1 | .TH CJPEG 1 "30 December 2009"
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2 | .SH NAME
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3 | cjpeg \- compress an image file to a JPEG file
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4 | .SH SYNOPSIS
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5 | .B cjpeg
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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 cjpeg
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16 | compresses the named image file, or the standard input if no file is
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17 | named, and produces a JPEG/JFIF file on the standard output.
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18 | The currently supported input file formats are: PPM (PBMPLUS color
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19 | format), PGM (PBMPLUS gray-scale format), BMP, Targa, and RLE (Utah Raster
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20 | Toolkit format). (RLE is supported only if the URT library is available.)
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21 | .SH OPTIONS
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22 | All switch names may be abbreviated; for example,
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23 | .B \-grayscale
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24 | may be written
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25 | .B \-gray
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26 | or
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27 | .BR \-gr .
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28 | Most of the "basic" switches can be abbreviated to as little as one letter.
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29 | Upper and lower case are equivalent (thus
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30 | .B \-BMP
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31 | is the same as
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32 | .BR \-bmp ).
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33 | British spellings are also accepted (e.g.,
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34 | .BR \-greyscale ),
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35 | though for brevity these are not mentioned below.
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36 | .PP
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37 | The basic switches are:
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38 | .TP
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39 | .BI \-quality " N[,...]"
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40 | Scale quantization tables to adjust image quality. Quality is 0 (worst) to
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41 | 100 (best); default is 75. (See below for more info.)
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42 | .TP
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43 | .B \-grayscale
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44 | Create monochrome JPEG file from color input. Be sure to use this switch when
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45 | compressing a grayscale BMP file, because
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46 | .B cjpeg
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47 | isn't bright enough to notice whether a BMP file uses only shades of gray.
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48 | By saying
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49 | .BR \-grayscale ,
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50 | you'll get a smaller JPEG file that takes less time to process.
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51 | .TP
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52 | .B \-optimize
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53 | Perform optimization of entropy encoding parameters. Without this, default
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54 | encoding parameters are used.
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55 | .B \-optimize
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56 | usually makes the JPEG file a little smaller, but
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57 | .B cjpeg
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58 | runs somewhat slower and needs much more memory. Image quality and speed of
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59 | decompression are unaffected by
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60 | .BR \-optimize .
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61 | .TP
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62 | .B \-progressive
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63 | Create progressive JPEG file (see below).
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64 | .TP
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65 | .BI \-scale " M/N"
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66 | Scale the output image by a factor M/N. Currently supported scale factors are
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67 | 8/N with all N from 1 to 16.
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68 | .TP
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69 | .B \-targa
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70 | Input file is Targa format. Targa files that contain an "identification"
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71 | field will not be automatically recognized by
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72 | .BR cjpeg ;
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73 | for such files you must specify
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74 | .B \-targa
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75 | to make
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76 | .B cjpeg
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77 | treat the input as Targa format.
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78 | For most Targa files, you won't need this switch.
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79 | .PP
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80 | The
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81 | .B \-quality
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82 | switch lets you trade off compressed file size against quality of the
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83 | reconstructed image: the higher the quality setting, the larger the JPEG file,
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84 | and the closer the output image will be to the original input. Normally you
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85 | want to use the lowest quality setting (smallest file) that decompresses into
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86 | something visually indistinguishable from the original image. For this
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87 | purpose the quality setting should be between 50 and 95; the default of 75 is
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88 | often about right. If you see defects at
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89 | .B \-quality
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90 | 75, then go up 5 or 10 counts at a time until you are happy with the output
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91 | image. (The optimal setting will vary from one image to another.)
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92 | .PP
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93 | .B \-quality
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94 | 100 will generate a quantization table of all 1's, minimizing loss in the
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95 | quantization step (but there is still information loss in subsampling, as well
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96 | as roundoff error). This setting is mainly of interest for experimental
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97 | purposes. Quality values above about 95 are
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98 | .B not
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99 | recommended for normal use; the compressed file size goes up dramatically for
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100 | hardly any gain in output image quality.
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101 | .PP
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102 | In the other direction, quality values below 50 will produce very small files
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103 | of low image quality. Settings around 5 to 10 might be useful in preparing an
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104 | index of a large image library, for example. Try
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105 | .B \-quality
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106 | 2 (or so) for some amusing Cubist effects. (Note: quality
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107 | values below about 25 generate 2-byte quantization tables, which are
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108 | considered optional in the JPEG standard.
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109 | .B cjpeg
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110 | emits a warning message when you give such a quality value, because some
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111 | other JPEG programs may be unable to decode the resulting file. Use
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112 | .B \-baseline
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113 | if you need to ensure compatibility at low quality values.)
