_COLOR_TYPE_PALETTE |
---|
2686 | (bit depths 1, 2, 4, 8) |
---|
2687 | PNG_COLOR_TYPE_RGB |
---|
2688 | (bit_depths 8, 16) |
---|
2689 | PNG_COLOR_TYPE_RGB_ALPHA |
---|
2690 | (bit_depths 8, 16) |
---|
2691 | |
---|
2692 | PNG_COLOR_MASK_PALETTE |
---|
2693 | PNG_COLOR_MASK_COLOR |
---|
2694 | PNG_COLOR_MASK_ALPHA |
---|
2695 | |
---|
2696 | interlace_type - PNG_INTERLACE_NONE or |
---|
2697 | PNG_INTERLACE_ADAM7 |
---|
2698 | |
---|
2699 | compression_type - (must be |
---|
2700 | PNG_COMPRESSION_TYPE_DEFAULT) |
---|
2701 | |
---|
2702 | filter_method - (must be PNG_FILTER_TYPE_DEFAULT |
---|
2703 | or, if you are writing a PNG to |
---|
2704 | be embedded in a MNG datastream, |
---|
2705 | can also be |
---|
2706 | PNG_INTRAPIXEL_DIFFERENCING) |
---|
2707 | |
---|
2708 | If you call png_set_IHDR(), the call must appear before any of the |
---|
2709 | other png_set_*() functions, because they might require access to some of |
---|
2710 | the IHDR settings. The remaining png_set_*() functions can be called |
---|
2711 | in any order. |
---|
2712 | |
---|
2713 | If you wish, you can reset the compression_type, interlace_type, or |
---|
2714 | filter_method later by calling png_set_IHDR() again; if you do this, the |
---|
2715 | width, height, bit_depth, and color_type must be the same in each call. |
---|
2716 | |
---|
2717 | png_set_PLTE(png_ptr, info_ptr, palette, |
---|
2718 | num_palette); |
---|
2719 | |
---|
2720 | palette - the palette for the file |
---|
2721 | (array of png_color) |
---|
2722 | num_palette - number of entries in the palette |
---|
2723 | |
---|
2724 | png_set_gAMA(png_ptr, info_ptr, file_gamma); |
---|
2725 | png_set_gAMA_fixed(png_ptr, info_ptr, int_file_gamma); |
---|
2726 | |
---|
2727 | file_gamma - the gamma at which the image was |
---|
2728 | created (PNG_INFO_gAMA) |
---|
2729 | |
---|
2730 | int_file_gamma - 100,000 times the gamma at which |
---|
2731 | the image was created |
---|
2732 | |
---|
2733 | png_set_cHRM(png_ptr, info_ptr, white_x, white_y, red_x, red_y, |
---|
2734 | green_x, green_y, blue_x, blue_y) |
---|
2735 | png_set_cHRM_XYZ(png_ptr, info_ptr, red_X, red_Y, red_Z, green_X, |
---|
2736 | green_Y, green_Z, blue_X, blue_Y, blue_Z) |
---|
2737 | png_set_cHRM_fixed(png_ptr, info_ptr, int_white_x, int_white_y, |
---|
2738 | int_red_x, int_red_y, int_green_x, int_green_y, |
---|
2739 | int_blue_x, int_blue_y) |
---|
2740 | png_set_cHRM_XYZ_fixed(png_ptr, info_ptr, int_red_X, int_red_Y, |
---|
2741 | int_red_Z, int_green_X, int_green_Y, int_green_Z, |
---|
2742 | int_blue_X, int_blue_Y, int_blue_Z) |
---|
2743 | |
---|
2744 | {white,red,green,blue}_{x,y} |
---|
2745 | A color space encoding specified using the chromaticities |
---|
2746 | of the end points and the white point. |
---|
2747 | |
---|
2748 | {red,green,blue}_{X,Y,Z} |
---|
2749 | A color space encoding specified using the encoding end |
---|
2750 | points - the CIE tristimulus specification of the intended |
---|
2751 | color of the red, green and blue channels in the PNG RGB |
---|
2752 | data. The white point is simply the sum of the three end |
---|
2753 | points. |
---|
2754 | |
---|
2755 | png_set_sRGB(png_ptr, info_ptr, srgb_intent); |
---|
2756 | |
---|
2757 | srgb_intent - the rendering intent |
---|
2758 | (PNG_INFO_sRGB) The presence of |
---|
2759 | the sRGB chunk means that the pixel |
---|
2760 | data is in the sRGB color space. |
---|
2761 | This chunk also implies specific |
---|
2762 | values of gAMA and cHRM. Rendering |
---|
2763 | intent is the CSS-1 property that |
---|
2764 | has been defined by the International |
---|
2765 | Color Consortium |
---|
2766 | (http://www.color.org). |
---|
2767 | It can be one of |
---|
2768 | PNG_sRGB_INTENT_SATURATION, |
---|
2769 | PNG_sRGB_INTENT_PERCEPTUAL, |
---|
2770 | PNG_sRGB_INTENT_ABSOLUTE, or |
---|
2771 | PNG_sRGB_INTENT_RELATIVE. |
---|
2772 | |
---|
2773 | |
---|
2774 | png_set_sRGB_gAMA_and_cHRM(png_ptr, info_ptr, |
---|
2775 | srgb_intent); |
---|
2776 | |
---|
2777 | srgb_intent - the rendering intent |
---|
2778 | (PNG_INFO_sRGB) The presence of the |
---|
2779 | sRGB chunk means that the pixel |
---|
2780 | data is in the sRGB color space. |
---|
2781 | This function also causes gAMA and |
---|
2782 | cHRM chunks with the specific values |
---|
2783 | that are consistent with sRGB to be |
---|
2784 | written. |
---|
2785 | |
---|
2786 | png_set_iCCP(png_ptr, info_ptr, name, compression_type, |
---|
2787 | profile, proflen); |
---|
2788 | |
---|
2789 | name - The profile name. |
---|
2790 | |
---|
2791 | compression_type - The compression type; always |
---|
2792 | PNG_COMPRESSION_TYPE_BASE for PNG 1.0. |
---|
2793 | You may give NULL to this argument to |
---|
2794 | ignore it. |
---|
2795 | |
---|
2796 | profile - International Color Consortium color |
---|
2797 | profile data. May contain NULs. |
---|
2798 | |
---|
2799 | proflen - length of profile data in bytes. |
---|
2800 | |
---|
2801 | png_set_sBIT(png_ptr, info_ptr, sig_bit); |
---|
2802 | |
---|
2803 | sig_bit - the number of significant bits for |
---|
2804 | (PNG_INFO_sBIT) each of the gray, red, |
---|
2805 | green, and blue channels, whichever are |
---|
2806 | appropriate for the given color type |
---|
2807 | (png_color_16) |
---|
2808 | |
---|
2809 | png_set_tRNS(png_ptr, info_ptr, trans_alpha, |
---|
2810 | num_trans, trans_color); |
---|
2811 | |
---|
2812 | trans_alpha - array of alpha (transparency) |
---|
2813 | entries for palette (PNG_INFO_tRNS) |
---|
2814 | |
---|
2815 | num_trans - number of transparent entries |
---|
2816 | (PNG_INFO_tRNS) |
---|
2817 | |
---|
2818 | trans_color - graylevel or color sample values |
---|
2819 | (in order red, green, blue) of the |
---|
2820 | single transparent color for |
---|
2821 | non-paletted images (PNG_INFO_tRNS) |
---|
2822 | |
---|
2823 | png_set_hIST(png_ptr, info_ptr, hist); |
---|
2824 | |
---|
2825 | hist - histogram of palette (array of |
---|
2826 | png_uint_16) (PNG_INFO_hIST) |
---|
2827 | |
---|
2828 | png_set_tIME(png_ptr, info_ptr, mod_time); |
---|
2829 | |
---|
2830 | mod_time - time image was last modified |
---|
2831 | (PNG_VALID_tIME) |
---|
2832 | |
---|
2833 | png_set_bKGD(png_ptr, info_ptr, background); |
---|
2834 | |
---|
2835 | background - background color (of type |
---|
2836 | png_color_16p) (PNG_VALID_bKGD) |
---|
2837 | |
---|
2838 | png_set_text(png_ptr, info_ptr, text_ptr, num_text); |
---|
2839 | |
---|
2840 | text_ptr - array of png_text holding image |
---|
2841 | comments |
---|
2842 | |
---|
2843 | text_ptr[i].compression - type of compression used |
---|
2844 | on "text" PNG_TEXT_COMPRESSION_NONE |
---|
2845 | PNG_TEXT_COMPRESSION_zTXt |
---|
2846 | PNG_ITXT_COMPRESSION_NONE |
---|
2847 | PNG_ITXT_COMPRESSION_zTXt |
---|
2848 | text_ptr[i].key - keyword for comment. Must contain |
---|
2849 | 1-79 characters. |
---|
2850 | text_ptr[i].text - text comments for current |
---|
2851 | keyword. Can be NULL or empty. |
---|
2852 | text_ptr[i].text_length - length of text string, |
---|
2853 | after decompression, 0 for iTXt |
---|
2854 | text_ptr[i].itxt_length - length of itxt string, |
---|
2855 | after decompression, 0 for tEXt/zTXt |
---|
2856 | text_ptr[i].lang - language of comment (NULL or |
---|
2857 | empty for unknown). |
---|
2858 | text_ptr[i].translated_keyword - keyword in UTF-8 (NULL |
---|
2859 | or empty for unknown). |
---|
2860 | |
---|
2861 | Note that the itxt_length, lang, and lang_key |
---|
2862 | members of the text_ptr structure only exist when the |
---|
2863 | library is built with iTXt chunk support. Prior to |
---|
2864 | libpng-1.4.0 the library was built by default without |
---|
2865 | iTXt support. Also note that when iTXt is supported, |
---|
2866 | they contain NULL pointers when the "compression" |
---|
2867 | field contains PNG_TEXT_COMPRESSION_NONE or |
---|
2868 | PNG_TEXT_COMPRESSION_zTXt. |
---|
2869 | |
---|
2870 | num_text - number of comments |
---|
2871 | |
---|
2872 | png_set_sPLT(png_ptr, info_ptr, &palette_ptr, |
---|
2873 | num_spalettes); |
---|
2874 | |
---|
2875 | palette_ptr - array of png_sPLT_struct structures |
---|
2876 | to be added to the list of palettes |
---|
2877 | in the info structure. |
---|
2878 | num_spalettes - number of palette structures to be |
---|
2879 | added. |
---|
2880 | |
---|
2881 | png_set_oFFs(png_ptr, info_ptr, offset_x, offset_y, |
---|
2882 | unit_type); |
---|
2883 | |
---|
2884 | offset_x - positive offset from the left |
---|
2885 | edge of the screen |
---|
2886 | |
---|
2887 | offset_y - positive offset from the top |
---|
2888 | edge of the screen |
---|
2889 | |
---|
2890 | unit_type - PNG_OFFSET_PIXEL, PNG_OFFSET_MICROMETER |
---|
2891 | |
---|
2892 | png_set_pHYs(png_ptr, info_ptr, res_x, res_y, |
---|
2893 | unit_type); |
---|
2894 | |
---|
2895 | res_x - pixels/unit physical resolution |
---|
2896 | in x direction |
---|
2897 | |
---|
2898 | res_y - pixels/unit physical resolution |
---|
2899 | in y direction |
---|
2900 | |
---|
2901 | unit_type - PNG_RESOLUTION_UNKNOWN, |
---|
2902 | PNG_RESOLUTION_METER |
---|
2903 | |
---|
2904 | png_set_sCAL(png_ptr, info_ptr, unit, width, height) |
---|
2905 | |
---|
2906 | unit - physical scale units (an integer) |
---|
2907 | |
---|
2908 | width - width of a pixel in physical scale units |
---|
2909 | |
---|
2910 | height - height of a pixel in physical scale units |
---|
2911 | (width and height are doubles) |
---|
2912 | |
---|
2913 | png_set_sCAL_s(png_ptr, info_ptr, unit, width, height) |
---|
2914 | |
---|
2915 | unit - physical scale units (an integer) |
---|
2916 | |
---|
2917 | width - width of a pixel in physical scale units |
---|
2918 | expressed as a string |
---|
2919 | |
---|
2920 | height - height of a pixel in physical scale units |
---|
2921 | (width and height are strings like "2.54") |
---|
2922 | |
---|
2923 | png_set_unknown_chunks(png_ptr, info_ptr, &unknowns, |
---|
2924 | num_unknowns) |
---|
2925 | |
---|
2926 | unknowns - array of png_unknown_chunk |
---|
2927 | structures holding unknown chunks |
---|
2928 | unknowns[i].name - name of unknown chunk |
---|
2929 | unknowns[i].data - data of unknown chunk |
---|
2930 | unknowns[i].size - size of unknown chunk's data |
---|
2931 | unknowns[i].location - position to write chunk in file |
---|
2932 | 0: do not write chunk |
---|
2933 | PNG_HAVE_IHDR: before PLTE |
---|
2934 | PNG_HAVE_PLTE: before IDAT |
---|
2935 | PNG_AFTER_IDAT: after IDAT |
---|
2936 | |
---|
2937 | The "location" member is set automatically according to |
---|
2938 | what part of the output file has already been written. |
---|
2939 | You can change its value after calling png_set_unknown_chunks() |
---|
2940 | as demonstrated in pngtest.c. Within each of the "locations", |
---|
2941 | the chunks are sequenced according to their position in the |
---|
2942 | structure (that is, the value of "i", which is the order in which |
---|
2943 | the chunk was either read from the input file or defined with |
---|
2944 | png_set_unknown_chunks). |
---|
2945 | |
---|
2946 | A quick word about text and num_text. text is an array of png_text |
---|
2947 | structures. num_text is the number of valid structures in the array. |
---|
2948 | Each png_text structure holds a language code, a keyword, a text value, |
---|
2949 | and a compression type. |
---|
2950 | |
---|
2951 | The compression types have the same valid numbers as the compression |
---|
2952 | types of the image data. Currently, the only valid number is zero. |
---|
2953 | However, you can store text either compressed or uncompressed, unlike |
---|
2954 | images, which always have to be compressed. So if you don't want the |
---|
2955 | text compressed, set the compression type to PNG_TEXT_COMPRESSION_NONE. |
---|
2956 | Because tEXt and zTXt chunks don't have a language field, if you |
---|
2957 | specify PNG_TEXT_COMPRESSION_NONE or PNG_TEXT_COMPRESSION_zTXt |
---|
2958 | any language code or translated keyword will not be written out. |
---|
2959 | |
---|
2960 | Until text gets around a few hundred bytes, it is not worth compressing it. |
---|
2961 | After the text has been written out to the file, the compression type |
---|
2962 | is set to PNG_TEXT_COMPRESSION_NONE_WR or PNG_TEXT_COMPRESSION_zTXt_WR, |
---|
2963 | so that it isn't written out again at the end (in case you are calling |
---|
2964 | png_write_end() with the same struct). |
---|
2965 | |
---|
2966 | The keywords that are given in the PNG Specification are: |
---|
2967 | |
---|
2968 | Title Short (one line) title or |
---|
2969 | caption for image |
---|
2970 | |
---|
2971 | Author Name of image's creator |
---|
2972 | |
---|
2973 | Description Description of image (possibly long) |
---|
2974 | |
---|
2975 | Copyright Copyright notice |
---|
2976 | |
---|
2977 | Creation Time Time of original image creation |
---|
2978 | (usually RFC 1123 format, see below) |
---|
2979 | |
---|
2980 | Software Software used to create the image |
---|
2981 | |
---|
2982 | Disclaimer Legal disclaimer |
---|
2983 | |
---|
2984 | Warning Warning of nature of content |
---|
2985 | |
---|
2986 | Source Device used to create the image |
---|
2987 | |
---|
2988 | Comment Miscellaneous comment; conversion |
---|
2989 | from other image format |
---|
2990 | |
---|
2991 | The keyword-text pairs work like this. Keywords should be short |
---|
