src/jpeg/jpegtran.1

   1 .TH JPEGTRAN 1 "3 August 1997"
   2 .SH NAME
   3 jpegtran \- lossless transformation of JPEG files
   4 .SH SYNOPSIS
   5 .B jpegtran
   6 [
   7 .I options
   8 ]
   9 [
  10 .I filename
  11 ]
  12 .LP
  13 .SH DESCRIPTION
  14 .LP
  15 .B jpegtran
  16 performs various useful transformations of JPEG files.
  17 It can translate the coded representation from one variant of JPEG to another,
  18 for example from baseline JPEG to progressive JPEG or vice versa.  It can also
  19 perform some rearrangements of the image data, for example turning an image
  20 from landscape to portrait format by rotation.
  21 .PP
  22 .B jpegtran
  23 works by rearranging the compressed data (DCT coefficients), without
  24 ever fully decoding the image.  Therefore, its transformations are lossless:
  25 there is no image degradation at all, which would not be true if you used
  26 .B djpeg
  27 followed by
  28 .B cjpeg
  29 to accomplish the same conversion.  But by the same token,
  30 .B jpegtran
  31 cannot perform lossy operations such as changing the image quality.
  32 .PP
  33 .B jpegtran
  34 reads the named JPEG/JFIF file, or the standard input if no file is
  35 named, and produces a JPEG/JFIF file on the standard output.
  36 .SH OPTIONS
  37 All switch names may be abbreviated; for example,
  38 .B \-optimize
  39 may be written
  40 .B \-opt
  41 or
  42 .BR \-o .
  43 Upper and lower case are equivalent.
  44 British spellings are also accepted (e.g.,
  45 .BR \-optimise ),
  46 though for brevity these are not mentioned below.
  47 .PP
  48 To specify the coded JPEG representation used in the output file,
  49 .B jpegtran
  50 accepts a subset of the switches recognized by
  51 .BR cjpeg :
  52 .TP
  53 .B \-optimize
  54 Perform optimization of entropy encoding parameters.
  55 .TP
  56 .B \-progressive
  57 Create progressive JPEG file.
  58 .TP
  59 .BI \-restart " N"
  60 Emit a JPEG restart marker every N MCU rows, or every N MCU blocks if "B" is
  61 attached to the number.
  62 .TP
  63 .BI \-scans " file"
  64 Use the scan script given in the specified text file.
  65 .PP
  66 See
  67 .BR cjpeg (1)
  68 for more details about these switches.
  69 If you specify none of these switches, you get a plain baseline-JPEG output
  70 file.  The quality setting and so forth are determined by the input file.
  71 .PP
  72 The image can be losslessly transformed by giving one of these switches:
  73 .TP
  74 .B \-flip horizontal
  75 Mirror image horizontally (left-right).
  76 .TP
  77 .B \-flip vertical
  78 Mirror image vertically (top-bottom).
  79 .TP
  80 .B \-rotate 90
  81 Rotate image 90 degrees clockwise.
  82 .TP
  83 .B \-rotate 180
  84 Rotate image 180 degrees.
  85 .TP
  86 .B \-rotate 270
  87 Rotate image 270 degrees clockwise (or 90 ccw).
  88 .TP
  89 .B \-transpose
  90 Transpose image (across UL-to-LR axis).
  91 .TP
  92 .B \-transverse
  93 Transverse transpose (across UR-to-LL axis).
  94 .PP
  95 The transpose transformation has no restrictions regarding image dimensions.
  96 The other transformations operate rather oddly if the image dimensions are not
  97 a multiple of the iMCU size (usually 8 or 16 pixels), because they can only
  98 transform complete blocks of DCT coefficient data in the desired way.
  99 .PP
 100 .BR jpegtran 's
 101 default behavior when transforming an odd-size image is designed
 102 to preserve exact reversibility and mathematical consistency of the
 103 transformation set.  As stated, transpose is able to flip the entire image
 104 area.  Horizontal mirroring leaves any partial iMCU column at the right edge
 105 untouched, but is able to flip all rows of the image.  Similarly, vertical
 106 mirroring leaves any partial iMCU row at the bottom edge untouched, but is
 107 able to flip all columns.  The other transforms can be built up as sequences
 108 of transpose and flip operations; for consistency, their actions on edge
 109 pixels are defined to be the same as the end result of the corresponding
 110 transpose-and-flip sequence.