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114 | .PP
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115 | The
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116 | .B \-quality
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117 | option has been extended in IJG version 7 for support of separate quality
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118 | settings for luminance and chrominance (or in general, for every provided
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119 | quantization table slot). This feature is useful for high-quality
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120 | applications which cannot accept the damage of color data by coarse
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121 | subsampling settings. You can now easily reduce the color data amount more
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122 | smoothly with finer control without separate subsampling. The resulting file
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123 | is fully compliant with standard JPEG decoders.
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124 | Note that the
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125 | .B \-quality
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126 | ratings refer to the quantization table slots, and that the last value is
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127 | replicated if there are more q-table slots than parameters. The default
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128 | q-table slots are 0 for luminance and 1 for chrominance with default tables as
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129 | given in the JPEG standard. This is compatible with the old behaviour in case
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130 | that only one parameter is given, which is then used for both luminance and
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131 | chrominance (slots 0 and 1). More or custom quantization tables can be set
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132 | with
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133 | .B \-qtables
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134 | and assigned to components with
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135 | .B \-qslots
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136 | parameter (see the "wizard" switches below).
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137 | .B Caution:
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138 | You must explicitly add
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139 | .BI \-sample " 1x1"
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140 | for efficient separate color
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141 | quality selection, since the default value used by library is 2x2!
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142 | .PP
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143 | The
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144 | .B \-progressive
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145 | switch creates a "progressive JPEG" file. In this type of JPEG file, the data
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146 | is stored in multiple scans of increasing quality. If the file is being
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147 | transmitted over a slow communications link, the decoder can use the first
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148 | scan to display a low-quality image very quickly, and can then improve the
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149 | display with each subsequent scan. The final image is exactly equivalent to a
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150 | standard JPEG file of the same quality setting, and the total file size is
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151 | about the same --- often a little smaller.
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152 | .PP
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153 | Switches for advanced users:
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154 | .TP
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155 | .B \-dct int
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156 | Use integer DCT method (default).
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157 | .TP
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158 | .B \-dct fast
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159 | Use fast integer DCT (less accurate).
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160 | .TP
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161 | .B \-dct float
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162 | Use floating-point DCT method.
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163 | The float method is very slightly more accurate than the int method, but is
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164 | much slower unless your machine has very fast floating-point hardware. Also
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165 | note that results of the floating-point method may vary slightly across
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166 | machines, while the integer methods should give the same results everywhere.
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167 | The fast integer method is much less accurate than the other two.
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168 | .TP
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169 | .B \-nosmooth
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170 | Don't use high-quality downsampling.
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171 | .TP
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172 | .BI \-restart " N"
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173 | Emit a JPEG restart marker every N MCU rows, or every N MCU blocks if "B" is
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174 | attached to the number.
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175 | .B \-restart 0
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176 | (the default) means no restart markers.
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177 | .TP
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178 | .BI \-smooth " N"
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179 | Smooth the input image to eliminate dithering noise. N, ranging from 1 to
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180 | 100, indicates the strength of smoothing. 0 (the default) means no smoothing.
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181 | .TP
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182 | .BI \-maxmemory " N"
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183 | Set limit for amount of memory to use in processing large images. Value is
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184 | in thousands of bytes, or millions of bytes if "M" is attached to the
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185 | number. For example,
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186 | .B \-max 4m
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187 | selects 4000000 bytes. If more space is needed, temporary files will be used.
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188 | .TP
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189 | .BI \-outfile " name"
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190 | Send output image to the named file, not to standard output.
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191 | .TP
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192 | .B \-verbose
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193 | Enable debug printout. More
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194 | .BR \-v 's
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195 | give more output. Also, version information is printed at startup.
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196 | .TP
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197 | .B \-debug
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198 | Same as
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199 | .BR \-verbose .
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200 | .PP
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201 | The
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202 | .B \-restart
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203 | option inserts extra markers that allow a JPEG decoder to resynchronize after
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204 | a transmission error. Without restart markers, any damage to a compressed
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205 | file will usually ruin the image from the point of the error to the end of the
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206 | image; with restart markers, the damage is usually confined to the portion of
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207 | the image up to the next restart marker. Of course, the restart markers
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208 | occupy extra space. We recommend
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209 | .B \-restart 1
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210 | for images that will be transmitted across unreliable networks such as Usenet.
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211 | .PP
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212 | The
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213 | .B \-smooth
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214 | option filters the input to eliminate fine-scale noise. This is often useful
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215 | when converting dithered images to JPEG: a moderate smoothing factor of 10 to
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216 | 50 gets rid of dithering patterns in the input file, resulting in a smaller
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217 | JPEG file and a better-looking image. Too large a smoothing factor will
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218 | visibly blur the image, however.