2992 | simple descriptions of what the comment is about. Some typical |
---|
2993 | keywords are found in the PNG specification, as is some recommendations |
---|
2994 | on keywords. You can repeat keywords in a file. You can even write |
---|
2995 | some text before the image and some after. For example, you may want |
---|
2996 | to put a description of the image before the image, but leave the |
---|
2997 | disclaimer until after, so viewers working over modem connections |
---|
2998 | don't have to wait for the disclaimer to go over the modem before |
---|
2999 | they start seeing the image. Finally, keywords should be full |
---|
3000 | words, not abbreviations. Keywords and text are in the ISO 8859-1 |
---|
3001 | (Latin-1) character set (a superset of regular ASCII) and can not |
---|
3002 | contain NUL characters, and should not contain control or other |
---|
3003 | unprintable characters. To make the comments widely readable, stick |
---|
3004 | with basic ASCII, and avoid machine specific character set extensions |
---|
3005 | like the IBM-PC character set. The keyword must be present, but |
---|
3006 | you can leave off the text string on non-compressed pairs. |
---|
3007 | Compressed pairs must have a text string, as only the text string |
---|
3008 | is compressed anyway, so the compression would be meaningless. |
---|
3009 | |
---|
3010 | PNG supports modification time via the png_time structure. Two |
---|
3011 | conversion routines are provided, png_convert_from_time_t() for |
---|
3012 | time_t and png_convert_from_struct_tm() for struct tm. The |
---|
3013 | time_t routine uses gmtime(). You don't have to use either of |
---|
3014 | these, but if you wish to fill in the png_time structure directly, |
---|
3015 | you should provide the time in universal time (GMT) if possible |
---|
3016 | instead of your local time. Note that the year number is the full |
---|
3017 | year (e.g. 1998, rather than 98 - PNG is year 2000 compliant!), and |
---|
3018 | that months start with 1. |
---|
3019 | |
---|
3020 | If you want to store the time of the original image creation, you should |
---|
3021 | use a plain tEXt chunk with the "Creation Time" keyword. This is |
---|
3022 | necessary because the "creation time" of a PNG image is somewhat vague, |
---|
3023 | depending on whether you mean the PNG file, the time the image was |
---|
3024 | created in a non-PNG format, a still photo from which the image was |
---|
3025 | scanned, or possibly the subject matter itself. In order to facilitate |
---|
3026 | machine-readable dates, it is recommended that the "Creation Time" |
---|
3027 | tEXt chunk use RFC 1123 format dates (e.g. "22 May 1997 18:07:10 GMT"), |
---|
3028 | although this isn't a requirement. Unlike the tIME chunk, the |
---|
3029 | "Creation Time" tEXt chunk is not expected to be automatically changed |
---|
3030 | by the software. To facilitate the use of RFC 1123 dates, a function |
---|
3031 | png_convert_to_rfc1123(png_timep) is provided to convert from PNG |
---|
3032 | time to an RFC 1123 format string. |
---|
3033 | |
---|
3034 | Writing unknown chunks |
---|
3035 | |
---|
3036 | You can use the png_set_unknown_chunks function to queue up chunks |
---|
3037 | for writing. You give it a chunk name, raw data, and a size; that's |
---|
3038 | all there is to it. The chunks will be written by the next following |
---|
3039 | png_write_info_before_PLTE, png_write_info, or png_write_end function. |
---|
3040 | Any chunks previously read into the info structure's unknown-chunk |
---|
3041 | list will also be written out in a sequence that satisfies the PNG |
---|
3042 | specification's ordering rules. |
---|
3043 | |
---|
3044 | The high-level write interface |
---|
3045 | |
---|
3046 | At this point there are two ways to proceed; through the high-level |
---|
3047 | write interface, or through a sequence of low-level write operations. |
---|
3048 | You can use the high-level interface if your image data is present |
---|
3049 | in the info structure. All defined output |
---|
3050 | transformations are permitted, enabled by the following masks. |
---|
3051 | |
---|
3052 | PNG_TRANSFORM_IDENTITY No transformation |
---|
3053 | PNG_TRANSFORM_PACKING Pack 1, 2 and 4-bit samples |
---|
3054 | PNG_TRANSFORM_PACKSWAP Change order of packed |
---|
3055 | pixels to LSB first |
---|
3056 | PNG_TRANSFORM_INVERT_MONO Invert monochrome images |
---|
3057 | PNG_TRANSFORM_SHIFT Normalize pixels to the |
---|
3058 | sBIT depth |
---|
3059 | PNG_TRANSFORM_BGR Flip RGB to BGR, RGBA |
---|
3060 | to BGRA |
---|
3061 | PNG_TRANSFORM_SWAP_ALPHA Flip RGBA to ARGB or GA |
---|
3062 | to AG |
---|
3063 | PNG_TRANSFORM_INVERT_ALPHA Change alpha from opacity |
---|
3064 | to transparency |
---|
3065 | PNG_TRANSFORM_SWAP_ENDIAN Byte-swap 16-bit samples |
---|
3066 | PNG_TRANSFORM_STRIP_FILLER Strip out filler |
---|
3067 | bytes (deprecated). |
---|
3068 | PNG_TRANSFORM_STRIP_FILLER_BEFORE Strip out leading |
---|
3069 | filler bytes |
---|
3070 | PNG_TRANSFORM_STRIP_FILLER_AFTER Strip out trailing |
---|
3071 | filler bytes |
---|
3072 | |
---|
3073 | If you have valid image data in the info structure (you can use |
---|
3074 | png_set_rows() to put image data in the info structure), simply do this: |
---|
3075 | |
---|
3076 | png_write_png(png_ptr, info_ptr, png_transforms, NULL) |
---|
3077 | |
---|
3078 | where png_transforms is an integer containing the bitwise OR of some set of |
---|
3079 | transformation flags. This call is equivalent to png_write_info(), |
---|
3080 | followed the set of transformations indicated by the transform mask, |
---|
3081 | then png_write_image(), and finally png_write_end(). |
---|
3082 | |
---|
3083 | (The final parameter of this call is not yet used. Someday it might point |
---|
3084 | to transformation parameters required by some future output transform.) |
---|
3085 | |
---|
3086 | You must use png_transforms and not call any png_set_transform() functions |
---|
3087 | when you use png_write_png(). |
---|
3088 | |
---|
3089 | The low-level write interface |
---|
3090 | |
---|
3091 | If you are going the low-level route instead, you are now ready to |
---|
3092 | write all the file information up to the actual image data. You do |
---|
3093 | this with a call to png_write_info(). |
---|
3094 | |
---|
3095 | png_write_info(png_ptr, info_ptr); |
---|
3096 | |
---|
3097 | Note that there is one transformation you may need to do before |
---|
3098 | png_write_info(). In PNG files, the alpha channel in an image is the |
---|
3099 | level of opacity. If your data is supplied as a level of transparency, |
---|
3100 | you can invert the alpha channel before you write it, so that 0 is |
---|
3101 | fully transparent and 255 (in 8-bit or paletted images) or 65535 |
---|
3102 | (in 16-bit images) is fully opaque, with |
---|
3103 | |
---|
3104 | png_set_invert_alpha(png_ptr); |
---|
3105 | |
---|
3106 | This must appear before png_write_info() instead of later with the |
---|
3107 | other transformations because in the case of paletted images the tRNS |
---|
3108 | chunk data has to be inverted before the tRNS chunk is written. If |
---|
3109 | your image is not a paletted image, the tRNS data (which in such cases |
---|
3110 | represents a single color to be rendered as transparent) won't need to |
---|
3111 | be changed, and you can safely do this transformation after your |
---|
3112 | png_write_info() call. |
---|
3113 | |
---|
3114 | If you need to write a private chunk that you want to appear before |
---|
3115 | the PLTE chunk when PLTE is present, you can write the PNG info in |
---|
3116 | two steps, and insert code to write your own chunk between them: |
---|
3117 | |
---|
3118 | png_write_info_before_PLTE(png_ptr, info_ptr); |
---|
3119 | png_set_unknown_chunks(png_ptr, info_ptr, ...); |
---|
3120 | png_write_info(png_ptr, info_ptr); |
---|
3121 | |
---|
3122 | After you've written the file information, you can set up the library |
---|
3123 | to handle any special transformations of the image data. The various |
---|
3124 | ways to transform the data will be described in the order that they |
---|
3125 | should occur. This is important, as some of these change the color |
---|
3126 | type and/or bit depth of the data, and some others only work on |
---|
3127 | certain color types and bit depths. Even though each transformation |
---|
3128 | checks to see if it has data that it can do something with, you should |
---|
3129 | make sure to only enable a transformation if it will be valid for the |
---|
3130 | data. For example, don't swap red and blue on grayscale data. |
---|
3131 | |
---|
3132 | PNG files store RGB pixels packed into 3 or 6 bytes. This code tells |
---|
3133 | the library to strip input data that has 4 or 8 bytes per pixel down |
---|
3134 | to 3 or 6 bytes (or strip 2 or 4-byte grayscale+filler data to 1 or 2 |
---|
3135 | bytes per pixel). |
---|
3136 | |
---|
3137 | png_set_filler(png_ptr, 0, PNG_FILLER_BEFORE); |
---|
3138 | |
---|
3139 | where the 0 is unused, and the location is either PNG_FILLER_BEFORE or |
---|
3140 | PNG_FILLER_AFTER, depending upon whether the filler byte in the pixel |
---|
3141 | is stored XRGB or RGBX. |
---|
3142 | |
---|
3143 | PNG files pack pixels of bit depths 1, 2, and 4 into bytes as small as |
---|
3144 | they can, resulting in, for example, 8 pixels per byte for 1 bit files. |
---|
3145 | If the data is supplied at 1 pixel per byte, use this code, which will |
---|
3146 | correctly pack the pixels into a single byte: |
---|
3147 | |
---|
3148 | png_set_packing(png_ptr); |
---|
3149 | |
---|
3150 | PNG files reduce possible bit depths to 1, 2, 4, 8, and 16. If your |
---|
3151 | data is of another bit depth, you can write an sBIT chunk into the |
---|
3152 | file so that decoders can recover the original data if desired. |
---|
3153 | |
---|
3154 | /* Set the true bit depth of the image data */ |
---|
3155 | if (color_type & PNG_COLOR_MASK_COLOR) |
---|
3156 | { |
---|
3157 | sig_bit.red = true_bit_depth; |
---|
3158 | sig_bit.green = true_bit_depth; |
---|
3159 | sig_bit.blue = true_bit_depth; |
---|
3160 | } |
---|
3161 | |
---|
3162 | else |
---|
3163 | { |
---|
3164 | sig_bit.gray = true_bit_depth; |
---|
3165 | } |
---|
3166 | |
---|
3167 | if (color_type & PNG_COLOR_MASK_ALPHA) |
---|
3168 | { |
---|
3169 | sig_bit.alpha = true_bit_depth; |
---|
3170 | } |
---|
3171 | |
---|
3172 | png_set_sBIT(png_ptr, info_ptr, &sig_bit); |
---|
3173 | |
---|
3174 | If the data is stored in the row buffer in a bit depth other than |
---|
3175 | one supported by PNG (e.g. 3 bit data in the range 0-7 for a 4-bit PNG), |
---|
3176 | this will scale the values to appear to be the correct bit depth as |
---|
3177 | is required by PNG. |
---|
3178 | |
---|
3179 | png_set_shift(png_ptr, &sig_bit); |
---|
3180 | |
---|
3181 | PNG files store 16-bit pixels in network byte order (big-endian, |
---|
3182 | ie. most significant bits first). This code would be used if they are |
---|
3183 | supplied the other way (little-endian, i.e. least significant bits |
---|
3184 | first, the way PCs store them): |
---|
3185 | |
---|
3186 | if (bit_depth > 8) |
---|
3187 | png_set_swap(png_ptr); |
---|
3188 | |
---|
3189 | If you are using packed-pixel images (1, 2, or 4 bits/pixel), and you |
---|
3190 | need to change the order the pixels are packed into bytes, you can use: |
---|
3191 | |
---|
3192 | if (bit_depth < 8) |
---|
3193 | png_set_packswap(png_ptr); |
---|
3194 | |
---|
3195 | PNG files store 3 color pixels in red, green, blue order. This code |
---|
3196 | would be used if they are supplied as blue, green, red: |
---|
3197 | |
---|
3198 | png_set_bgr(png_ptr); |
---|
3199 | |
---|
3200 | PNG files describe monochrome as black being zero and white being |
---|
3201 | one. This code would be used if the pixels are supplied with this reversed |
---|
3202 | (black being one and white being zero): |
---|
3203 | |
---|
3204 | png_set_invert_mono(png_ptr); |
---|
3205 | |
---|
3206 | Finally, you can write your own transformation function if none of |
---|
3207 | the existing ones meets your needs. This is done by setting a callback |
---|
3208 | with |
---|
3209 | |
---|
3210 | png_set_write_user_transform_fn(png_ptr, |
---|
3211 | write_transform_fn); |
---|
3212 | |
---|
3213 | You must supply the function |
---|
3214 | |
---|
3215 | void write_transform_fn(png_structp png_ptr, png_row_infop |
---|
3216 | row_info, png_bytep data) |
---|
3217 | |
---|
3218 | See pngtest.c for a working example. Your function will be called |
---|
3219 | before any of the other transformations are processed. If supported |
---|
3220 | libpng also supplies an information routine that may be called from |
---|
3221 | your callback: |
---|
3222 | |
---|
3223 | png_get_current_row_number(png_ptr); |
---|
3224 | png_get_current_pass_number(png_ptr); |
---|
3225 | |
---|
3226 | This returns the current row passed to the transform. With interlaced |
---|
3227 | images the value returned is the row in the input sub-image image. Use |
---|
3228 | PNG_ROW_FROM_PASS_ROW(row, pass) and PNG_COL_FROM_PASS_COL(col, pass) to |
---|
3229 | find the output pixel (x,y) given an interlaced sub-image pixel (row,col,pass). |
---|
3230 | |
---|
3231 | The discussion of interlace handling above contains more information on how to |
