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jpayne@68 1 .TH JPEGTRAN 1 "28 August 2019"
jpayne@68 2 .SH NAME
jpayne@68 3 jpegtran \- lossless transformation of JPEG files
jpayne@68 4 .SH SYNOPSIS
jpayne@68 5 .B jpegtran
jpayne@68 6 [
jpayne@68 7 .I options
jpayne@68 8 ]
jpayne@68 9 [
jpayne@68 10 .I filename
jpayne@68 11 ]
jpayne@68 12 .LP
jpayne@68 13 .SH DESCRIPTION
jpayne@68 14 .LP
jpayne@68 15 .B jpegtran
jpayne@68 16 performs various useful transformations of JPEG files.
jpayne@68 17 It can translate the coded representation from one variant of JPEG to another,
jpayne@68 18 for example from baseline JPEG to progressive JPEG or vice versa. It can also
jpayne@68 19 perform some rearrangements of the image data, for example turning an image
jpayne@68 20 from landscape to portrait format by rotation.
jpayne@68 21 .PP
jpayne@68 22 For EXIF files and JPEG files containing Exif data, you may prefer to use
jpayne@68 23 .B exiftran
jpayne@68 24 instead.
jpayne@68 25 .PP
jpayne@68 26 .B jpegtran
jpayne@68 27 works by rearranging the compressed data (DCT coefficients), without
jpayne@68 28 ever fully decoding the image. Therefore, its transformations are lossless:
jpayne@68 29 there is no image degradation at all, which would not be true if you used
jpayne@68 30 .B djpeg
jpayne@68 31 followed by
jpayne@68 32 .B cjpeg
jpayne@68 33 to accomplish the same conversion. But by the same token,
jpayne@68 34 .B jpegtran
jpayne@68 35 cannot perform lossy operations such as changing the image quality. However,
jpayne@68 36 while the image data is losslessly transformed, metadata can be removed. See
jpayne@68 37 the
jpayne@68 38 .B \-copy
jpayne@68 39 option for specifics.
jpayne@68 40 .PP
jpayne@68 41 .B jpegtran
jpayne@68 42 reads the named JPEG/JFIF file, or the standard input if no file is
jpayne@68 43 named, and produces a JPEG/JFIF file on the standard output.
jpayne@68 44 .SH OPTIONS
jpayne@68 45 All switch names may be abbreviated; for example,
jpayne@68 46 .B \-optimize
jpayne@68 47 may be written
jpayne@68 48 .B \-opt
jpayne@68 49 or
jpayne@68 50 .BR \-o .
jpayne@68 51 Upper and lower case are equivalent.
jpayne@68 52 British spellings are also accepted (e.g.,
jpayne@68 53 .BR \-optimise ),
jpayne@68 54 though for brevity these are not mentioned below.
jpayne@68 55 .PP
jpayne@68 56 To specify the coded JPEG representation used in the output file,
jpayne@68 57 .B jpegtran
jpayne@68 58 accepts a subset of the switches recognized by
jpayne@68 59 .BR cjpeg :
jpayne@68 60 .TP
jpayne@68 61 .B \-optimize
jpayne@68 62 Perform optimization of entropy encoding parameters.
jpayne@68 63 .TP
jpayne@68 64 .B \-progressive
jpayne@68 65 Create progressive JPEG file.
jpayne@68 66 .TP
jpayne@68 67 .BI \-restart " N"
jpayne@68 68 Emit a JPEG restart marker every N MCU rows, or every N MCU blocks if "B" is
jpayne@68 69 attached to the number.
jpayne@68 70 .TP
jpayne@68 71 .B \-arithmetic
jpayne@68 72 Use arithmetic coding.
jpayne@68 73 .TP
jpayne@68 74 .BI \-scans " file"
jpayne@68 75 Use the scan script given in the specified text file.
jpayne@68 76 .PP
jpayne@68 77 See
jpayne@68 78 .BR cjpeg (1)
jpayne@68 79 for more details about these switches.
jpayne@68 80 If you specify none of these switches, you get a plain baseline-JPEG output
jpayne@68 81 file. The quality setting and so forth are determined by the input file.
jpayne@68 82 .PP
jpayne@68 83 The image can be losslessly transformed by giving one of these switches:
jpayne@68 84 .TP
jpayne@68 85 .B \-flip horizontal
jpayne@68 86 Mirror image horizontally (left-right).
jpayne@68 87 .TP
jpayne@68 88 .B \-flip vertical
jpayne@68 89 Mirror image vertically (top-bottom).