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219 | .PP
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220 | Switches for wizards:
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221 | .TP
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222 | .B \-arithmetic
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223 | Use arithmetic coding.
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224 | .B Caution:
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225 | arithmetic coded JPEG is not yet widely implemented, so many decoders will be
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226 | unable to view an arithmetic coded JPEG file at all.
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227 | .TP
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228 | .B \-baseline
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229 | Force baseline-compatible quantization tables to be generated. This clamps
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230 | quantization values to 8 bits even at low quality settings. (This switch is
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231 | poorly named, since it does not ensure that the output is actually baseline
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232 | JPEG. For example, you can use
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233 | .B \-baseline
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234 | and
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235 | .B \-progressive
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236 | together.)
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237 | .TP
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238 | .BI \-qtables " file"
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239 | Use the quantization tables given in the specified text file.
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240 | .TP
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241 | .BI \-qslots " N[,...]"
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242 | Select which quantization table to use for each color component.
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243 | .TP
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244 | .BI \-sample " HxV[,...]"
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245 | Set JPEG sampling factors for each color component.
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246 | .TP
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247 | .BI \-scans " file"
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248 | Use the scan script given in the specified text file.
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249 | .PP
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250 | The "wizard" switches are intended for experimentation with JPEG. If you
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251 | don't know what you are doing, \fBdon't use them\fR. These switches are
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252 | documented further in the file wizard.txt.
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253 | .SH EXAMPLES
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254 | .LP
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255 | This example compresses the PPM file foo.ppm with a quality factor of
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256 | 60 and saves the output as foo.jpg:
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257 | .IP
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258 | .B cjpeg \-quality
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259 | .I 60 foo.ppm
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260 | .B >
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261 | .I foo.jpg
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262 | .SH HINTS
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263 | Color GIF files are not the ideal input for JPEG; JPEG is really intended for
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264 | compressing full-color (24-bit) images. In particular, don't try to convert
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265 | cartoons, line drawings, and other images that have only a few distinct
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266 | colors. GIF works great on these, JPEG does not. If you want to convert a
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267 | GIF to JPEG, you should experiment with
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268 | .BR cjpeg 's
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269 | .B \-quality
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270 | and
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271 | .B \-smooth
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272 | options to get a satisfactory conversion.
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273 | .B \-smooth 10
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274 | or so is often helpful.
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275 | .PP
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276 | Avoid running an image through a series of JPEG compression/decompression
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277 | cycles. Image quality loss will accumulate; after ten or so cycles the image
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278 | may be noticeably worse than it was after one cycle. It's best to use a
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279 | lossless format while manipulating an image, then convert to JPEG format when
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280 | you are ready to file the image away.
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281 | .PP
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282 | The
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283 | .B \-optimize
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284 | option to
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285 | .B cjpeg
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286 | is worth using when you are making a "final" version for posting or archiving.
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287 | It's also a win when you are using low quality settings to make very small
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288 | JPEG files; the percentage improvement is often a lot more than it is on
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289 | larger files. (At present,
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290 | .B \-optimize
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291 | mode is always selected when generating progressive JPEG files.)
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292 | .SH ENVIRONMENT
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293 | .TP
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294 | .B JPEGMEM
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295 | If this environment variable is set, its value is the default memory limit.
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296 | The value is specified as described for the
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297 | .B \-maxmemory
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298 | switch.
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299 | .B JPEGMEM
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300 | overrides the default value specified when the program was compiled, and
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301 | itself is overridden by an explicit
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302 | .BR \-maxmemory .
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303 | .SH SEE ALSO
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304 | .BR djpeg (1),
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305 | .BR jpegtran (1),
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306 | .BR rdjpgcom (1),
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307 | .BR wrjpgcom (1)
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308 | .br
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309 | .BR ppm (5),
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310 | .BR pgm (5)
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311 | .br
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312 | Wallace, Gregory K. "The JPEG Still Picture Compression Standard",
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313 | Communications of the ACM, April 1991 (vol. 34, no. 4), pp. 30-44.
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314 | .SH AUTHOR
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315 | Independent JPEG Group
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316 | .SH BUGS
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317 | GIF input files are no longer supported, to avoid the Unisys LZW patent.
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318 | (Conversion of GIF files to JPEG is usually a bad idea anyway.)
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319 | .PP
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320 | Not all variants of BMP and Targa file formats are supported.
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321 | .PP
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322 | The
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323 | .B \-targa
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324 | switch is not a bug, it's a feature. (It would be a bug if the Targa format
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325 | designers had not been clueless.)
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