---|
3232 | use these values. |
---|
3233 | |
---|
3234 | You can also set up a pointer to a user structure for use by your |
---|
3235 | callback function. |
---|
3236 | |
---|
3237 | png_set_user_transform_info(png_ptr, user_ptr, 0, 0); |
---|
3238 | |
---|
3239 | The user_channels and user_depth parameters of this function are ignored |
---|
3240 | when writing; you can set them to zero as shown. |
---|
3241 | |
---|
3242 | You can retrieve the pointer via the function png_get_user_transform_ptr(). |
---|
3243 | For example: |
---|
3244 | |
---|
3245 | voidp write_user_transform_ptr = |
---|
3246 | png_get_user_transform_ptr(png_ptr); |
---|
3247 | |
---|
3248 | It is possible to have libpng flush any pending output, either manually, |
---|
3249 | or automatically after a certain number of lines have been written. To |
---|
3250 | flush the output stream a single time call: |
---|
3251 | |
---|
3252 | png_write_flush(png_ptr); |
---|
3253 | |
---|
3254 | and to have libpng flush the output stream periodically after a certain |
---|
3255 | number of scanlines have been written, call: |
---|
3256 | |
---|
3257 | png_set_flush(png_ptr, nrows); |
---|
3258 | |
---|
3259 | Note that the distance between rows is from the last time png_write_flush() |
---|
3260 | was called, or the first row of the image if it has never been called. |
---|
3261 | So if you write 50 lines, and then png_set_flush 25, it will flush the |
---|
3262 | output on the next scanline, and every 25 lines thereafter, unless |
---|
3263 | png_write_flush() is called before 25 more lines have been written. |
---|
3264 | If nrows is too small (less than about 10 lines for a 640 pixel wide |
---|
3265 | RGB image) the image compression may decrease noticeably (although this |
---|
3266 | may be acceptable for real-time applications). Infrequent flushing will |
---|
3267 | only degrade the compression performance by a few percent over images |
---|
3268 | that do not use flushing. |
---|
3269 | |
---|
3270 | Writing the image data |
---|
3271 | |
---|
3272 | That's it for the transformations. Now you can write the image data. |
---|
3273 | The simplest way to do this is in one function call. If you have the |
---|
3274 | whole image in memory, you can just call png_write_image() and libpng |
---|
3275 | will write the image. You will need to pass in an array of pointers to |
---|
3276 | each row. This function automatically handles interlacing, so you don't |
---|
3277 | need to call png_set_interlace_handling() or call this function multiple |
---|
3278 | times, or any of that other stuff necessary with png_write_rows(). |
---|
3279 | |
---|
3280 | png_write_image(png_ptr, row_pointers); |
---|
3281 | |
---|
3282 | where row_pointers is: |
---|
3283 | |
---|
3284 | png_byte *row_pointers[height]; |
---|
3285 | |
---|
3286 | You can point to void or char or whatever you use for pixels. |
---|
3287 | |
---|
3288 | If you don't want to write the whole image at once, you can |
---|
3289 | use png_write_rows() instead. If the file is not interlaced, |
---|
3290 | this is simple: |
---|
3291 | |
---|
3292 | png_write_rows(png_ptr, row_pointers, |
---|
3293 | number_of_rows); |
---|
3294 | |
---|
3295 | row_pointers is the same as in the png_write_image() call. |
---|
3296 | |
---|
3297 | If you are just writing one row at a time, you can do this with |
---|
3298 | a single row_pointer instead of an array of row_pointers: |
---|
3299 | |
---|
3300 | png_bytep row_pointer = row; |
---|
3301 | |
---|
3302 | png_write_row(png_ptr, row_pointer); |
---|
3303 | |
---|
3304 | When the file is interlaced, things can get a good deal more complicated. |
---|
3305 | The only currently (as of the PNG Specification version 1.2, dated July |
---|
3306 | 1999) defined interlacing scheme for PNG files is the "Adam7" interlace |
---|
3307 | scheme, that breaks down an image into seven smaller images of varying |
---|
3308 | size. libpng will build these images for you, or you can do them |
---|
3309 | yourself. If you want to build them yourself, see the PNG specification |
---|
3310 | for details of which pixels to write when. |
---|
3311 | |
---|
3312 | If you don't want libpng to handle the interlacing details, just |
---|
3313 | use png_set_interlace_handling() and call png_write_rows() the |
---|
3314 | correct number of times to write all the sub-images |
---|
3315 | (png_set_interlace_handling() returns the number of sub-images.) |
---|
3316 | |
---|
3317 | If you want libpng to build the sub-images, call this before you start |
---|
3318 | writing any rows: |
---|
3319 | |
---|
3320 | number_of_passes = png_set_interlace_handling(png_ptr); |
---|
3321 | |
---|
3322 | This will return the number of passes needed. Currently, this is seven, |
---|
3323 | but may change if another interlace type is added. |
---|
3324 | |
---|
3325 | Then write the complete image number_of_passes times. |
---|
3326 | |
---|
3327 | png_write_rows(png_ptr, row_pointers, number_of_rows); |
---|
3328 | |
---|
3329 | Think carefully before you write an interlaced image. Typically code that |
---|
3330 | reads such images reads all the image data into memory, uncompressed, before |
---|
3331 | doing any processing. Only code that can display an image on the fly can |
---|
3332 | take advantage of the interlacing and even then the image has to be exactly |
---|
3333 | the correct size for the output device, because scaling an image requires |
---|
3334 | adjacent pixels and these are not available until all the passes have been |
---|
3335 | read. |
---|
3336 | |
---|
3337 | If you do write an interlaced image you will hardly ever need to handle |
---|
3338 | the interlacing yourself. Call png_set_interlace_handling() and use the |
---|
3339 | approach described above. |
---|
3340 | |
---|
3341 | The only time it is conceivable that you will really need to write an |
---|
3342 | interlaced image pass-by-pass is when you have read one pass by pass and |
---|
3343 | made some pixel-by-pixel transformation to it, as described in the read |
---|
3344 | code above. In this case use the PNG_PASS_ROWS and PNG_PASS_COLS macros |
---|
3345 | to determine the size of each sub-image in turn and simply write the rows |
---|
3346 | you obtained from the read code. |
---|
3347 | |
---|
3348 | Finishing a sequential write |
---|
3349 | |
---|
3350 | After you are finished writing the image, you should finish writing |
---|
3351 | the file. If you are interested in writing comments or time, you should |
---|
3352 | pass an appropriately filled png_info pointer. If you are not interested, |
---|
3353 | you can pass NULL. |
---|
3354 | |
---|
3355 | png_write_end(png_ptr, info_ptr); |
---|
3356 | |
---|
3357 | When you are done, you can free all memory used by libpng like this: |
---|
3358 | |
---|
3359 | png_destroy_write_struct(&png_ptr, &info_ptr); |
---|
3360 | |
---|
3361 | It is also possible to individually free the info_ptr members that |
---|
3362 | point to libpng-allocated storage with the following function: |
---|
3363 | |
---|
3364 | png_free_data(png_ptr, info_ptr, mask, seq) |
---|
3365 | |
---|
3366 | mask - identifies data to be freed, a mask |
---|
3367 | containing the bitwise OR of one or |
---|
3368 | more of |
---|
3369 | PNG_FREE_PLTE, PNG_FREE_TRNS, |
---|
3370 | PNG_FREE_HIST, PNG_FREE_ICCP, |
---|
3371 | PNG_FREE_PCAL, PNG_FREE_ROWS, |
---|
3372 | PNG_FREE_SCAL, PNG_FREE_SPLT, |
---|
3373 | PNG_FREE_TEXT, PNG_FREE_UNKN, |
---|
3374 | or simply PNG_FREE_ALL |
---|
3375 | |
---|
3376 | seq - sequence number of item to be freed |
---|
3377 | (-1 for all items) |
---|
3378 | |
---|
3379 | This function may be safely called when the relevant storage has |
---|
3380 | already been freed, or has not yet been allocated, or was allocated |
---|
3381 | by the user and not by libpng, and will in those cases do nothing. |
---|
3382 | The "seq" parameter is ignored if only one item of the selected data |
---|
3383 | type, such as PLTE, is allowed. If "seq" is not -1, and multiple items |
---|
3384 | are allowed for the data type identified in the mask, such as text or |
---|
3385 | sPLT, only the n'th item in the structure is freed, where n is "seq". |
---|
3386 | |
---|
3387 | If you allocated data such as a palette that you passed in to libpng |
---|
3388 | with png_set_*, you must not free it until just before the call to |
---|
3389 | png_destroy_write_struct(). |
---|
3390 | |
---|
3391 | The default behavior is only to free data that was allocated internally |
---|
3392 | by libpng. This can be changed, so that libpng will not free the data, |
---|
3393 | or so that it will free data that was allocated by the user with png_malloc() |
---|
3394 | or png_zalloc() and passed in via a png_set_*() function, with |
---|
3395 | |
---|
3396 | png_data_freer(png_ptr, info_ptr, freer, mask) |
---|
3397 | |
---|
3398 | freer - one of |
---|
3399 | PNG_DESTROY_WILL_FREE_DATA |
---|
3400 | PNG_SET_WILL_FREE_DATA |
---|
3401 | PNG_USER_WILL_FREE_DATA |
---|
3402 | |
---|
3403 | mask - which data elements are affected |
---|
3404 | same choices as in png_free_data() |
---|
3405 | |
---|
3406 | For example, to transfer responsibility for some data from a read structure |
---|
3407 | to a write structure, you could use |
---|
3408 | |
---|
3409 | png_data_freer(read_ptr, read_info_ptr, |
---|
3410 | PNG_USER_WILL_FREE_DATA, |
---|
3411 | PNG_FREE_PLTE|PNG_FREE_tRNS|PNG_FREE_hIST) |
---|
3412 | |
---|
3413 | png_data_freer(write_ptr, write_info_ptr, |
---|
3414 | PNG_DESTROY_WILL_FREE_DATA, |
---|
3415 | PNG_FREE_PLTE|PNG_FREE_tRNS|PNG_FREE_hIST) |
---|
3416 | |
---|
3417 | thereby briefly reassigning responsibility for freeing to the user but |
---|
3418 | immediately afterwards reassigning it once more to the write_destroy |
---|
3419 | function. Having done this, it would then be safe to destroy the read |
---|
3420 | structure and continue to use the PLTE, tRNS, and hIST data in the write |
---|
3421 | structure. |
---|
3422 | |
---|
3423 | This function only affects data that has already been allocated. |
---|
3424 | You can call this function before calling after the png_set_*() functions |
---|
3425 | to control whether the user or png_destroy_*() is supposed to free the data. |
---|
3426 | When the user assumes responsibility for libpng-allocated data, the |
---|
3427 | application must use |
---|
3428 | png_free() to free it, and when the user transfers responsibility to libpng |
---|
3429 | for data that the user has allocated, the user must have used png_malloc() |
---|
3430 | or png_zalloc() to allocate it. |
---|
3431 | |
---|
3432 | If you allocated text_ptr.text, text_ptr.lang, and text_ptr.translated_keyword |
---|
3433 | separately, do not transfer responsibility for freeing text_ptr to libpng, |
---|
3434 | because when libpng fills a png_text structure it combines these members with |
---|
3435 | the key member, and png_free_data() will free only text_ptr.key. Similarly, |
---|
3436 | if you transfer responsibility for free'ing text_ptr from libpng to your |
---|
3437 | application, your application must not separately free those members. |
---|
3438 | For a more compact example of writing a PNG image, see the file example.c. |
---|
3439 | |
---|
3440 | V. Modifying/Customizing libpng: |
---|
3441 | |
---|
3442 | There are two issues here. The first is changing how libpng does |
---|
3443 | standard things like memory allocation, input/output, and error handling. |
---|
3444 | The second deals with more complicated things like adding new chunks, |
---|
3445 | adding new transformations, and generally changing how libpng works. |
---|
3446 | Both of those are compile-time issues; that is, they are generally |
---|
3447 | determined at the time the code is written, and there is rarely a need |
---|
3448 | to provide the user with a means of changing them. |
---|
3449 | |
---|
3450 | Memory allocation, input/output, and error handling |
---|
3451 | |
---|
3452 | All of the memory allocation, input/output, and error handling in libpng |
---|
3453 | goes through callbacks that are user-settable. The default routines are |
---|
3454 | in pngmem.c, pngrio.c, pngwio.c, and pngerror.c, respectively. To change |
---|
3455 | these functions, call the appropriate png_set_*_fn() function. |
---|
3456 | |
---|
3457 | Memory allocation is done through the functions png_malloc(), png_calloc(), |
---|
3458 | and png_free(). These currently just call the standard C functions. |
---|
3459 | png_calloc() calls png_malloc() and then clears the newly |
---|
3460 | allocated memory to zero. There is limited support for certain systems |
---|
3461 | with segmented memory architectures and the types of pointers declared by |
---|
3462 | png.h match this; you will have to use appropriate pointers in your |
---|
3463 | application. Since it is |
---|
3464 | unlikely that the method of handling memory allocation on a platform |
---|
3465 | will change between applications, these functions must be modified in |
---|
3466 | the library at compile time. If you prefer to use a different method |
---|
3467 | of allocating and freeing data, you can use png_create_read_struct_2() or |
---|
3468 | png_create_write_struct_2() to register your own functions as described |
---|
3469 | above. These functions also provide a void pointer that can be retrieved |
