1 | .TH JPEGTRAN 1 "28 December 2009"
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2 | .SH NAME
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3 | jpegtran \- lossless transformation of JPEG files
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4 | .SH SYNOPSIS
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5 | .B jpegtran
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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 jpegtran
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16 | performs various useful transformations of JPEG files.
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17 | It can translate the coded representation from one variant of JPEG to another,
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18 | for example from baseline JPEG to progressive JPEG or vice versa. It can also
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19 | perform some rearrangements of the image data, for example turning an image
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20 | from landscape to portrait format by rotation.
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21 | .PP
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22 | .B jpegtran
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23 | works by rearranging the compressed data (DCT coefficients), without
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24 | ever fully decoding the image. Therefore, its transformations are lossless:
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25 | there is no image degradation at all, which would not be true if you used
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26 | .B djpeg
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27 | followed by
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28 | .B cjpeg
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29 | to accomplish the same conversion. But by the same token,
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30 | .B jpegtran
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31 | cannot perform lossy operations such as changing the image quality.
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32 | .PP
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33 | .B jpegtran
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34 | reads the named JPEG/JFIF file, or the standard input if no file is
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35 | named, and produces a JPEG/JFIF file on the standard output.
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36 | .SH OPTIONS
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37 | All switch names may be abbreviated; for example,
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38 | .B \-optimize
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39 | may be written
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40 | .B \-opt
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41 | or
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42 | .BR \-o .
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43 | Upper and lower case are equivalent.
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44 | British spellings are also accepted (e.g.,
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45 | .BR \-optimise ),
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46 | though for brevity these are not mentioned below.
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47 | .PP
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48 | To specify the coded JPEG representation used in the output file,
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49 | .B jpegtran
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50 | accepts a subset of the switches recognized by
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51 | .BR cjpeg :
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52 | .TP
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53 | .B \-optimize
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54 | Perform optimization of entropy encoding parameters.
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55 | .TP
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56 | .B \-progressive
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57 | Create progressive JPEG file.
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58 | .TP
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59 | .BI \-restart " N"
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60 | Emit a JPEG restart marker every N MCU rows, or every N MCU blocks if "B" is
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61 | attached to the number.
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62 | .TP
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63 | .B \-arithmetic
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64 | Use arithmetic coding.
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65 | .TP
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66 | .BI \-scans " file"
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67 | Use the scan script given in the specified text file.
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68 | .PP
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69 | See
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70 | .BR cjpeg (1)
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71 | for more details about these switches.
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72 | If you specify none of these switches, you get a plain baseline-JPEG output
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73 | file. The quality setting and so forth are determined by the input file.
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74 | .PP
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75 | The image can be losslessly transformed by giving one of these switches:
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76 | .TP
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77 | .B \-flip horizontal
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78 | Mirror image horizontally (left-right).
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79 | .TP
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80 | .B \-flip vertical
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81 | Mirror image vertically (top-bottom).
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82 | .TP
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83 | .B \-rotate 90
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84 | Rotate image 90 degrees clockwise.
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85 | .TP
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86 | .B \-rotate 180
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87 | Rotate image 180 degrees.
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88 | .TP
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89 | .B \-rotate 270
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90 | Rotate image 270 degrees clockwise (or 90 ccw).
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91 | .TP
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92 | .B \-transpose
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93 | Transpose image (across UL-to-LR axis).
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94 | .TP
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95 | .B \-transverse
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96 | Transverse transpose (across UR-to-LL axis).
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97 | .IP
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98 | The transpose transformation has no restrictions regarding image dimensions.
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99 | The other transformations operate rather oddly if the image dimensions are not
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100 | a multiple of the iMCU size (usually 8 or 16 pixels), because they can only
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101 | transform complete blocks of DCT coefficient data in the desired way.