jpayne@68 90 .TP
jpayne@68 91 .B \-rotate 90
jpayne@68 92 Rotate image 90 degrees clockwise.
jpayne@68 93 .TP
jpayne@68 94 .B \-rotate 180
jpayne@68 95 Rotate image 180 degrees.
jpayne@68 96 .TP
jpayne@68 97 .B \-rotate 270
jpayne@68 98 Rotate image 270 degrees clockwise (or 90 ccw).
jpayne@68 99 .TP
jpayne@68 100 .B \-transpose
jpayne@68 101 Transpose image (across UL-to-LR axis).
jpayne@68 102 .TP
jpayne@68 103 .B \-transverse
jpayne@68 104 Transverse transpose (across UR-to-LL axis).
jpayne@68 105 .IP
jpayne@68 106 The transpose transformation has no restrictions regarding image dimensions.
jpayne@68 107 The other transformations operate rather oddly if the image dimensions are not
jpayne@68 108 a multiple of the iMCU size (usually 8 or 16 pixels), because they can only
jpayne@68 109 transform complete blocks of DCT coefficient data in the desired way.
jpayne@68 110 .IP
jpayne@68 111 .BR jpegtran 's
jpayne@68 112 default behavior when transforming an odd-size image is designed
jpayne@68 113 to preserve exact reversibility and mathematical consistency of the
jpayne@68 114 transformation set. As stated, transpose is able to flip the entire image
jpayne@68 115 area. Horizontal mirroring leaves any partial iMCU column at the right edge
jpayne@68 116 untouched, but is able to flip all rows of the image. Similarly, vertical
jpayne@68 117 mirroring leaves any partial iMCU row at the bottom edge untouched, but is
jpayne@68 118 able to flip all columns. The other transforms can be built up as sequences
jpayne@68 119 of transpose and flip operations; for consistency, their actions on edge
jpayne@68 120 pixels are defined to be the same as the end result of the corresponding
jpayne@68 121 transpose-and-flip sequence.
jpayne@68 122 .IP
jpayne@68 123 For practical use, you may prefer to discard any untransformable edge pixels
jpayne@68 124 rather than having a strange-looking strip along the right and/or bottom edges
jpayne@68 125 of a transformed image. To do this, add the
jpayne@68 126 .B \-trim
jpayne@68 127 switch:
jpayne@68 128 .TP
jpayne@68 129 .B \-trim
jpayne@68 130 Drop non-transformable edge blocks.
jpayne@68 131 .IP
jpayne@68 132 Obviously, a transformation with
jpayne@68 133 .B \-trim
jpayne@68 134 is not reversible, so strictly speaking
jpayne@68 135 .B jpegtran
jpayne@68 136 with this switch is not lossless. Also, the expected mathematical
jpayne@68 137 equivalences between the transformations no longer hold. For example,
jpayne@68 138 .B \-rot 270 -trim
jpayne@68 139 trims only the bottom edge, but
jpayne@68 140 .B \-rot 90 -trim
jpayne@68 141 followed by
jpayne@68 142 .B \-rot 180 -trim
jpayne@68 143 trims both edges.
jpayne@68 144 .IP
jpayne@68 145 If you are only interested in perfect transformation, add the
jpayne@68 146 .B \-perfect
jpayne@68 147 switch:
jpayne@68 148 .TP
jpayne@68 149 .B \-perfect
jpayne@68 150 Fails with an error if the transformation is not perfect.
jpayne@68 151 .IP
jpayne@68 152 For example you may want to do
jpayne@68 153 .IP
jpayne@68 154 .B (jpegtran \-rot 90 -perfect
jpayne@68 155 .I foo.jpg
jpayne@68 156 .B || djpeg
jpayne@68 157 .I foo.jpg
jpayne@68 158 .B | pnmflip \-r90 | cjpeg)
jpayne@68 159 .IP
jpayne@68 160 to do a perfect rotation if available or an approximated one if not.
jpayne@68 161 .PP
jpayne@68 162 We also offer a lossless-crop option, which discards data outside a given
jpayne@68 163 image region but losslessly preserves what is inside. Like the rotate and
jpayne@68 164 flip transforms, lossless crop is restricted by the current JPEG format: the
jpayne@68 165 upper left corner of the selected region must fall on an iMCU boundary. If
jpayne@68 166 this does not hold for the given crop parameters, we silently move the upper
jpayne@68 167 left corner up and/or left to make it so, simultaneously increasing the
jpayne@68 168 region dimensions to keep the lower right crop corner unchanged. (Thus, the
jpayne@68 169 output image covers at least the requested region, but may cover more.)