---|
3470 | via |
---|
3471 | |
---|
3472 | mem_ptr=png_get_mem_ptr(png_ptr); |
---|
3473 | |
---|
3474 | Your replacement memory functions must have prototypes as follows: |
---|
3475 | |
---|
3476 | png_voidp malloc_fn(png_structp png_ptr, |
---|
3477 | png_alloc_size_t size); |
---|
3478 | |
---|
3479 | void free_fn(png_structp png_ptr, png_voidp ptr); |
---|
3480 | |
---|
3481 | Your malloc_fn() must return NULL in case of failure. The png_malloc() |
---|
3482 | function will normally call png_error() if it receives a NULL from the |
---|
3483 | system memory allocator or from your replacement malloc_fn(). |
---|
3484 | |
---|
3485 | Your free_fn() will never be called with a NULL ptr, since libpng's |
---|
3486 | png_free() checks for NULL before calling free_fn(). |
---|
3487 | |
---|
3488 | Input/Output in libpng is done through png_read() and png_write(), |
---|
3489 | which currently just call fread() and fwrite(). The FILE * is stored in |
---|
3490 | png_struct and is initialized via png_init_io(). If you wish to change |
---|
3491 | the method of I/O, the library supplies callbacks that you can set |
---|
3492 | through the function png_set_read_fn() and png_set_write_fn() at run |
---|
3493 | time, instead of calling the png_init_io() function. These functions |
---|
3494 | also provide a void pointer that can be retrieved via the function |
---|
3495 | png_get_io_ptr(). For example: |
---|
3496 | |
---|
3497 | png_set_read_fn(png_structp read_ptr, |
---|
3498 | voidp read_io_ptr, png_rw_ptr read_data_fn) |
---|
3499 | |
---|
3500 | png_set_write_fn(png_structp write_ptr, |
---|
3501 | voidp write_io_ptr, png_rw_ptr write_data_fn, |
---|
3502 | png_flush_ptr output_flush_fn); |
---|
3503 | |
---|
3504 | voidp read_io_ptr = png_get_io_ptr(read_ptr); |
---|
3505 | voidp write_io_ptr = png_get_io_ptr(write_ptr); |
---|
3506 | |
---|
3507 | The replacement I/O functions must have prototypes as follows: |
---|
3508 | |
---|
3509 | void user_read_data(png_structp png_ptr, |
---|
3510 | png_bytep data, png_size_t length); |
---|
3511 | |
---|
3512 | void user_write_data(png_structp png_ptr, |
---|
3513 | png_bytep data, png_size_t length); |
---|
3514 | |
---|
3515 | void user_flush_data(png_structp png_ptr); |
---|
3516 | |
---|
3517 | The user_read_data() function is responsible for detecting and |
---|
3518 | handling end-of-data errors. |
---|
3519 | |
---|
3520 | Supplying NULL for the read, write, or flush functions sets them back |
---|
3521 | to using the default C stream functions, which expect the io_ptr to |
---|
3522 | point to a standard *FILE structure. It is probably a mistake |
---|
3523 | to use NULL for one of write_data_fn and output_flush_fn but not both |
---|
3524 | of them, unless you have built libpng with PNG_NO_WRITE_FLUSH defined. |
---|
3525 | It is an error to read from a write stream, and vice versa. |
---|
3526 | |
---|
3527 | Error handling in libpng is done through png_error() and png_warning(). |
---|
3528 | Errors handled through png_error() are fatal, meaning that png_error() |
---|
3529 | should never return to its caller. Currently, this is handled via |
---|
3530 | setjmp() and longjmp() (unless you have compiled libpng with |
---|
3531 | PNG_NO_SETJMP, in which case it is handled via PNG_ABORT()), |
---|
3532 | but you could change this to do things like exit() if you should wish, |
---|
3533 | as long as your function does not return. |
---|
3534 | |
---|
3535 | On non-fatal errors, png_warning() is called |
---|
3536 | to print a warning message, and then control returns to the calling code. |
---|
3537 | By default png_error() and png_warning() print a message on stderr via |
---|
3538 | fprintf() unless the library is compiled with PNG_NO_CONSOLE_IO defined |
---|
3539 | (because you don't want the messages) or PNG_NO_STDIO defined (because |
---|
3540 | fprintf() isn't available). If you wish to change the behavior of the error |
---|
3541 | functions, you will need to set up your own message callbacks. These |
---|
3542 | functions are normally supplied at the time that the png_struct is created. |
---|
3543 | It is also possible to redirect errors and warnings to your own replacement |
---|
3544 | functions after png_create_*_struct() has been called by calling: |
---|
3545 | |
---|
3546 | png_set_error_fn(png_structp png_ptr, |
---|
3547 | png_voidp error_ptr, png_error_ptr error_fn, |
---|
3548 | png_error_ptr warning_fn); |
---|
3549 | |
---|
3550 | png_voidp error_ptr = png_get_error_ptr(png_ptr); |
---|
3551 | |
---|
3552 | If NULL is supplied for either error_fn or warning_fn, then the libpng |
---|
3553 | default function will be used, calling fprintf() and/or longjmp() if a |
---|
3554 | problem is encountered. The replacement error functions should have |
---|
3555 | parameters as follows: |
---|
3556 | |
---|
3557 | void user_error_fn(png_structp png_ptr, |
---|
3558 | png_const_charp error_msg); |
---|
3559 | |
---|
3560 | void user_warning_fn(png_structp png_ptr, |
---|
3561 | png_const_charp warning_msg); |
---|
3562 | |
---|
3563 | The motivation behind using setjmp() and longjmp() is the C++ throw and |
---|
3564 | catch exception handling methods. This makes the code much easier to write, |
---|
3565 | as there is no need to check every return code of every function call. |
---|
3566 | However, there are some uncertainties about the status of local variables |
---|
3567 | after a longjmp, so the user may want to be careful about doing anything |
---|
3568 | after setjmp returns non-zero besides returning itself. Consult your |
---|
3569 | compiler documentation for more details. For an alternative approach, you |
---|
3570 | may wish to use the "cexcept" facility (see http://cexcept.sourceforge.net), |
---|
3571 | which is illustrated in pngvalid.c and in contrib/visupng. |
---|
3572 | |
---|
3573 | Custom chunks |
---|
3574 | |
---|
3575 | If you need to read or write custom chunks, you may need to get deeper |
---|
3576 | into the libpng code. The library now has mechanisms for storing |
---|
3577 | and writing chunks of unknown type; you can even declare callbacks |
---|
3578 | for custom chunks. However, this may not be good enough if the |
---|
3579 | library code itself needs to know about interactions between your |
---|
3580 | chunk and existing `intrinsic' chunks. |
---|
3581 | |
---|
3582 | If you need to write a new intrinsic chunk, first read the PNG |
---|
3583 | specification. Acquire a first level of understanding of how it works. |
---|
3584 | Pay particular attention to the sections that describe chunk names, |
---|
3585 | and look at how other chunks were designed, so you can do things |
---|
3586 | similarly. Second, check out the sections of libpng that read and |
---|
3587 | write chunks. Try to find a chunk that is similar to yours and use |
---|
3588 | it as a template. More details can be found in the comments inside |
---|
3589 | the code. It is best to handle private or unknown chunks in a generic method, |
---|
3590 | via callback functions, instead of by modifying libpng functions. This |
---|
3591 | is illustrated in pngtest.c, which uses a callback function to handle a |
---|
3592 | private "vpAg" chunk and the new "sTER" chunk, which are both unknown to |
---|
3593 | libpng. |
---|
3594 | |
---|
3595 | If you wish to write your own transformation for the data, look through |
---|
3596 | the part of the code that does the transformations, and check out some of |
---|
3597 | the simpler ones to get an idea of how they work. Try to find a similar |
---|
3598 | transformation to the one you want to add and copy off of it. More details |
---|
3599 | can be found in the comments inside the code itself. |
---|
3600 | |
---|
3601 | Configuring for 16-bit platforms |
---|
3602 | |
---|
3603 | You will want to look into zconf.h to tell zlib (and thus libpng) that |
---|
3604 | it cannot allocate more then 64K at a time. Even if you can, the memory |
---|
3605 | won't be accessible. So limit zlib and libpng to 64K by defining MAXSEG_64K. |
---|
3606 | |
---|
3607 | Configuring for DOS |
---|
3608 | |
---|
3609 | For DOS users who only have access to the lower 640K, you will |
---|
3610 | have to limit zlib's memory usage via a png_set_compression_mem_level() |
---|
3611 | call. See zlib.h or zconf.h in the zlib library for more information. |
---|
3612 | |
---|
3613 | Configuring for Medium Model |
---|
3614 | |
---|
3615 | Libpng's support for medium model has been tested on most of the popular |
---|
3616 | compilers. Make sure MAXSEG_64K gets defined, USE_FAR_KEYWORD gets |
---|
3617 | defined, and FAR gets defined to far in pngconf.h, and you should be |
---|
3618 | all set. Everything in the library (except for zlib's structure) is |
---|
3619 | expecting far data. You must use the typedefs with the p or pp on |
---|
3620 | the end for pointers (or at least look at them and be careful). Make |
---|
3621 | note that the rows of data are defined as png_bytepp, which is |
---|
3622 | an "unsigned char far * far *". |
---|
3623 | |
---|
3624 | Configuring for gui/windowing platforms: |
---|
3625 | |
---|
3626 | You will need to write new error and warning functions that use the GUI |
---|
3627 | interface, as described previously, and set them to be the error and |
---|
3628 | warning functions at the time that png_create_*_struct() is called, |
---|
3629 | in order to have them available during the structure initialization. |
---|
3630 | They can be changed later via png_set_error_fn(). On some compilers, |
---|
3631 | you may also have to change the memory allocators (png_malloc, etc.). |
---|
3632 | |
---|
3633 | Configuring for compiler xxx: |
---|
3634 | |
---|
3635 | All includes for libpng are in pngconf.h. If you need to add, change |
---|
3636 | or delete an include, this is the place to do it. |
---|
3637 | The includes that are not needed outside libpng are placed in pngpriv.h, |
---|
3638 | which is only used by the routines inside libpng itself. |
---|
3639 | The files in libpng proper only include pngpriv.h and png.h, which |
---|
3640 | in turn includes pngconf.h and, as of libpng-1.5.0, pnglibconf.h. |
---|
3641 | As of libpng-1.5.0, pngpriv.h also includes three other private header |
---|
3642 | files, pngstruct.h, pnginfo.h, and pngdebug.h, which contain material |
---|
3643 | that previously appeared in the public headers. |
---|
3644 | |
---|
3645 | Configuring zlib: |
---|
3646 | |
---|
3647 | There are special functions to configure the compression. Perhaps the |
---|
3648 | most useful one changes the compression level, which currently uses |
---|
3649 | input compression values in the range 0 - 9. The library normally |
---|
3650 | uses the default compression level (Z_DEFAULT_COMPRESSION = 6). Tests |
---|
3651 | have shown that for a large majority of images, compression values in |
---|
3652 | the range 3-6 compress nearly as well as higher levels, and do so much |
---|
3653 | faster. For online applications it may be desirable to have maximum speed |
---|
3654 | (Z_BEST_SPEED = 1). With versions of zlib after v0.99, you can also |
---|
3655 | specify no compression (Z_NO_COMPRESSION = 0), but this would create |
---|
3656 | files larger than just storing the raw bitmap. You can specify the |
---|
3657 | compression level by calling: |
---|
3658 | |
---|
3659 | #include zlib.h |
---|
3660 | png_set_compression_level(png_ptr, level); |
---|
3661 | |
---|
3662 | Another useful one is to reduce the memory level used by the library. |
---|
3663 | The memory level defaults to 8, but it can be lowered if you are |
---|
3664 | short on memory (running DOS, for example, where you only have 640K). |
---|
3665 | Note that the memory level does have an effect on compression; among |
---|
3666 | other things, lower levels will result in sections of incompressible |
---|
3667 | data being emitted in smaller stored blocks, with a correspondingly |
---|
3668 | larger relative overhead of up to 15% in the worst case. |
---|
3669 | |
---|
3670 | #include zlib.h |
---|
3671 | png_set_compression_mem_level(png_ptr, level); |
---|
3672 | |
---|
3673 | The other functions are for configuring zlib. They are not recommended |
---|
3674 | for normal use and may result in writing an invalid PNG file. See |
---|
3675 | zlib.h for more information on what these mean. |
---|
3676 | |
---|
3677 | #include zlib.h |
---|
3678 | png_set_compression_strategy(png_ptr, |
---|
3679 | strategy); |
---|
3680 | |
---|
3681 | png_set_compression_window_bits(png_ptr, |
---|
3682 | window_bits); |
---|
3683 | |
---|
3684 | png_set_compression_method(png_ptr, method); |
---|
3685 | |
---|
3686 | png_set_compression_buffer_size(png_ptr, size); |
---|
3687 | |
---|
3688 | As of libpng version 1.5.4, additional APIs became |
---|
3689 | available to set these separately for non-IDAT |
---|
3690 | compressed chunks such as zTXt, iTXt, and iCCP: |
---|
3691 | |
---|
3692 | #include zlib.h |
---|
3693 | #if PNG_LIBPNG_VER >= 10504 |
---|
3694 | png_set_text_compression_level(png_ptr, level); |
---|
3695 | |
---|
3696 | png_set_text_compression_mem_level(png_ptr, level); |
---|
3697 | |
---|
3698 | png_set_text_compression_strategy(png_ptr, |
---|
3699 | strategy); |
---|
3700 | |
---|
3701 | png_set_text_compression_window_bits(png_ptr, |
---|