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102 | .IP
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103 | .BR jpegtran 's
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104 | default behavior when transforming an odd-size image is designed
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105 | to preserve exact reversibility and mathematical consistency of the
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106 | transformation set. As stated, transpose is able to flip the entire image
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107 | area. Horizontal mirroring leaves any partial iMCU column at the right edge
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108 | untouched, but is able to flip all rows of the image. Similarly, vertical
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109 | mirroring leaves any partial iMCU row at the bottom edge untouched, but is
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110 | able to flip all columns. The other transforms can be built up as sequences
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111 | of transpose and flip operations; for consistency, their actions on edge
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112 | pixels are defined to be the same as the end result of the corresponding
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113 | transpose-and-flip sequence.
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114 | .IP
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115 | For practical use, you may prefer to discard any untransformable edge pixels
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116 | rather than having a strange-looking strip along the right and/or bottom edges
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117 | of a transformed image. To do this, add the
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118 | .B \-trim
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119 | switch:
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120 | .TP
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121 | .B \-trim
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122 | Drop non-transformable edge blocks.
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123 | .IP
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124 | Obviously, a transformation with
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125 | .B \-trim
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126 | is not reversible, so strictly speaking
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127 | .B jpegtran
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128 | with this switch is not lossless. Also, the expected mathematical
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129 | equivalences between the transformations no longer hold. For example,
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130 | .B \-rot 270 -trim
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131 | trims only the bottom edge, but
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132 | .B \-rot 90 -trim
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133 | followed by
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134 | .B \-rot 180 -trim
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135 | trims both edges.
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136 | .IP
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137 | If you are only interested in perfect transformation, add the
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138 | .B \-perfect
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139 | switch:
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140 | .TP
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141 | .B \-perfect
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142 | Fails with an error if the transformation is not perfect.
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143 | .IP
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144 | For example you may want to do
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145 | .IP
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146 | .B (jpegtran \-rot 90 -perfect
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147 | .I foo.jpg
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148 | .B || djpeg
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149 | .I foo.jpg
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150 | .B | pnmflip \-r90 | cjpeg)
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151 | .IP
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152 | to do a perfect rotation if available or an approximated one if not.
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153 | .PP
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154 | We also offer a lossless-crop option, which discards data outside a given
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155 | image region but losslessly preserves what is inside. Like the rotate and
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156 | flip transforms, lossless crop is restricted by the current JPEG format: the
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157 | upper left corner of the selected region must fall on an iMCU boundary. If
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158 | this does not hold for the given crop parameters, we silently move the upper
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159 | left corner up and/or left to make it so, simultaneously increasing the region
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160 | dimensions to keep the lower right crop corner unchanged. (Thus, the output
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161 | image covers at least the requested region, but may cover more.)
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162 |
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163 | The image can be losslessly cropped by giving the switch:
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164 | .TP
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165 | .B \-crop WxH+X+Y
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166 | Crop to a rectangular subarea of width W, height H starting at point X,Y.
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167 | .PP
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168 | Other not-strictly-lossless transformation switches are:
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169 | .TP
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170 | .B \-grayscale
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171 | Force grayscale output.
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172 | .IP
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173 | This option discards the chrominance channels if the input image is YCbCr
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174 | (ie, a standard color JPEG), resulting in a grayscale JPEG file. The
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175 | luminance channel is preserved exactly, so this is a better method of reducing
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176 | to grayscale than decompression, conversion, and recompression. This switch
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177 | is particularly handy for fixing a monochrome picture that was mistakenly
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178 | encoded as a color JPEG. (In such a case, the space savings from getting rid
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179 | of the near-empty chroma channels won't be large; but the decoding time for
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180 | a grayscale JPEG is substantially less than that for a color JPEG.)
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181 | .TP
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182 | .BI \-scale " M/N"
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183 | Scale the output image by a factor M/N.
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184 | .IP
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185 | Currently supported scale factors are M/N with all M from 1 to 16, where N is
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186 | the source DCT size, which is 8 for baseline JPEG. If the /N part is omitted,
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187 | then M specifies the DCT scaled size to be applied on the given input. For
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188 | baseline JPEG this is equivalent to M/8 scaling, since the source DCT size
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189 | for baseline JPEG is 8.
