1 | .TH DJPEG 1 "28 March 2009" |
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2 | .SH NAME |
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3 | djpeg \- decompress a JPEG file to an image file |
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4 | .SH SYNOPSIS |
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5 | .B djpeg |
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6 | [ |
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7 | .I options |
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8 | ] |
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9 | [ |
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10 | .I filename |
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11 | ] |
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12 | .LP |
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13 | .SH DESCRIPTION |
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14 | .LP |
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15 | .B djpeg |
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16 | decompresses the named JPEG file, or the standard input if no file is named, |
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17 | and produces an image file on the standard output. PBMPLUS (PPM/PGM), BMP, |
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18 | GIF, Targa, or RLE (Utah Raster Toolkit) output format can be selected. |
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19 | (RLE is supported only if the URT library is available.) |
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20 | .SH OPTIONS |
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21 | All switch names may be abbreviated; for example, |
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22 | .B \-grayscale |
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23 | may be written |
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24 | .B \-gray |
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25 | or |
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26 | .BR \-gr . |
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27 | Most of the "basic" switches can be abbreviated to as little as one letter. |
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28 | Upper and lower case are equivalent (thus |
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29 | .B \-BMP |
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30 | is the same as |
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31 | .BR \-bmp ). |
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32 | British spellings are also accepted (e.g., |
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33 | .BR \-greyscale ), |
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34 | though for brevity these are not mentioned below. |
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35 | .PP |
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36 | The basic switches are: |
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37 | .TP |
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38 | .BI \-colors " N" |
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39 | Reduce image to at most N colors. This reduces the number of colors used in |
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40 | the output image, so that it can be displayed on a colormapped display or |
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41 | stored in a colormapped file format. For example, if you have an 8-bit |
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42 | display, you'd need to reduce to 256 or fewer colors. |
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43 | .TP |
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44 | .BI \-quantize " N" |
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45 | Same as |
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46 | .BR \-colors . |
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47 | .B \-colors |
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48 | is the recommended name, |
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49 | .B \-quantize |
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50 | is provided only for backwards compatibility. |
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51 | .TP |
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52 | .B \-fast |
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53 | Select recommended processing options for fast, low quality output. (The |
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54 | default options are chosen for highest quality output.) Currently, this is |
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55 | equivalent to \fB\-dct fast \-nosmooth \-onepass \-dither ordered\fR. |
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56 | .TP |
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57 | .B \-grayscale |
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58 | Force gray-scale output even if JPEG file is color. Useful for viewing on |
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59 | monochrome displays; also, |
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60 | .B djpeg |
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61 | runs noticeably faster in this mode. |
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62 | .TP |
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63 | .BI \-scale " M/N" |
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64 | Scale the output image by a factor M/N. Currently supported scale factors are |
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65 | M/8 with all M from 1 to 16. If the /N part is omitted, then M specifies the |
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66 | DCT scaled size to be applied on the given input, which is currently |
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67 | equivalent to M/8 scaling, since the source DCT size is currently always 8. |
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68 | Scaling is handy if the image is larger than your screen; also, |
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69 | .B djpeg |
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70 | runs much faster when scaling down the output. |
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71 | .TP |
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72 | .B \-bmp |
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73 | Select BMP output format (Windows flavor). 8-bit colormapped format is |
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74 | emitted if |
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75 | .B \-colors |
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76 | or |
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77 | .B \-grayscale |
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78 | is specified, or if the JPEG file is gray-scale; otherwise, 24-bit full-color |
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79 | format is emitted. |
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80 | .TP |
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81 | .B \-gif |
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82 | Select GIF output format. Since GIF does not support more than 256 colors, |
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83 | .B \-colors 256 |
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84 | is assumed (unless you specify a smaller number of colors). |
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85 | .TP |
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86 | .B \-os2 |
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87 | Select BMP output format (OS/2 1.x flavor). 8-bit colormapped format is |
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88 | emitted if |
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89 | .B \-colors |
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90 | or |
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91 | .B \-grayscale |
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92 | is specified, or if the JPEG file is gray-scale; otherwise, 24-bit full-color |
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93 | format is emitted. |
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94 | .TP |
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95 | .B \-pnm |
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96 | Select PBMPLUS (PPM/PGM) output format (this is the default format). |
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97 | PGM is emitted if the JPEG file is gray-scale or if |
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98 | .B \-grayscale |
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99 | is specified; otherwise PPM is emitted. |
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100 | .TP |
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101 | .B \-rle |
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102 | Select RLE output format. (Requires URT library.) |
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103 | .TP |
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104 | .B \-targa |
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105 | Select Targa output format. Gray-scale format is emitted if the JPEG file is |
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106 | gray-scale or if |
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107 | .B \-grayscale |
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108 | is specified; otherwise, colormapped format is emitted if |
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109 | .B \-colors |
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110 | is specified; otherwise, 24-bit full-color format is emitted. |
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111 | .PP |
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112 | Switches for advanced users: |
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113 | .TP |
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114 | .B \-dct int |
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115 | Use integer DCT method (default). |
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116 | .TP |
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117 | .B \-dct fast |
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118 | Use fast integer DCT (less accurate). |
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119 | .TP |
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120 | .B \-dct float |
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121 | Use floating-point DCT method. |
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122 | The float method is very slightly more accurate than the int method, but is |
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123 | much slower unless your machine has very fast floating-point hardware. Also |
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124 | note that results of the floating-point method may vary slightly across |
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125 | machines, while the integer methods should give the same results everywhere. |
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126 | The fast integer method is much less accurate than the other two. |