3702 | window_bits); |
---|
3703 | |
---|
3704 | png_set_text_compression_method(png_ptr, method); |
---|
3705 | #endif |
---|
3706 | |
---|
3707 | Controlling row filtering |
---|
3708 | |
---|
3709 | If you want to control whether libpng uses filtering or not, which |
---|
3710 | filters are used, and how it goes about picking row filters, you |
---|
3711 | can call one of these functions. The selection and configuration |
---|
3712 | of row filters can have a significant impact on the size and |
---|
3713 | encoding speed and a somewhat lesser impact on the decoding speed |
---|
3714 | of an image. Filtering is enabled by default for RGB and grayscale |
---|
3715 | images (with and without alpha), but not for paletted images nor |
---|
3716 | for any images with bit depths less than 8 bits/pixel. |
---|
3717 | |
---|
3718 | The 'method' parameter sets the main filtering method, which is |
---|
3719 | currently only '0' in the PNG 1.2 specification. The 'filters' |
---|
3720 | parameter sets which filter(s), if any, should be used for each |
---|
3721 | scanline. Possible values are PNG_ALL_FILTERS and PNG_NO_FILTERS |
---|
3722 | to turn filtering on and off, respectively. |
---|
3723 | |
---|
3724 | Individual filter types are PNG_FILTER_NONE, PNG_FILTER_SUB, |
---|
3725 | PNG_FILTER_UP, PNG_FILTER_AVG, PNG_FILTER_PAETH, which can be bitwise |
---|
3726 | ORed together with '|' to specify one or more filters to use. |
---|
3727 | These filters are described in more detail in the PNG specification. |
---|
3728 | If you intend to change the filter type during the course of writing |
---|
3729 | the image, you should start with flags set for all of the filters |
---|
3730 | you intend to use so that libpng can initialize its internal |
---|
3731 | structures appropriately for all of the filter types. (Note that this |
---|
3732 | means the first row must always be adaptively filtered, because libpng |
---|
3733 | currently does not allocate the filter buffers until png_write_row() |
---|
3734 | is called for the first time.) |
---|
3735 | |
---|
3736 | filters = PNG_FILTER_NONE | PNG_FILTER_SUB |
---|
3737 | PNG_FILTER_UP | PNG_FILTER_AVG | |
---|
3738 | PNG_FILTER_PAETH | PNG_ALL_FILTERS; |
---|
3739 | |
---|
3740 | png_set_filter(png_ptr, PNG_FILTER_TYPE_BASE, |
---|
3741 | filters); |
---|
3742 | The second parameter can also be |
---|
3743 | PNG_INTRAPIXEL_DIFFERENCING if you are |
---|
3744 | writing a PNG to be embedded in a MNG |
---|
3745 | datastream. This parameter must be the |
---|
3746 | same as the value of filter_method used |
---|
3747 | in png_set_IHDR(). |
---|
3748 | |
---|
3749 | It is also possible to influence how libpng chooses from among the |
---|
3750 | available filters. This is done in one or both of two ways - by |
---|
3751 | telling it how important it is to keep the same filter for successive |
---|
3752 | rows, and by telling it the relative computational costs of the filters. |
---|
3753 | |
---|
3754 | double weights[3] = {1.5, 1.3, 1.1}, |
---|
3755 | costs[PNG_FILTER_VALUE_LAST] = |
---|
3756 | {1.0, 1.3, 1.3, 1.5, 1.7}; |
---|
3757 | |
---|
3758 | png_set_filter_heuristics(png_ptr, |
---|
3759 | PNG_FILTER_HEURISTIC_WEIGHTED, 3, |
---|
3760 | weights, costs); |
---|
3761 | |
---|
3762 | The weights are multiplying factors that indicate to libpng that the |
---|
3763 | row filter should be the same for successive rows unless another row filter |
---|
3764 | is that many times better than the previous filter. In the above example, |
---|
3765 | if the previous 3 filters were SUB, SUB, NONE, the SUB filter could have a |
---|
3766 | "sum of absolute differences" 1.5 x 1.3 times higher than other filters |
---|
3767 | and still be chosen, while the NONE filter could have a sum 1.1 times |
---|
3768 | higher than other filters and still be chosen. Unspecified weights are |
---|
3769 | taken to be 1.0, and the specified weights should probably be declining |
---|
3770 | like those above in order to emphasize recent filters over older filters. |
---|
3771 | |
---|
3772 | The filter costs specify for each filter type a relative decoding cost |
---|
3773 | to be considered when selecting row filters. This means that filters |
---|
3774 | with higher costs are less likely to be chosen over filters with lower |
---|
3775 | costs, unless their "sum of absolute differences" is that much smaller. |
---|
3776 | The costs do not necessarily reflect the exact computational speeds of |
---|
3777 | the various filters, since this would unduly influence the final image |
---|
3778 | size. |
---|
3779 | |
---|
3780 | Note that the numbers above were invented purely for this example and |
---|
3781 | are given only to help explain the function usage. Little testing has |
---|
3782 | been done to find optimum values for either the costs or the weights. |
---|
3783 | |
---|
3784 | Removing unwanted object code |
---|
3785 | |
---|
3786 | There are a bunch of #define's in pngconf.h that control what parts of |
---|
3787 | libpng are compiled. All the defines end in _SUPPORTED. If you are |
---|
3788 | never going to use a capability, you can change the #define to #undef |
---|
3789 | before recompiling libpng and save yourself code and data space, or |
---|
3790 | you can turn off individual capabilities with defines that begin with |
---|
3791 | PNG_NO_. |
---|
3792 | |
---|
3793 | In libpng-1.5.0 and later, the #define's are in pnglibconf.h instead. |
---|
3794 | |
---|
3795 | You can also turn all of the transforms and ancillary chunk capabilities |
---|
3796 | off en masse with compiler directives that define |
---|
3797 | PNG_NO_READ[or WRITE]_TRANSFORMS, or PNG_NO_READ[or WRITE]_ANCILLARY_CHUNKS, |
---|
3798 | or all four, |
---|
3799 | along with directives to turn on any of the capabilities that you do |
---|
3800 | want. The PNG_NO_READ[or WRITE]_TRANSFORMS directives disable the extra |
---|
3801 | transformations but still leave the library fully capable of reading |
---|
3802 | and writing PNG files with all known public chunks. Use of the |
---|
3803 | PNG_NO_READ[or WRITE]_ANCILLARY_CHUNKS directive produces a library |
---|
3804 | that is incapable of reading or writing ancillary chunks. If you are |
---|
3805 | not using the progressive reading capability, you can turn that off |
---|
3806 | with PNG_NO_PROGRESSIVE_READ (don't confuse this with the INTERLACING |
---|
3807 | capability, which you'll still have). |
---|
3808 | |
---|
3809 | All the reading and writing specific code are in separate files, so the |
---|
3810 | linker should only grab the files it needs. However, if you want to |
---|
3811 | make sure, or if you are building a stand alone library, all the |
---|
3812 | reading files start with "pngr" and all the writing files start with "pngw". |
---|
3813 | The files that don't match either (like png.c, pngtrans.c, etc.) |
---|
3814 | are used for both reading and writing, and always need to be included. |
---|
3815 | The progressive reader is in pngpread.c |
---|
3816 | |
---|
3817 | If you are creating or distributing a dynamically linked library (a .so |
---|
3818 | or DLL file), you should not remove or disable any parts of the library, |
---|
3819 | as this will cause applications linked with different versions of the |
---|
3820 | library to fail if they call functions not available in your library. |
---|
3821 | The size of the library itself should not be an issue, because only |
---|
3822 | those sections that are actually used will be loaded into memory. |
---|
3823 | |
---|
3824 | Requesting debug printout |
---|
3825 | |
---|
3826 | The macro definition PNG_DEBUG can be used to request debugging |
---|
3827 | printout. Set it to an integer value in the range 0 to 3. Higher |
---|
3828 | numbers result in increasing amounts of debugging information. The |
---|
3829 | information is printed to the "stderr" file, unless another file |
---|
3830 | name is specified in the PNG_DEBUG_FILE macro definition. |
---|
3831 | |
---|
3832 | When PNG_DEBUG > 0, the following functions (macros) become available: |
---|
3833 | |
---|
3834 | png_debug(level, message) |
---|
3835 | png_debug1(level, message, p1) |
---|
3836 | png_debug2(level, message, p1, p2) |
---|
3837 | |
---|
3838 | in which "level" is compared to PNG_DEBUG to decide whether to print |
---|
3839 | the message, "message" is the formatted string to be printed, |
---|
3840 | and p1 and p2 are parameters that are to be embedded in the string |
---|
3841 | according to printf-style formatting directives. For example, |
---|
3842 | |
---|
3843 | png_debug1(2, "foo=%d\n", foo); |
---|
3844 | |
---|
3845 | is expanded to |
---|
3846 | |
---|
3847 | if (PNG_DEBUG > 2) |
---|
3848 | fprintf(PNG_DEBUG_FILE, "foo=%d\n", foo); |
---|
3849 | |
---|
3850 | When PNG_DEBUG is defined but is zero, the macros aren't defined, but you |
---|
3851 | can still use PNG_DEBUG to control your own debugging: |
---|
3852 | |
---|
3853 | #ifdef PNG_DEBUG |
---|
3854 | fprintf(stderr, ... |
---|
3855 | #endif |
---|
3856 | |
---|
3857 | When PNG_DEBUG = 1, the macros are defined, but only png_debug statements |
---|
3858 | having level = 0 will be printed. There aren't any such statements in |
---|
3859 | this version of libpng, but if you insert some they will be printed. |
---|
3860 | |
---|
3861 | VI. MNG support |
---|
3862 | |
---|
3863 | The MNG specification (available at http://www.libpng.org/pub/mng) allows |
---|
3864 | certain extensions to PNG for PNG images that are embedded in MNG datastreams. |
---|
3865 | Libpng can support some of these extensions. To enable them, use the |
---|
3866 | png_permit_mng_features() function: |
---|
3867 | |
---|
3868 | feature_set = png_permit_mng_features(png_ptr, mask) |
---|
3869 | |
---|
3870 | mask is a png_uint_32 containing the bitwise OR of the |
---|
3871 | features you want to enable. These include |
---|
3872 | PNG_FLAG_MNG_EMPTY_PLTE |
---|
3873 | PNG_FLAG_MNG_FILTER_64 |
---|
3874 | PNG_ALL_MNG_FEATURES |
---|
3875 | |
---|
3876 | feature_set is a png_uint_32 that is the bitwise AND of |
---|
3877 | your mask with the set of MNG features that is |
---|
3878 | supported by the version of libpng that you are using. |
---|
3879 | |
---|
3880 | It is an error to use this function when reading or writing a standalone |
---|
3881 | PNG file with the PNG 8-byte signature. The PNG datastream must be wrapped |
---|
3882 | in a MNG datastream. As a minimum, it must have the MNG 8-byte signature |
---|
3883 | and the MHDR and MEND chunks. Libpng does not provide support for these |
---|
3884 | or any other MNG chunks; your application must provide its own support for |
---|
3885 | them. You may wish to consider using libmng (available at |
---|
3886 | http://www.libmng.com) instead. |
---|
3887 | |
---|
3888 | VII. Changes to Libpng from version 0.88 |
---|
3889 | |
---|
3890 | It should be noted that versions of libpng later than 0.96 are not |
---|
3891 | distributed by the original libpng author, Guy Schalnat, nor by |
---|
3892 | Andreas Dilger, who had taken over from Guy during 1996 and 1997, and |
---|
3893 | distributed versions 0.89 through 0.96, but rather by another member |
---|
3894 | of the original PNG Group, Glenn Randers-Pehrson. Guy and Andreas are |
---|
3895 | still alive and well, but they have moved on to other things. |
---|
3896 | |
---|
3897 | The old libpng functions png_read_init(), png_write_init(), |
---|
3898 | png_info_init(), png_read_destroy(), and png_write_destroy() have been |
---|
3899 | moved to PNG_INTERNAL in version 0.95 to discourage their use. These |
---|
3900 | functions will be removed from libpng version 1.4.0. |
---|
3901 | |
---|
3902 | The preferred method of creating and initializing the libpng structures is |
---|
3903 | via the png_create_read_struct(), png_create_write_struct(), and |
---|
3904 | png_create_info_struct() because they isolate the size of the structures |
---|
3905 | from the application, allow version error checking, and also allow the |
---|
3906 | use of custom error handling routines during the initialization, which |
---|
3907 | the old functions do not. The functions png_read_destroy() and |
---|
3908 | png_write_destroy() do not actually free the memory that libpng |
---|
3909 | allocated for these structs, but just reset the data structures, so they |
---|
3910 | can be used instead of png_destroy_read_struct() and |
---|
3911 | png_destroy_write_struct() if you feel there is too much system overhead |
---|
3912 | allocating and freeing the png_struct for each image read. |
---|
3913 | |
---|
3914 | Setting the error callbacks via png_set_message_fn() before |
---|
3915 | png_read_init() as was suggested in libpng-0.88 is no longer supported |
---|
3916 | because this caused applications that do not use custom error functions |
---|
3917 | to fail if the png_ptr was not initialized to zero. It is still possible |
---|
3918 | to set the error callbacks AFTER png_read_init(), or to change them with |
---|
3919 | png_set_error_fn(), which is essentially the same function, but with a new |
---|
3920 | name to force compilation errors with applications that try to use the old |
---|
3921 | method. |
---|
3922 | |
---|
3923 | Starting with version 1.0.7, you can find out which version of the library |
---|
3924 | you are using at run-time: |