jpayne@68 170 The adjustment of the region dimensions may be optionally disabled by
jpayne@68 171 attaching an 'f' character ("force") to the width or height number.
jpayne@68 172 .PP
jpayne@68 173 The image can be losslessly cropped by giving the switch:
jpayne@68 174 .TP
jpayne@68 175 .B \-crop WxH+X+Y
jpayne@68 176 Crop to a rectangular subarea of width W, height H starting at point X,Y.
jpayne@68 177 .PP
jpayne@68 178 Crop extension: The width or height parameters can be made larger than the
jpayne@68 179 source image. In this case the extra area is filled in with zero (neutral
jpayne@68 180 gray). A larger width parameter has two more options: Attaching an 'f'
jpayne@68 181 character ("flatten") to the width number will fill in the extra area with
jpayne@68 182 the DC of the adjacent block, instead of gray out. Attaching an 'r'
jpayne@68 183 character ("reflect") to the width number will fill in the extra area with
jpayne@68 184 repeated reflections of the source region, instead of gray out.
jpayne@68 185 .PP
jpayne@68 186 A complementary lossless-wipe option is provided to discard (gray out) data
jpayne@68 187 inside a given image region while losslessly preserving what is outside:
jpayne@68 188 .TP
jpayne@68 189 .B \-wipe WxH+X+Y
jpayne@68 190 Wipe (gray out) a rectangular subarea of width W, height H starting at point
jpayne@68 191 X,Y.
jpayne@68 192 .PP
jpayne@68 193 Attaching an 'f' character ("flatten") to the width number will fill the
jpayne@68 194 region with the average of adjacent blocks, instead of gray out. In case
jpayne@68 195 the wipe region and outside area form two horizontally adjacent rectangles,
jpayne@68 196 attaching an 'r' character ("reflect") to the width number will fill the
jpayne@68 197 region with repeated reflections of the outside area, instead of gray out.
jpayne@68 198 .PP
jpayne@68 199 Another option is lossless-drop, which replaces data at a given image
jpayne@68 200 position by another image:
jpayne@68 201 .TP
jpayne@68 202 .B \-drop +X+Y filename
jpayne@68 203 Drop another image
jpayne@68 204 .PP
jpayne@68 205 Both source images must have the same subsampling values. It is best if
jpayne@68 206 they also have the same quantization, otherwise quantization adaption occurs.
jpayne@68 207 The trim option can be used with the drop option to requantize the drop file
jpayne@68 208 to the source file.
jpayne@68 209 .PP
jpayne@68 210 Other not-strictly-lossless transformation switches are:
jpayne@68 211 .TP
jpayne@68 212 .B \-grayscale
jpayne@68 213 Force grayscale output.
jpayne@68 214 .IP
jpayne@68 215 This option discards the chrominance channels if the input image is YCbCr
jpayne@68 216 (ie, a standard color JPEG), resulting in a grayscale JPEG file. The
jpayne@68 217 luminance channel is preserved exactly, so this is a better method of reducing
jpayne@68 218 to grayscale than decompression, conversion, and recompression. This switch
jpayne@68 219 is particularly handy for fixing a monochrome picture that was mistakenly
jpayne@68 220 encoded as a color JPEG. (In such a case, the space savings from getting rid
jpayne@68 221 of the near-empty chroma channels won't be large; but the decoding time for
jpayne@68 222 a grayscale JPEG is substantially less than that for a color JPEG.)
jpayne@68 223 .TP
jpayne@68 224 .BI \-scale " M/N"
jpayne@68 225 Scale the output image by a factor M/N.
jpayne@68 226 .IP
jpayne@68 227 Currently supported scale factors are M/N with all M from 1 to 16, where N is
jpayne@68 228 the source DCT size, which is 8 for baseline JPEG. If the /N part is omitted,
jpayne@68 229 then M specifies the DCT scaled size to be applied on the given input. For
jpayne@68 230 baseline JPEG this is equivalent to M/8 scaling, since the source DCT size
jpayne@68 231 for baseline JPEG is 8.
jpayne@68 232 .B Caution:
jpayne@68 233 An implementation of the JPEG SmartScale extension is required for this
jpayne@68 234 feature. SmartScale enabled JPEG is not yet widely implemented, so many
jpayne@68 235 decoders will be unable to view a SmartScale extended JPEG file at all.
jpayne@68 236 .PP
jpayne@68 237 .B jpegtran
jpayne@68 238 also recognizes these switches that control what to do with "extra" markers,
jpayne@68 239 such as comment blocks:
jpayne@68 240 .TP
jpayne@68 241 .B \-copy none
jpayne@68 242 Copy no extra markers from source file. This setting suppresses all
jpayne@68 243 comments and other metadata in the source file.