 111 .PP
 112 For practical use, you may prefer to discard any untransformable edge pixels
 113 rather than having a strange-looking strip along the right and/or bottom edges
 114 of a transformed image.  To do this, add the
 115 .B \-trim
 116 switch:
 117 .TP
 118 .B \-trim
 119 Drop non-transformable edge blocks.
 120 .PP
 121 Obviously, a transformation with
 122 .B \-trim
 123 is not reversible, so strictly speaking
 124 .B jpegtran
 125 with this switch is not lossless.  Also, the expected mathematical
 126 equivalences between the transformations no longer hold.  For example,
 127 .B \-rot 270 -trim
 128 trims only the bottom edge, but
 129 .B \-rot 90 -trim
 130 followed by
 131 .B \-rot 180 -trim
 132 trims both edges.
 133 .PP
 134 Another not-strictly-lossless transformation switch is:
 135 .TP
 136 .B \-grayscale
 137 Force grayscale output.
 138 .PP
 139 This option discards the chrominance channels if the input image is YCbCr
 140 (ie, a standard color JPEG), resulting in a grayscale JPEG file.  The
 141 luminance channel is preserved exactly, so this is a better method of reducing
 142 to grayscale than decompression, conversion, and recompression.  This switch
 143 is particularly handy for fixing a monochrome picture that was mistakenly
 144 encoded as a color JPEG.  (In such a case, the space savings from getting rid
 145 of the near-empty chroma channels won't be large; but the decoding time for
 146 a grayscale JPEG is substantially less than that for a color JPEG.)
 147 .PP
 148 .B jpegtran
 149 also recognizes these switches that control what to do with "extra" markers,
 150 such as comment blocks:
 151 .TP
 152 .B \-copy none
 153 Copy no extra markers from source file.  This setting suppresses all
 154 comments and other excess baggage present in the source file.
 155 .TP
 156 .B \-copy comments
 157 Copy only comment markers.  This setting copies comments from the source file,
 158 but discards any other inessential data.
 159 .TP
 160 .B \-copy all
 161 Copy all extra markers.  This setting preserves miscellaneous markers
 162 found in the source file, such as JFIF thumbnails and Photoshop settings.
 163 In some files these extra markers can be sizable.
 164 .PP
 165 The default behavior is
 166 .BR "\-copy comments" .
 167 (Note: in IJG releases v6 and v6a,
 168 .B jpegtran
 169 always did the equivalent of
 170 .BR "\-copy none" .)
 171 .PP
 172 Additional switches recognized by jpegtran are:
 173 .TP
 174 .BI \-maxmemory " N"
 175 Set limit for amount of memory to use in processing large images.  Value is
 176 in thousands of bytes, or millions of bytes if "M" is attached to the
 177 number.  For example,
 178 .B \-max 4m
 179 selects 4000000 bytes.  If more space is needed, temporary files will be used.
 180 .TP
 181 .BI \-outfile " name"
 182 Send output image to the named file, not to standard output.
 183 .TP
 184 .B \-verbose
 185 Enable debug printout.  More
 186 .BR \-v 's
 187 give more output.  Also, version information is printed at startup.
 188 .TP
 189 .B \-debug
 190 Same as
 191 .BR \-verbose .
 192 .SH EXAMPLES
 193 .LP
 194 This example converts a baseline JPEG file to progressive form:
 195 .IP
 196 .B jpegtran \-progressive
 197 .I foo.jpg
 198 .B >
 199 .I fooprog.jpg
 200 .PP
 201 This example rotates an image 90 degrees clockwise, discarding any
 202 unrotatable edge pixels:
 203 .IP
 204 .B jpegtran \-rot 90 -trim
 205 .I foo.jpg
 206 .B >
 207 .I foo90.jpg
 208 .SH ENVIRONMENT
 209 .TP
 210 .B JPEGMEM
 211 If this environment variable is set, its value is the default memory limit.
 212 The value is specified as described for the
 213 .B \-maxmemory
 214 switch.
 215 .B JPEGMEM
 216 overrides the default value specified when the program was compiled, and
 217 itself is overridden by an explicit
 218 .BR \-maxmemory .
 219 .SH SEE ALSO
 220 .BR cjpeg (1),
 221 .BR djpeg (1),
 222 .BR rdjpgcom (1),
 223 .BR wrjpgcom (1)
 224 .br
 225 Wallace, Gregory K.  "The JPEG Still Picture Compression Standard",
 226 Communications of the ACM, April 1991 (vol. 34, no. 4), pp. 30-44.
 227 .SH AUTHOR
 228 Independent JPEG Group
 229 .SH BUGS
 230 Arithmetic coding is not supported for legal reasons.
 231 .PP
 232 The transform options can't transform odd-size images perfectly.  Use
 233 .B \-trim
 234 if you don't like the results without it.
 235 .PP
 236 The entire image is read into memory and then written out again, even in
 237 cases where this isn't really necessary.  Expect swapping on large images,
 238 especially when using the more complex transform options.