---|
3925 | |
---|
3926 | png_uint_32 libpng_vn = png_access_version_number(); |
---|
3927 | |
---|
3928 | The number libpng_vn is constructed from the major version, minor |
---|
3929 | version with leading zero, and release number with leading zero, |
---|
3930 | (e.g., libpng_vn for version 1.0.7 is 10007). |
---|
3931 | |
---|
3932 | Note that this function does not take a png_ptr, so you can call it |
---|
3933 | before you've created one. |
---|
3934 | |
---|
3935 | You can also check which version of png.h you used when compiling your |
---|
3936 | application: |
---|
3937 | |
---|
3938 | png_uint_32 application_vn = PNG_LIBPNG_VER; |
---|
3939 | |
---|
3940 | VIII. Changes to Libpng from version 1.0.x to 1.2.x |
---|
3941 | |
---|
3942 | Support for user memory management was enabled by default. To |
---|
3943 | accomplish this, the functions png_create_read_struct_2(), |
---|
3944 | png_create_write_struct_2(), png_set_mem_fn(), png_get_mem_ptr(), |
---|
3945 | png_malloc_default(), and png_free_default() were added. |
---|
3946 | |
---|
3947 | Support for the iTXt chunk has been enabled by default as of |
---|
3948 | version 1.2.41. |
---|
3949 | |
---|
3950 | Support for certain MNG features was enabled. |
---|
3951 | |
---|
3952 | Support for numbered error messages was added. However, we never got |
---|
3953 | around to actually numbering the error messages. The function |
---|
3954 | png_set_strip_error_numbers() was added (Note: the prototype for this |
---|
3955 | function was inadvertently removed from png.h in PNG_NO_ASSEMBLER_CODE |
---|
3956 | builds of libpng-1.2.15. It was restored in libpng-1.2.36). |
---|
3957 | |
---|
3958 | The png_malloc_warn() function was added at libpng-1.2.3. This issues |
---|
3959 | a png_warning and returns NULL instead of aborting when it fails to |
---|
3960 | acquire the requested memory allocation. |
---|
3961 | |
---|
3962 | Support for setting user limits on image width and height was enabled |
---|
3963 | by default. The functions png_set_user_limits(), png_get_user_width_max(), |
---|
3964 | and png_get_user_height_max() were added at libpng-1.2.6. |
---|
3965 | |
---|
3966 | The png_set_add_alpha() function was added at libpng-1.2.7. |
---|
3967 | |
---|
3968 | The function png_set_expand_gray_1_2_4_to_8() was added at libpng-1.2.9. |
---|
3969 | Unlike png_set_gray_1_2_4_to_8(), the new function does not expand the |
---|
3970 | tRNS chunk to alpha. The png_set_gray_1_2_4_to_8() function is |
---|
3971 | deprecated. |
---|
3972 | |
---|
3973 | A number of macro definitions in support of runtime selection of |
---|
3974 | assembler code features (especially Intel MMX code support) were |
---|
3975 | added at libpng-1.2.0: |
---|
3976 | |
---|
3977 | PNG_ASM_FLAG_MMX_SUPPORT_COMPILED |
---|
3978 | PNG_ASM_FLAG_MMX_SUPPORT_IN_CPU |
---|
3979 | PNG_ASM_FLAG_MMX_READ_COMBINE_ROW |
---|
3980 | PNG_ASM_FLAG_MMX_READ_INTERLACE |
---|
3981 | PNG_ASM_FLAG_MMX_READ_FILTER_SUB |
---|
3982 | PNG_ASM_FLAG_MMX_READ_FILTER_UP |
---|
3983 | PNG_ASM_FLAG_MMX_READ_FILTER_AVG |
---|
3984 | PNG_ASM_FLAG_MMX_READ_FILTER_PAETH |
---|
3985 | PNG_ASM_FLAGS_INITIALIZED |
---|
3986 | PNG_MMX_READ_FLAGS |
---|
3987 | PNG_MMX_FLAGS |
---|
3988 | PNG_MMX_WRITE_FLAGS |
---|
3989 | PNG_MMX_FLAGS |
---|
3990 | |
---|
3991 | We added the following functions in support of runtime |
---|
3992 | selection of assembler code features: |
---|
3993 | |
---|
3994 | png_get_mmx_flagmask() |
---|
3995 | png_set_mmx_thresholds() |
---|
3996 | png_get_asm_flags() |
---|
3997 | png_get_mmx_bitdepth_threshold() |
---|
3998 | png_get_mmx_rowbytes_threshold() |
---|
3999 | png_set_asm_flags() |
---|
4000 | |
---|
4001 | We replaced all of these functions with simple stubs in libpng-1.2.20, |
---|
4002 | when the Intel assembler code was removed due to a licensing issue. |
---|
4003 | |
---|
4004 | These macros are deprecated: |
---|
4005 | |
---|
4006 | PNG_READ_TRANSFORMS_NOT_SUPPORTED |
---|
4007 | PNG_PROGRESSIVE_READ_NOT_SUPPORTED |
---|
4008 | PNG_NO_SEQUENTIAL_READ_SUPPORTED |
---|
4009 | PNG_WRITE_TRANSFORMS_NOT_SUPPORTED |
---|
4010 | PNG_READ_ANCILLARY_CHUNKS_NOT_SUPPORTED |
---|
4011 | PNG_WRITE_ANCILLARY_CHUNKS_NOT_SUPPORTED |
---|
4012 | |
---|
4013 | They have been replaced, respectively, by: |
---|
4014 | |
---|
4015 | PNG_NO_READ_TRANSFORMS |
---|
4016 | PNG_NO_PROGRESSIVE_READ |
---|
4017 | PNG_NO_SEQUENTIAL_READ |
---|
4018 | PNG_NO_WRITE_TRANSFORMS |
---|
4019 | PNG_NO_READ_ANCILLARY_CHUNKS |
---|
4020 | PNG_NO_WRITE_ANCILLARY_CHUNKS |
---|
4021 | |
---|
4022 | PNG_MAX_UINT was replaced with PNG_UINT_31_MAX. It has been |
---|
4023 | deprecated since libpng-1.0.16 and libpng-1.2.6. |
---|
4024 | |
---|
4025 | The function |
---|
4026 | png_check_sig(sig, num) |
---|
4027 | was replaced with |
---|
4028 | !png_sig_cmp(sig, 0, num) |
---|
4029 | It has been deprecated since libpng-0.90. |
---|
4030 | |
---|
4031 | The function |
---|
4032 | png_set_gray_1_2_4_to_8() |
---|
4033 | which also expands tRNS to alpha was replaced with |
---|
4034 | png_set_expand_gray_1_2_4_to_8() |
---|
4035 | which does not. It has been deprecated since libpng-1.0.18 and 1.2.9. |
---|
4036 | |
---|
4037 | IX. Changes to Libpng from version 1.0.x/1.2.x to 1.4.x |
---|
4038 | |
---|
4039 | Private libpng prototypes and macro definitions were moved from |
---|
4040 | png.h and pngconf.h into a new pngpriv.h header file. |
---|
4041 | |
---|
4042 | Functions png_set_benign_errors(), png_benign_error(), and |
---|
4043 | png_chunk_benign_error() were added. |
---|
4044 | |
---|
4045 | Support for setting the maximum amount of memory that the application |
---|
4046 | will allocate for reading chunks was added, as a security measure. |
---|
4047 | The functions png_set_chunk_cache_max() and png_get_chunk_cache_max() |
---|
4048 | were added to the library. |
---|
4049 | |
---|
4050 | We implemented support for I/O states by adding png_ptr member io_state |
---|
4051 | and functions png_get_io_chunk_name() and png_get_io_state() in pngget.c |
---|
4052 | |
---|
4053 | We added PNG_TRANSFORM_GRAY_TO_RGB to the available high-level |
---|
4054 | input transforms. |
---|
4055 | |
---|
4056 | Checking for and reporting of errors in the IHDR chunk is more thorough. |
---|
4057 | |
---|
4058 | Support for global arrays was removed, to improve thread safety. |
---|
4059 | |
---|
4060 | Some obsolete/deprecated macros and functions have been removed. |
---|
4061 | |
---|
4062 | Typecasted NULL definitions such as |
---|
4063 | #define png_voidp_NULL (png_voidp)NULL |
---|
4064 | were eliminated. If you used these in your application, just use |
---|
4065 | NULL instead. |
---|
4066 | |
---|
4067 | The png_struct and info_struct members "trans" and "trans_values" were |
---|
4068 | changed to "trans_alpha" and "trans_color", respectively. |
---|
4069 | |
---|
4070 | The obsolete, unused pnggccrd.c and pngvcrd.c files and related makefiles |
---|
4071 | were removed. |
---|
4072 | |
---|
4073 | The PNG_1_0_X and PNG_1_2_X macros were eliminated. |
---|
4074 | |
---|
4075 | The PNG_LEGACY_SUPPORTED macro was eliminated. |
---|
4076 | |
---|
4077 | Many WIN32_WCE #ifdefs were removed. |
---|
4078 | |
---|
4079 | The functions png_read_init(info_ptr), png_write_init(info_ptr), |
---|
4080 | png_info_init(info_ptr), png_read_destroy(), and png_write_destroy() |
---|
4081 | have been removed. They have been deprecated since libpng-0.95. |
---|
4082 | |
---|
4083 | The png_permit_empty_plte() was removed. It has been deprecated |
---|
4084 | since libpng-1.0.9. Use png_permit_mng_features() instead. |
---|
4085 | |
---|
4086 | We removed the obsolete stub functions png_get_mmx_flagmask(), |
---|
4087 | png_set_mmx_thresholds(), png_get_asm_flags(), |
---|
4088 | png_get_mmx_bitdepth_threshold(), png_get_mmx_rowbytes_threshold(), |
---|
4089 | png_set_asm_flags(), and png_mmx_supported() |
---|
4090 | |
---|
4091 | We removed the obsolete png_check_sig(), png_memcpy_check(), and |
---|
4092 | png_memset_check() functions. Instead use !png_sig_cmp(), memcpy(), |
---|
4093 | and memset(), respectively. |
---|
4094 | |
---|
4095 | The function png_set_gray_1_2_4_to_8() was removed. It has been |
---|
4096 | deprecated since libpng-1.0.18 and 1.2.9, when it was replaced with |
---|
4097 | png_set_expand_gray_1_2_4_to_8() because the former function also |
---|
4098 | expanded any tRNS chunk to an alpha channel. |
---|
4099 | |
---|
4100 | Macros for png_get_uint_16, png_get_uint_32, and png_get_int_32 |
---|
4101 | were added and are used by default instead of the corresponding |
---|
4102 | functions. Unfortunately, |
---|
4103 | from libpng-1.4.0 until 1.4.4, the png_get_uint_16 macro (but not the |
---|
4104 | function) incorrectly returned a value of type png_uint_32. |
---|
4105 | |
---|
4106 | We changed the prototype for png_malloc() from |
---|
4107 | png_malloc(png_structp png_ptr, png_uint_32 size) |
---|
4108 | to |
---|
4109 | png_malloc(png_structp png_ptr, png_alloc_size_t size) |
---|
4110 | |
---|
4111 | This also applies to the prototype for the user replacement malloc_fn(). |
---|
4112 | |
---|
4113 | The png_calloc() function was added and is used in place of |
---|
4114 | of "png_malloc(); memset();" except in the case in png_read_png() |
---|
4115 | where the array consists of pointers; in this case a "for" loop is used |
---|
4116 | after the png_malloc() to set the pointers to NULL, to give robust. |
---|
4117 | behavior in case the application runs out of memory part-way through |
---|
4118 | the process. |
---|
4119 | |
---|
4120 | We changed the prototypes of png_get_compression_buffer_size() and |
---|
4121 | png_set_compression_buffer_size() to work with png_size_t instead of |
---|
4122 | png_uint_32. |
---|
4123 | |
---|
4124 | Support for numbered error messages was removed by default, since we |
---|
4125 | never got around to actually numbering the error messages. The function |
---|
4126 | png_set_strip_error_numbers() was removed from the library by default. |
---|
4127 | |
---|
4128 | The png_zalloc() and png_zfree() functions are no longer exported. |
---|
4129 | The png_zalloc() function no longer zeroes out the memory that it |
---|
4130 | allocates. |
---|
4131 | |
---|
4132 | Support for dithering was disabled by default in libpng-1.4.0, because |
---|
4133 | it has not been well tested and doesn't actually "dither". |
---|
4134 | The code was not |
---|
4135 | removed, however, and could be enabled by building libpng with |
---|
4136 | PNG_READ_DITHER_SUPPORTED defined. In libpng-1.4.2, this support |
---|
4137 | was reenabled, but the function was renamed png_set_quantize() to |
---|
4138 | reflect more accurately what it actually does. At the same time, |
---|
4139 | the PNG_DITHER_[RED,GREEN_BLUE]_BITS macros were also renamed to |
---|
4140 | PNG_QUANTIZE_[RED,GREEN,BLUE]_BITS, and PNG_READ_DITHER_SUPPORTED |
---|
4141 | was renamed to PNG_READ_QUANTIZE_SUPPORTED. |
---|
4142 | |
---|
4143 | We removed the trailing '.' from the warning and error messages. |
---|
4144 | |
---|
4145 | X. Changes to Libpng from version 1.4.x to 1.5.x |
---|
4146 | |
---|
4147 | From libpng-1.4.0 until 1.4.4, the png_get_uint_16 macro (but not the |
---|
4148 | function) incorrectly returned a value of type png_uint_32. |
---|
4149 | |
---|
4150 | Checking for invalid palette index on read or write was added at libpng |
---|
4151 | 1.5.10. When an invalid index is found, libpng issues a benign error. |
---|
4152 | This is enabled by default but can be disabled in each png_ptr with |
---|
4153 | |
---|
4154 | png_set_check_for_invalid_index(png_ptr, allowed); |
---|
4155 | |
---|
4156 | allowed - one of |
---|
4157 | 0: disable |
---|
4158 | 1: enable |
---|
4159 | |
---|
4160 | A. Changes that affect users of libpng |
---|
4161 | |
---|
4162 | There are no substantial API changes between the non-deprecated parts of |
---|
4163 | the 1.4.5 API and the 1.5.0 API; however, the ability to directly access |
---|
4164 | the main libpng control structures, png_struct and png_info, deprecated |
---|
4165 | in earlier versions of libpng, has been completely removed from |
---|
4166 | libpng 1.5. |
---|
4167 | |
---|
4168 | We no longer include zlib.h in png.h. Applications that need access |
---|
4169 | to information in zlib.h will need to add the '#include "zlib.h"' |
---|
4170 | directive. It does not matter whether it is placed prior to or after |
---|
4171 | the '"#include png.h"' directive. |
---|
4172 | |
---|
4173 | We moved the png_strcpy(), png_strncpy(), png_strlen(), png_memcpy(), |
---|
4174 | png_memcmp(), png_sprintf, and png_memcpy() macros into a private |
---|
4175 | header file (pngpriv.h) that is not accessible to applications. |
---|
4176 | |
---|
4177 | In png_get_iCCP, the type of "profile" was changed from png_charpp |
---|
4178 | to png_bytepp, and in png_set_iCCP, from png_charp to png_const_bytep. |
---|
4179 | |
---|
4180 | There are changes of form in png.h, including new and changed macros to |
---|
4181 | declare parts of the API. Some API functions with arguments that are |
---|
4182 | pointers to data not modified within the function have been corrected to |
---|
4183 | declare these arguments with PNG_CONST. |
---|
4184 | |
---|
4185 | Much of the internal use of C macros to control the library build has also |
