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