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127 | .TP |
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128 | .B \-dither fs |
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129 | Use Floyd-Steinberg dithering in color quantization. |
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130 | .TP |
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131 | .B \-dither ordered |
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132 | Use ordered dithering in color quantization. |
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133 | .TP |
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134 | .B \-dither none |
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135 | Do not use dithering in color quantization. |
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136 | By default, Floyd-Steinberg dithering is applied when quantizing colors; this |
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137 | is slow but usually produces the best results. Ordered dither is a compromise |
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138 | between speed and quality; no dithering is fast but usually looks awful. Note |
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139 | that these switches have no effect unless color quantization is being done. |
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140 | Ordered dither is only available in |
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141 | .B \-onepass |
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142 | mode. |
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143 | .TP |
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144 | .BI \-map " file" |
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145 | Quantize to the colors used in the specified image file. This is useful for |
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146 | producing multiple files with identical color maps, or for forcing a |
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147 | predefined set of colors to be used. The |
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148 | .I file |
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149 | must be a GIF or PPM file. This option overrides |
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150 | .B \-colors |
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151 | and |
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152 | .BR \-onepass . |
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153 | .TP |
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154 | .B \-nosmooth |
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155 | Don't use high-quality upsampling. |
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156 | .TP |
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157 | .B \-onepass |
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158 | Use one-pass instead of two-pass color quantization. The one-pass method is |
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159 | faster and needs less memory, but it produces a lower-quality image. |
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160 | .B \-onepass |
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161 | is ignored unless you also say |
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162 | .B \-colors |
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163 | .IR N . |
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164 | Also, the one-pass method is always used for gray-scale output (the two-pass |
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165 | method is no improvement then). |
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166 | .TP |
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167 | .BI \-maxmemory " N" |
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168 | Set limit for amount of memory to use in processing large images. Value is |
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169 | in thousands of bytes, or millions of bytes if "M" is attached to the |
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170 | number. For example, |
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171 | .B \-max 4m |
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172 | selects 4000000 bytes. If more space is needed, temporary files will be used. |
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173 | .TP |
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174 | .BI \-outfile " name" |
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175 | Send output image to the named file, not to standard output. |
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176 | .TP |
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177 | .B \-verbose |
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178 | Enable debug printout. More |
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179 | .BR \-v 's |
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180 | give more output. Also, version information is printed at startup. |
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181 | .TP |
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182 | .B \-debug |
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183 | Same as |
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184 | .BR \-verbose . |
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185 | .SH EXAMPLES |
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186 | .LP |
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187 | This example decompresses the JPEG file foo.jpg, quantizes it to |
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188 | 256 colors, and saves the output in 8-bit BMP format in foo.bmp: |
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189 | .IP |
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190 | .B djpeg \-colors 256 \-bmp |
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191 | .I foo.jpg |
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192 | .B > |
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193 | .I foo.bmp |
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194 | .SH HINTS |
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195 | To get a quick preview of an image, use the |
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196 | .B \-grayscale |
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197 | and/or |
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198 | .B \-scale |
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199 | switches. |
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200 | .B \-grayscale \-scale 1/8 |
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201 | is the fastest case. |
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202 | .PP |
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203 | Several options are available that trade off image quality to gain speed. |
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204 | .B \-fast |
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205 | turns on the recommended settings. |
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206 | .PP |
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207 | .B \-dct fast |
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208 | and/or |
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209 | .B \-nosmooth |
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210 | gain speed at a small sacrifice in quality. |
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211 | When producing a color-quantized image, |
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212 | .B \-onepass \-dither ordered |
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213 | is fast but much lower quality than the default behavior. |
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214 | .B \-dither none |
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215 | may give acceptable results in two-pass mode, but is seldom tolerable in |
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216 | one-pass mode. |
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217 | .PP |
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218 | If you are fortunate enough to have very fast floating point hardware, |
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219 | \fB\-dct float\fR may be even faster than \fB\-dct fast\fR. But on most |
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220 | machines \fB\-dct float\fR is slower than \fB\-dct int\fR; in this case it is |
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221 | not worth using, because its theoretical accuracy advantage is too small to be |
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222 | significant in practice. |
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223 | .SH ENVIRONMENT |
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224 | .TP |
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225 | .B JPEGMEM |
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226 | If this environment variable is set, its value is the default memory limit. |
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227 | The value is specified as described for the |
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228 | .B \-maxmemory |
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229 | switch. |
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230 | .B JPEGMEM |
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231 | overrides the default value specified when the program was compiled, and |
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232 | itself is overridden by an explicit |
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233 | .BR \-maxmemory . |
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234 | .SH SEE ALSO |
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235 | .BR cjpeg (1), |
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236 | .BR jpegtran (1), |
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237 | .BR rdjpgcom (1), |
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238 | .BR wrjpgcom (1) |
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239 | .br |
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240 | .BR ppm (5), |
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241 | .BR pgm (5) |
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242 | .br |
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243 | Wallace, Gregory K. "The JPEG Still Picture Compression Standard", |
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244 | Communications of the ACM, April 1991 (vol. 34, no. 4), pp. 30-44. |
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245 | .SH AUTHOR |
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246 | Independent JPEG Group |
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247 | .SH BUGS |
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248 | To avoid the Unisys LZW patent, |
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249 | .B djpeg |
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250 | produces uncompressed GIF files. These are larger than they should be, but |
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251 | are readable by standard GIF decoders. |
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