---|
4186 | changed and some of this is visible in the exported header files, in |
---|
4187 | particular the use of macros to control data and API elements visible |
---|
4188 | during application compilation may require significant revision to |
---|
4189 | application code. (It is extremely rare for an application to do this.) |
---|
4190 | |
---|
4191 | Any program that compiled against libpng 1.4 and did not use deprecated |
---|
4192 | features or access internal library structures should compile and work |
---|
4193 | against libpng 1.5, except for the change in the prototype for |
---|
4194 | png_get_iCCP() and png_set_iCCP() API functions mentioned above. |
---|
4195 | |
---|
4196 | libpng 1.5.0 adds PNG_ PASS macros to help in the reading and writing of |
---|
4197 | interlaced images. The macros return the number of rows and columns in |
---|
4198 | each pass and information that can be used to de-interlace and (if |
---|
4199 | absolutely necessary) interlace an image. |
---|
4200 | |
---|
4201 | libpng 1.5.0 adds an API png_longjmp(png_ptr, value). This API calls |
---|
4202 | the application-provided png_longjmp_ptr on the internal, but application |
---|
4203 | initialized, longjmp buffer. It is provided as a convenience to avoid |
---|
4204 | the need to use the png_jmpbuf macro, which had the unnecessary side |
---|
4205 | effect of resetting the internal png_longjmp_ptr value. |
---|
4206 | |
---|
4207 | libpng 1.5.0 includes a complete fixed point API. By default this is |
---|
4208 | present along with the corresponding floating point API. In general the |
---|
4209 | fixed point API is faster and smaller than the floating point one because |
---|
4210 | the PNG file format used fixed point, not floating point. This applies |
---|
4211 | even if the library uses floating point in internal calculations. A new |
---|
4212 | macro, PNG_FLOATING_ARITHMETIC_SUPPORTED, reveals whether the library |
---|
4213 | uses floating point arithmetic (the default) or fixed point arithmetic |
---|
4214 | internally for performance critical calculations such as gamma correction. |
---|
4215 | In some cases, the gamma calculations may produce slightly different |
---|
4216 | results. This has changed the results in png_rgb_to_gray and in alpha |
---|
4217 | composition (png_set_background for example). This applies even if the |
---|
4218 | original image was already linear (gamma == 1.0) and, therefore, it is |
---|
4219 | not necessary to linearize the image. This is because libpng has *not* |
---|
4220 | been changed to optimize that case correctly, yet. |
---|
4221 | |
---|
4222 | Fixed point support for the sCAL chunk comes with an important caveat; |
---|
4223 | the sCAL specification uses a decimal encoding of floating point values |
---|
4224 | and the accuracy of PNG fixed point values is insufficient for |
---|
4225 | representation of these values. Consequently a "string" API |
---|
4226 | (png_get_sCAL_s and png_set_sCAL_s) is the only reliable way of reading |
---|
4227 | arbitrary sCAL chunks in the absence of either the floating point API or |
---|
4228 | internal floating point calculations. |
---|
4229 | |
---|
4230 | Applications no longer need to include the optional distribution header |
---|
4231 | file pngusr.h or define the corresponding macros during application |
---|
4232 | build in order to see the correct variant of the libpng API. From 1.5.0 |
---|
4233 | application code can check for the corresponding _SUPPORTED macro: |
---|
4234 | |
---|
4235 | #ifdef PNG_INCH_CONVERSIONS_SUPPORTED |
---|
4236 | /* code that uses the inch conversion APIs. */ |
---|
4237 | #endif |
---|
4238 | |
---|
4239 | This macro will only be defined if the inch conversion functions have been |
---|
4240 | compiled into libpng. The full set of macros, and whether or not support |
---|
4241 | has been compiled in, are available in the header file pnglibconf.h. |
---|
4242 | This header file is specific to the libpng build. Notice that prior to |
---|
4243 | 1.5.0 the _SUPPORTED macros would always have the default definition unless |
---|
4244 | reset by pngusr.h or by explicit settings on the compiler command line. |
---|
4245 | These settings may produce compiler warnings or errors in 1.5.0 because |
---|
4246 | of macro redefinition. |
---|
4247 | |
---|
4248 | From libpng-1.4.0 until 1.4.4, the png_get_uint_16 macro (but not the |
---|
4249 | function) incorrectly returned a value of type png_uint_32. libpng 1.5.0 |
---|
4250 | is consistent with the implementation in 1.4.5 and 1.2.x (where the macro |
---|
4251 | did not exist.) |
---|
4252 | |
---|
4253 | Applications can now choose whether to use these macros or to call the |
---|
4254 | corresponding function by defining PNG_USE_READ_MACROS or |
---|
4255 | PNG_NO_USE_READ_MACROS before including png.h. Notice that this is |
---|
4256 | only supported from 1.5.0 -defining PNG_NO_USE_READ_MACROS prior to 1.5.0 |
---|
4257 | will lead to a link failure. |
---|
4258 | |
---|
4259 | Prior to libpng-1.5.4, the zlib compressor used the same set of parameters |
---|
4260 | when compressing the IDAT data and textual data such as zTXt and iCCP. |
---|
4261 | In libpng-1.5.4 we reinitialized the zlib stream for each type of data. |
---|
4262 | We added five png_set_text_*() functions for setting the parameters to |
---|
4263 | use with textual data. |
---|
4264 | |
---|
4265 | Prior to libpng-1.5.4, the PNG_READ_16_TO_8_ACCURATE_SCALE_SUPPORTED |
---|
4266 | option was off by default, and slightly inaccurate scaling occurred. |
---|
4267 | This option can no longer be turned off, and the choice of accurate |
---|
4268 | or inaccurate 16-to-8 scaling is by using the new png_set_scale_16_to_8() |
---|
4269 | API for accurate scaling or the old png_set_strip_16_to_8() API for simple |
---|
4270 | chopping. |
---|
4271 | |
---|
4272 | Prior to libpng-1.5.4, the png_set_user_limits() function could only be |
---|
4273 | used to reduce the width and height limits from the value of |
---|
4274 | PNG_USER_WIDTH_MAX and PNG_USER_HEIGHT_MAX, although this document said |
---|
4275 | that it could be used to override them. Now this function will reduce or |
---|
4276 | increase the limits. |
---|
4277 | |
---|
4278 | Starting in libpng-1.5.10, the user limits can be set en masse with the |
---|
4279 | configuration option PNG_SAFE_LIMITS_SUPPORTED. If this option is enabled, |
---|
4280 | a set of "safe" limits is applied in pngpriv.h. These can be overridden by |
---|
4281 | application calls to png_set_user_limits(), png_set_user_chunk_cache_max(), |
---|
4282 | and/or png_set_user_malloc_max() that increase or decrease the limits. Also, |
---|
4283 | in libpng-1.5.10 the default width and height limits were increased |
---|
4284 | from 1,000,000 to 0x7ffffff (i.e., made unlimited). Therefore, the |
---|
4285 | limits are now |
---|
4286 | default safe |
---|
4287 | png_user_width_max 0x7fffffff 1,000,000 |
---|
4288 | png_user_height_max 0x7fffffff 1,000,000 |
---|
4289 | png_user_chunk_cache_max 0 (unlimited) 128 |
---|
4290 | png_user_chunk_malloc_max 0 (unlimited) 8,000,000 |
---|
4291 | |
---|
4292 | B. Changes to the build and configuration of libpng |
---|
4293 | |
---|
4294 | Details of internal changes to the library code can be found in the CHANGES |
---|
4295 | file and in the GIT repository logs. These will be of no concern to the vast |
---|
4296 | majority of library users or builders; however, the few who configure libpng |
---|
4297 | to a non-default feature set may need to change how this is done. |
---|
4298 | |
---|
4299 | There should be no need for library builders to alter build scripts if |
---|
4300 | these use the distributed build support - configure or the makefiles - |
---|
4301 | however, users of the makefiles may care to update their build scripts |
---|
4302 | to build pnglibconf.h where the corresponding makefile does not do so. |
---|
4303 | |
---|
4304 | Building libpng with a non-default configuration has changed completely. |
---|
4305 | The old method using pngusr.h should still work correctly even though the |
---|
4306 | way pngusr.h is used in the build has been changed; however, library |
---|
4307 | builders will probably want to examine the changes to take advantage of |
---|
4308 | new capabilities and to simplify their build system. |
---|
4309 | |
---|
4310 | B.1 Specific changes to library configuration capabilities |
---|
4311 | |
---|
4312 | The library now supports a complete fixed point implementation and can |
---|
4313 | thus be used on systems that have no floating point support or very |
---|
4314 | limited or slow support. Previously gamma correction, an essential part |
---|
4315 | of complete PNG support, required reasonably fast floating point. |
---|
4316 | |
---|
4317 | As part of this the choice of internal implementation has been made |
---|
4318 | independent of the choice of fixed versus floating point APIs and all the |
---|
4319 | missing fixed point APIs have been implemented. |
---|
4320 | |
---|
4321 | The exact mechanism used to control attributes of API functions has |
---|
4322 | changed. A single set of operating system independent macro definitions |
---|
4323 | is used and operating system specific directives are defined in |
---|
4324 | pnglibconf.h |
---|
4325 | |
---|
4326 | As part of this the mechanism used to choose procedure call standards on |
---|
4327 | those systems that allow a choice has been changed. At present this only |
---|
4328 | affects certain Microsoft (DOS, Windows) and IBM (OS/2) operating systems |
---|
4329 | running on Intel processors. As before, PNGAPI is defined where required |
---|
4330 | to control the exported API functions; however, two new macros, PNGCBAPI |
---|
4331 | and PNGCAPI, are used instead for callback functions (PNGCBAPI) and |
---|
4332 | (PNGCAPI) for functions that must match a C library prototype (currently |
---|
4333 | only png_longjmp_ptr, which must match the C longjmp function.) The new |
---|
4334 | approach is documented in pngconf.h |
---|
4335 | |
---|
4336 | Despite these changes, libpng 1.5.0 only supports the native C function |
---|
4337 | calling standard on those platforms tested so far (__cdecl on Microsoft |
---|
4338 | Windows). This is because the support requirements for alternative |
---|
4339 | calling conventions seem to no longer exist. Developers who find it |
---|
4340 | necessary to set PNG_API_RULE to 1 should advise the mailing list |
---|
4341 | (png-mng-implement) of this and library builders who use Openwatcom and |
---|
4342 | therefore set PNG_API_RULE to 2 should also contact the mailing list. |
---|
4343 | |
---|
4344 | A new test program, pngvalid, is provided in addition to pngtest. |
---|
4345 | pngvalid validates the arithmetic accuracy of the gamma correction |
---|
4346 | calculations and includes a number of validations of the file format. |
---|
4347 | A subset of the full range of tests is run when "make check" is done |
---|
4348 | (in the 'configure' build.) pngvalid also allows total allocated memory |
---|
4349 | usage to be evaluated and performs additional memory overwrite validation. |
---|
4350 | |
---|
4351 | Many changes to individual feature macros have been made. The following |
---|
4352 | are the changes most likely to be noticed by library builders who |
---|
4353 | configure libpng: |
---|
4354 | |
---|
4355 | 1) All feature macros now have consistent naming: |
---|
4356 | |
---|
4357 | #define PNG_NO_feature turns the feature off |
---|
4358 | #define PNG_feature_SUPPORTED turns the feature on |
---|
4359 | |
---|
4360 | pnglibconf.h contains one line for each feature macro which is either: |
---|
4361 | |
---|
4362 | #define PNG_feature_SUPPORTED |
---|
4363 | |
---|
4364 | if the feature is supported or: |
---|
4365 | |
---|
4366 | /*#undef PNG_feature_SUPPORTED*/ |
---|
4367 | |
---|
4368 | if it is not. Library code consistently checks for the 'SUPPORTED' macro. |
---|
4369 | It does not, and libpng applications should not, check for the 'NO' macro |
---|
4370 | which will not normally be defined even if the feature is not supported. |
---|
4371 | The 'NO' macros are only used internally for setting or not setting the |
---|
4372 | corresponding 'SUPPORTED' macros. |
---|
4373 | |
---|
4374 | Compatibility with the old names is provided as follows: |
---|
4375 | |
---|
4376 | PNG_INCH_CONVERSIONS turns on PNG_INCH_CONVERSIONS_SUPPORTED |
---|
4377 | |
---|
4378 | And the following definitions disable the corresponding feature: |
---|
4379 | |
---|
4380 | PNG_SETJMP_NOT_SUPPORTED disables SETJMP |
---|
4381 | PNG_READ_TRANSFORMS_NOT_SUPPORTED disables READ_TRANSFORMS |
---|
4382 | PNG_NO_READ_COMPOSITED_NODIV disables READ_COMPOSITE_NODIV |
---|
4383 | PNG_WRITE_TRANSFORMS_NOT_SUPPORTED disables WRITE_TRANSFORMS |
---|
4384 | PNG_READ_ANCILLARY_CHUNKS_NOT_SUPPORTED disables READ_ANCILLARY_CHUNKS |
---|
4385 | PNG_WRITE_ANCILLARY_CHUNKS_NOT_SUPPORTED disables WRITE_ANCILLARY_CHUNKS |
---|
4386 | |
---|
4387 | Library builders should remove use of the above, inconsistent, names. |
---|
4388 | |
---|
4389 | 2) Warning and error message formatting was previously conditional on |
---|
4390 | the STDIO feature. The library has been changed to use the |
---|
4391 | CONSOLE_IO feature instead. This means that if CONSOLE_IO is disabled |