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190 | .B Caution:
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191 | An implementation of the JPEG SmartScale extension is required for this
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192 | feature. SmartScale enabled JPEG is not yet widely implemented, so many
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193 | decoders will be unable to view a SmartScale extended JPEG file at all.
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194 | .PP
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195 | .B jpegtran
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196 | also recognizes these switches that control what to do with "extra" markers,
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197 | such as comment blocks:
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198 | .TP
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199 | .B \-copy none
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200 | Copy no extra markers from source file. This setting suppresses all
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201 | comments and other excess baggage present in the source file.
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202 | .TP
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203 | .B \-copy comments
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204 | Copy only comment markers. This setting copies comments from the source file,
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205 | but discards any other inessential (for image display) data.
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206 | .TP
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207 | .B \-copy all
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208 | Copy all extra markers. This setting preserves miscellaneous markers
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209 | found in the source file, such as JFIF thumbnails, Exif data, and Photoshop
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210 | settings. In some files these extra markers can be sizable.
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211 | .IP
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212 | The default behavior is
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213 | .BR "\-copy comments" .
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214 | (Note: in IJG releases v6 and v6a,
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215 | .B jpegtran
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216 | always did the equivalent of
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217 | .BR "\-copy none" .)
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218 | .PP
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219 | Additional switches recognized by jpegtran are:
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220 | .TP
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221 | .BI \-maxmemory " N"
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222 | Set limit for amount of memory to use in processing large images. Value is
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223 | in thousands of bytes, or millions of bytes if "M" is attached to the
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224 | number. For example,
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225 | .B \-max 4m
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226 | selects 4000000 bytes. If more space is needed, temporary files will be used.
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227 | .TP
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228 | .BI \-outfile " name"
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229 | Send output image to the named file, not to standard output.
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230 | .TP
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231 | .B \-verbose
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232 | Enable debug printout. More
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233 | .BR \-v 's
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234 | give more output. Also, version information is printed at startup.
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235 | .TP
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236 | .B \-debug
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237 | Same as
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238 | .BR \-verbose .
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239 | .SH EXAMPLES
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240 | .LP
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241 | This example converts a baseline JPEG file to progressive form:
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242 | .IP
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243 | .B jpegtran \-progressive
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244 | .I foo.jpg
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245 | .B >
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246 | .I fooprog.jpg
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247 | .PP
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248 | This example rotates an image 90 degrees clockwise, discarding any
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249 | unrotatable edge pixels:
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250 | .IP
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251 | .B jpegtran \-rot 90 -trim
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252 | .I foo.jpg
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253 | .B >
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254 | .I foo90.jpg
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255 | .SH ENVIRONMENT
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256 | .TP
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257 | .B JPEGMEM
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258 | If this environment variable is set, its value is the default memory limit.
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259 | The value is specified as described for the
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260 | .B \-maxmemory
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261 | switch.
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262 | .B JPEGMEM
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263 | overrides the default value specified when the program was compiled, and
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264 | itself is overridden by an explicit
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265 | .BR \-maxmemory .
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266 | .SH SEE ALSO
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267 | .BR cjpeg (1),
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268 | .BR djpeg (1),
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269 | .BR rdjpgcom (1),
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270 | .BR wrjpgcom (1)
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271 | .br
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272 | Wallace, Gregory K. "The JPEG Still Picture Compression Standard",
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273 | Communications of the ACM, April 1991 (vol. 34, no. 4), pp. 30-44.
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274 | .SH AUTHOR
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275 | Independent JPEG Group
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276 | .SH BUGS
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277 | The transform options can't transform odd-size images perfectly. Use
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278 | .B \-trim
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279 | or
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280 | .B \-perfect
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281 | if you don't like the results.
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282 | .PP
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283 | The entire image is read into memory and then written out again, even in
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284 | cases where this isn't really necessary. Expect swapping on large images,
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285 | especially when using the more complex transform options.
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