jpayne@68 244 .TP
jpayne@68 245 .B \-copy comments
jpayne@68 246 Copy only comment markers. This setting copies comments from the source file,
jpayne@68 247 but discards any other metadata.
jpayne@68 248 .TP
jpayne@68 249 .B \-copy all
jpayne@68 250 Copy all extra markers. This setting preserves metadata
jpayne@68 251 found in the source file, such as JFIF thumbnails, Exif data, and Photoshop
jpayne@68 252 settings. In some files these extra markers can be sizable. Note that this
jpayne@68 253 option will copy thumbnails as-is; they will not be transformed.
jpayne@68 254 .IP
jpayne@68 255 The default behavior is
jpayne@68 256 .BR "\-copy comments" .
jpayne@68 257 (Note: in IJG releases v6 and v6a,
jpayne@68 258 .B jpegtran
jpayne@68 259 always did the equivalent of
jpayne@68 260 .BR "\-copy none" .)
jpayne@68 261 .PP
jpayne@68 262 Additional switches recognized by jpegtran are:
jpayne@68 263 .TP
jpayne@68 264 .BI \-maxmemory " N"
jpayne@68 265 Set limit for amount of memory to use in processing large images. Value is
jpayne@68 266 in thousands of bytes, or millions of bytes if "M" is attached to the
jpayne@68 267 number. For example,
jpayne@68 268 .B \-max 4m
jpayne@68 269 selects 4000000 bytes. If more space is needed, temporary files will be used.
jpayne@68 270 .TP
jpayne@68 271 .BI \-outfile " name"
jpayne@68 272 Send output image to the named file, not to standard output.
jpayne@68 273 .TP
jpayne@68 274 .B \-verbose
jpayne@68 275 Enable debug printout. More
jpayne@68 276 .BR \-v 's
jpayne@68 277 give more output. Also, version information is printed at startup.
jpayne@68 278 .TP
jpayne@68 279 .B \-debug
jpayne@68 280 Same as
jpayne@68 281 .BR \-verbose .
jpayne@68 282 .SH EXAMPLES
jpayne@68 283 .LP
jpayne@68 284 This example converts a baseline JPEG file to progressive form:
jpayne@68 285 .IP
jpayne@68 286 .B jpegtran \-progressive
jpayne@68 287 .I foo.jpg
jpayne@68 288 .B >
jpayne@68 289 .I fooprog.jpg
jpayne@68 290 .PP
jpayne@68 291 This example rotates an image 90 degrees clockwise, discarding any
jpayne@68 292 unrotatable edge pixels:
jpayne@68 293 .IP
jpayne@68 294 .B jpegtran \-rot 90 -trim
jpayne@68 295 .I foo.jpg
jpayne@68 296 .B >
jpayne@68 297 .I foo90.jpg
jpayne@68 298 .SH ENVIRONMENT
jpayne@68 299 .TP
jpayne@68 300 .B JPEGMEM
jpayne@68 301 If this environment variable is set, its value is the default memory limit.
jpayne@68 302 The value is specified as described for the
jpayne@68 303 .B \-maxmemory
jpayne@68 304 switch.
jpayne@68 305 .B JPEGMEM
jpayne@68 306 overrides the default value specified when the program was compiled, and
jpayne@68 307 itself is overridden by an explicit
jpayne@68 308 .BR \-maxmemory .
jpayne@68 309 .SH SEE ALSO
jpayne@68 310 .BR cjpeg (1),
jpayne@68 311 .BR djpeg (1),
jpayne@68 312 .BR rdjpgcom (1),
jpayne@68 313 .BR wrjpgcom (1)
jpayne@68 314 .br
jpayne@68 315 Wallace, Gregory K. "The JPEG Still Picture Compression Standard",
jpayne@68 316 Communications of the ACM, April 1991 (vol. 34, no. 4), pp. 30-44.
jpayne@68 317 .SH AUTHOR
jpayne@68 318 Independent JPEG Group
jpayne@68 319 .SH BUGS
jpayne@68 320 The transform options can't transform odd-size images perfectly. Use
jpayne@68 321 .B \-trim
jpayne@68 322 or
jpayne@68 323 .B \-perfect
jpayne@68 324 if you don't like the results.
jpayne@68 325 .PP
jpayne@68 326 The entire image is read into memory and then written out again, even in
jpayne@68 327 cases where this isn't really necessary. Expect swapping on large images,
jpayne@68 328 especially when using the more complex transform options.