---|
4392 | the library no longer uses the printf(3) functions, even though the |
---|
4393 | default read/write implementations use (FILE) style stdio.h functions. |
---|
4394 | |
---|
4395 | 3) Three feature macros now control the fixed/floating point decisions: |
---|
4396 | |
---|
4397 | PNG_FLOATING_POINT_SUPPORTED enables the floating point APIs |
---|
4398 | |
---|
4399 | PNG_FIXED_POINT_SUPPORTED enables the fixed point APIs; however, in |
---|
4400 | practice these are normally required internally anyway (because the PNG |
---|
4401 | file format is fixed point), therefore in most cases PNG_NO_FIXED_POINT |
---|
4402 | merely stops the function from being exported. |
---|
4403 | |
---|
4404 | PNG_FLOATING_ARITHMETIC_SUPPORTED chooses between the internal floating |
---|
4405 | point implementation or the fixed point one. Typically the fixed point |
---|
4406 | implementation is larger and slower than the floating point implementation |
---|
4407 | on a system that supports floating point; however, it may be faster on a |
---|
4408 | system which lacks floating point hardware and therefore uses a software |
---|
4409 | emulation. |
---|
4410 | |
---|
4411 | 4) Added PNG_{READ,WRITE}_INT_FUNCTIONS_SUPPORTED. This allows the |
---|
4412 | functions to read and write ints to be disabled independently of |
---|
4413 | PNG_USE_READ_MACROS, which allows libpng to be built with the functions |
---|
4414 | even though the default is to use the macros - this allows applications |
---|
4415 | to choose at app buildtime whether or not to use macros (previously |
---|
4416 | impossible because the functions weren't in the default build.) |
---|
4417 | |
---|
4418 | B.2 Changes to the configuration mechanism |
---|
4419 | |
---|
4420 | Prior to libpng-1.5.0 library builders who needed to configure libpng |
---|
4421 | had either to modify the exported pngconf.h header file to add system |
---|
4422 | specific configuration or had to write feature selection macros into |
---|
4423 | pngusr.h and cause this to be included into pngconf.h by defining |
---|
4424 | PNG_USER_CONFIG. The latter mechanism had the disadvantage that an |
---|
4425 | application built without PNG_USER_CONFIG defined would see the |
---|
4426 | unmodified, default, libpng API and thus would probably fail to link. |
---|
4427 | |
---|
4428 | These mechanisms still work in the configure build and in any makefile |
---|
4429 | build that builds pnglibconf.h, although the feature selection macros |
---|
4430 | have changed somewhat as described above. In 1.5.0, however, pngusr.h is |
---|
4431 | processed only once, when the exported header file pnglibconf.h is built. |
---|
4432 | pngconf.h no longer includes pngusr.h, therefore pngusr.h is ignored after the |
---|
4433 | build of pnglibconf.h and it is never included in an application build. |
---|
4434 | |
---|
4435 | The rarely used alternative of adding a list of feature macros to the |
---|
4436 | CFLAGS setting in the build also still works; however, the macros will be |
---|
4437 | copied to pnglibconf.h and this may produce macro redefinition warnings |
---|
4438 | when the individual C files are compiled. |
---|
4439 | |
---|
4440 | All configuration now only works if pnglibconf.h is built from |
---|
4441 | scripts/pnglibconf.dfa. This requires the program awk. Brian Kernighan |
---|
4442 | (the original author of awk) maintains C source code of that awk and this |
---|
4443 | and all known later implementations (often called by subtly different |
---|
4444 | names - nawk and gawk for example) are adequate to build pnglibconf.h. |
---|
4445 | The Sun Microsystems (now Oracle) program 'awk' is an earlier version |
---|
4446 | and does not work; this may also apply to other systems that have a |
---|
4447 | functioning awk called 'nawk'. |
---|
4448 | |
---|
4449 | Configuration options are now documented in scripts/pnglibconf.dfa. This |
---|
4450 | file also includes dependency information that ensures a configuration is |
---|
4451 | consistent; that is, if a feature is switched off dependent features are |
---|
4452 | also removed. As a recommended alternative to using feature macros in |
---|
4453 | pngusr.h a system builder may also define equivalent options in pngusr.dfa |
---|
4454 | (or, indeed, any file) and add that to the configuration by setting |
---|
4455 | DFA_XTRA to the file name. The makefiles in contrib/pngminim illustrate |
---|
4456 | how to do this, and a case where pngusr.h is still required. |
---|
4457 | |
---|
4458 | XI. Detecting libpng |
---|
4459 | |
---|
4460 | The png_get_io_ptr() function has been present since libpng-0.88, has never |
---|
4461 | changed, and is unaffected by conditional compilation macros. It is the |
---|
4462 | best choice for use in configure scripts for detecting the presence of any |
---|
4463 | libpng version since 0.88. In an autoconf "configure.in" you could use |
---|
4464 | |
---|
4465 | AC_CHECK_LIB(png, png_get_io_ptr, ... |
---|
4466 | |
---|
4467 | XII. Source code repository |
---|
4468 | |
---|
4469 | Since about February 2009, version 1.2.34, libpng has been under "git" source |
---|
4470 | control. The git repository was built from old libpng-x.y.z.tar.gz files |
---|
4471 | going back to version 0.70. You can access the git repository (read only) |
---|
4472 | at |
---|
4473 | |
---|
4474 | git://libpng.git.sourceforge.net/gitroot/libpng |
---|
4475 | |
---|
4476 | or you can browse it via "gitweb" at |
---|
4477 | |
---|
4478 | http://libpng.git.sourceforge.net/git/gitweb.cgi?p=libpng |
---|
4479 | |
---|
4480 | Patches can be sent to glennrp at users.sourceforge.net or to |
---|
4481 | png-mng-implement at lists.sourceforge.net or you can upload them to |
---|
4482 | the libpng bug tracker at |
---|
4483 | |
---|
4484 | http://libpng.sourceforge.net |
---|
4485 | |
---|
4486 | We also accept patches built from the tar or zip distributions, and |
---|
4487 | simple verbal discriptions of bug fixes, reported either to the |
---|
4488 | SourceForge bug tracker, to the png-mng-implement at lists.sf.net |
---|
4489 | mailing list, or directly to glennrp. |
---|
4490 | |
---|
4491 | XIII. Coding style |
---|
4492 | |
---|
4493 | Our coding style is similar to the "Allman" style, with curly |
---|
4494 | braces on separate lines: |
---|
4495 | |
---|
4496 | if (condition) |
---|
4497 | { |
---|
4498 | action; |
---|
4499 | } |
---|
4500 | |
---|
4501 | else if (another condition) |
---|
4502 | { |
---|
4503 | another action; |
---|
4504 | } |
---|
4505 | |
---|
4506 | The braces can be omitted from simple one-line actions: |
---|
4507 | |
---|
4508 | if (condition) |
---|
4509 | return (0); |
---|
4510 | |
---|
4511 | We use 3-space indentation, except for continued statements which |
---|
4512 | are usually indented the same as the first line of the statement |
---|
4513 | plus four more spaces. |
---|
4514 | |
---|
4515 | For macro definitions we use 2-space indentation, always leaving the "#" |
---|
4516 | in the first column. |
---|
4517 | |
---|
4518 | #ifndef PNG_NO_FEATURE |
---|
4519 | # ifndef PNG_FEATURE_SUPPORTED |
---|
4520 | # define PNG_FEATURE_SUPPORTED |
---|
4521 | # endif |
---|
4522 | #endif |
---|
4523 | |
---|
4524 | Comments appear with the leading "/*" at the same indentation as |
---|
4525 | the statement that follows the comment: |
---|
4526 | |
---|
4527 | /* Single-line comment */ |
---|
4528 | statement; |
---|
4529 | |
---|
4530 | /* This is a multiple-line |
---|
4531 | * comment. |
---|
4532 | */ |
---|
4533 | statement; |
---|
4534 | |
---|
4535 | Very short comments can be placed after the end of the statement |
---|
4536 | to which they pertain: |
---|
4537 | |
---|
4538 | statement; /* comment */ |
---|
4539 | |
---|
4540 | We don't use C++ style ("//") comments. We have, however, |
---|
4541 | used them in the past in some now-abandoned MMX assembler |
---|
4542 | code. |
---|
4543 | |
---|
4544 | Functions and their curly braces are not indented, and |
---|
4545 | exported functions are marked with PNGAPI: |
---|
4546 | |
---|
4547 | /* This is a public function that is visible to |
---|
4548 | * application programmers. It does thus-and-so. |
---|
4549 | */ |
---|
4550 | void PNGAPI |
---|
4551 | png_exported_function(png_ptr, png_info, foo) |
---|
4552 | { |
---|
4553 | body; |
---|
4554 | } |
---|
4555 | |
---|
4556 | The prototypes for all exported functions appear in png.h, |
---|
4557 | above the comment that says |
---|
4558 | |
---|
4559 | /* Maintainer: Put new public prototypes here ... */ |
---|
4560 | |
---|
4561 | We mark all non-exported functions with "/* PRIVATE */"": |
---|
4562 | |
---|
4563 | void /* PRIVATE */ |
---|
4564 | png_non_exported_function(png_ptr, png_info, foo) |
---|
4565 | { |
---|
4566 | body; |
---|
4567 | } |
---|
4568 | |
---|
4569 | The prototypes for non-exported functions (except for those in |
---|
4570 | pngtest) appear in |
---|
4571 | pngpriv.h |
---|
4572 | above the comment that says |
---|
4573 | |
---|
4574 | /* Maintainer: Put new private prototypes here ^ and in libpngpf.3 */ |
---|
4575 | |
---|
4576 | To avoid polluting the global namespace, the names of all exported |
---|
4577 | functions and variables begin with "png_", and all publicly visible C |
---|
4578 | preprocessor macros begin with "PNG". We request that applications that |
---|
4579 | use libpng *not* begin any of their own symbols with either of these strings. |
---|
4580 | |
---|
4581 | We put a space after each comma and after each semicolon |
---|
4582 | in "for" statements, and we put spaces before and after each |
---|
4583 | C binary operator and after "for" or "while", and before |
---|
4584 | "?". We don't put a space between a typecast and the expression |
---|
4585 | being cast, nor do we put one between a function name and the |
---|
4586 | left parenthesis that follows it: |
---|
4587 | |
---|
4588 | for (i = 2; i > 0; --i) |
---|
4589 | y[i] = a(x) + (int)b; |
---|
4590 | |
---|
4591 | We prefer #ifdef and #ifndef to #if defined() and if !defined() |
---|
4592 | when there is only one macro being tested. |
---|
4593 | |
---|
4594 | We prefer to express integers that are used as bit masks in hex format, |
---|
4595 | with an even number of lower-case hex digits (e.g., 0x00, 0xff, 0x0100). |
---|
4596 | |
---|
4597 | We do not use the TAB character for indentation in the C sources. |
---|
4598 | |
---|
4599 | Lines do not exceed 80 characters. |
---|
4600 | |
---|
4601 | Other rules can be inferred by inspecting the libpng source. |
---|
4602 | |
---|
4603 | XIV. Y2K Compliance in libpng |
---|
4604 | |
---|
4605 | July 11, 2012 |
---|
4606 | |
---|
4607 | Since the PNG Development group is an ad-hoc body, we can't make |
---|
4608 | an official declaration. |
---|
4609 | |
---|
4610 | This is your unofficial assurance that libpng from version 0.71 and |
---|
4611 | upward through 1.5.12 are Y2K compliant. It is my belief that earlier |
---|
4612 | versions were also Y2K compliant. |
---|
4613 | |
---|
4614 | Libpng only has two year fields. One is a 2-byte unsigned integer that |
---|
4615 | will hold years up to 65535. The other holds the date in text |
---|
4616 | format, and will hold years up to 9999. |
---|
4617 | |
---|
4618 | The integer is |
---|
4619 | "png_uint_16 year" in png_time_struct. |
---|
4620 | |
---|
4621 | The string is |
---|
4622 | "char time_buffer[29]" in png_struct. This will no |
---|
4623 | longer be used in libpng-1.6.x and will be removed from libpng-1.7.0. |
---|
4624 | |
---|
4625 | There are seven time-related functions: |
---|
4626 | |
---|
4627 | png_convert_to_rfc_1123() in png.c |
---|
4628 | (formerly png_convert_to_rfc_1152() in error) |
---|
4629 | png_convert_from_struct_tm() in pngwrite.c, called |
---|
4630 | in pngwrite.c |
---|
4631 | png_convert_from_time_t() in pngwrite.c |
---|
4632 | png_get_tIME() in pngget.c |
---|
4633 | png_handle_tIME() in pngrutil.c, called in pngread.c |
---|
4634 | png_set_tIME() in pngset.c |
---|
4635 | png_write_tIME() in pngwutil.c, called in pngwrite.c |
---|
4636 | |
---|
4637 | All appear to handle dates properly in a Y2K environment. The |
---|
4638 | png_convert_from_time_t() function calls gmtime() to convert from system |
---|
4639 | clock time, which returns (year - 1900), which we properly convert to |
---|
4640 | the full 4-digit year. There is a possibility that applications using |
---|
4641 | libpng are not passing 4-digit years into the png_convert_to_rfc_1123() |
---|
4642 | function, or that they are incorrectly passing only a 2-digit year |
---|
4643 | instead of "year - 1900" into the png_convert_from_struct_tm() function, |
---|
4644 | but this is not under our control. The libpng documentation has always |
---|
4645 | stated that it works with 4-digit years, and the APIs have been |
---|
4646 | documented as such. |
---|
4647 | |
---|
4648 | The tIME chunk itself is also Y2K compliant. It uses a 2-byte unsigned |
---|
4649 | integer to hold the year, and can hold years as large as 65535. |
---|
4650 | |
---|
4651 | zlib, upon which libpng depends, is also Y2K compliant. It contains |
---|
4652 | no date-related code. |
---|
4653 | |
---|
4654 | |
---|
4655 | Glenn Randers-Pehrson |
---|
4656 | libpng maintainer |
---|
4657 | PNG Development Group |
---|