JPEGTRAN(1)                                           JPEGTRAN(1)

       jpegtran - lossless transformation of JPEG files

       jpegtran [ options ] [ filename ]

       jpegtran  performs  various useful transformations of JPEG
       files.  It can translate the coded representation from one
       variant of JPEG to another, for example from baseline JPEG
       to progressive JPEG or vice versa.  It  can  also  perform
       some rearrangements of the image data, for example turning
       an image from landscape to portrait format by rotation.

       jpegtran works by rearranging  the  compressed  data  (DCT
       coefficients),  without  ever  fully  decoding  the image.
       Therefore, its transformations are lossless: there  is  no
       image  degradation  at all, which would not be true if you
       used djpeg followed by cjpeg to accomplish the  same  con-
       version.   But  by the same token, jpegtran cannot perform
       lossy operations such as changing the image quality.

       jpegtran reads the named JPEG/JFIF file, or  the  standard
       input  if  no file is named, and produces a JPEG/JFIF file
       on the standard output.

       All switch names may be abbreviated; for  example,  -opti-
       mize  may be written -opt or -o.  Upper and lower case are
       equivalent.  British spellings are  also  accepted  (e.g.,
       -optimise),  though  for  brevity  these are not mentioned

       To specify the coded JPEG representation used in the  out-
       put file, jpegtran accepts a subset of the switches recog-
       nized by cjpeg:

              Perform optimization of  entropy  encoding  parame-

              Create progressive JPEG file.

       -restart N
              Emit  a  JPEG  restart  marker every N MCU rows, or
              every N MCU blocks if "B" is attached to  the  num-

       -scans file
              Use  the  scan  script  given in the specified text

       See cjpeg(1) for more details about  these  switches.   If
       you  specify none of these switches, you get a plain base-
       line-JPEG output file.  The quality setting and  so  forth
       are determined by the input file.

       The  image  can be losslessly transformed by giving one of
       these switches:

       -flip horizontal
              Mirror image horizontally (left-right).

       -flip vertical
              Mirror image vertically (top-bottom).

       -rotate 90
              Rotate image 90 degrees clockwise.

       -rotate 180
              Rotate image 180 degrees.

       -rotate 270
              Rotate image 270 degrees clockwise (or 90 ccw).

              Transpose image (across UL-to-LR axis).

              Transverse transpose (across UR-to-LL axis).

       The transpose transformation has no restrictions regarding
       image   dimensions.   The  other  transformations  operate
       rather oddly if the image dimensions are not a multiple of
       the  iMCU  size (usually 8 or 16 pixels), because they can
       only transform complete blocks of DCT coefficient data  in
       the desired way.

       jpegtran's  default behavior when transforming an odd-size
       image is designed  to  preserve  exact  reversibility  and
       mathematical  consistency  of  the transformation set.  As
       stated, transpose is able to flip the entire  image  area.
       Horizontal mirroring leaves any partial iMCU column at the
       right edge untouched, but is able to flip all rows of  the
       image.   Similarly,  vertical mirroring leaves any partial
       iMCU row at the bottom edge untouched, but is able to flip
       all  columns.   The  other  transforms  can be built up as
       sequences of transpose and flip  operations;  for  consis-
       tency,  their actions on edge pixels are defined to be the
       same as the end result of the corresponding transpose-and-
       flip sequence.

       For  practical use, you may prefer to discard any untrans-
       formable edge pixels rather than having a  strange-looking
       strip along the right and/or bottom edges of a transformed
       image.  To do this, add the -trim switch:

       -trim  Drop non-transformable edge blocks.

       Obviously, a transformation with -trim is not  reversible,
       so  strictly  speaking  jpegtran  with  this switch is not
       lossless.  Also, the  expected  mathematical  equivalences
       between  the transformations no longer hold.  For example,
       -rot 270 -trim trims only the bottom  edge,  but  -rot  90
       -trim followed by -rot 180 -trim trims both edges.

       Another not-strictly-lossless transformation switch is:

              Force grayscale output.

       This option discards the chrominance channels if the input
       image is YCbCr (ie, a standard color JPEG), resulting in a
       grayscale  JPEG  file.  The luminance channel is preserved
       exactly, so  this  is  a  better  method  of  reducing  to
       grayscale  than  decompression, conversion, and recompres-
       sion.  This switch is  particularly  handy  for  fixing  a
       monochrome  picture that was mistakenly encoded as a color
       JPEG.  (In such a case, the space savings from getting rid
       of  the near-empty chroma channels won't be large; but the
       decoding time for a grayscale JPEG is  substantially  less
       than that for a color JPEG.)

       jpegtran  also recognizes these switches that control what
       to do with "extra" markers, such as comment blocks:

       -copy none
              Copy no extra markers from source file.  This  set-
              ting  suppresses all comments and other excess bag-
              gage present in the source file.

       -copy comments
              Copy only comment  markers.   This  setting  copies
              comments  from  the  source  file, but discards any
              other inessential data.

       -copy all
              Copy all extra  markers.   This  setting  preserves
              miscellaneous  markers  found  in  the source file,
              such as JFIF thumbnails and Photoshop settings.  In
              some files these extra markers can be sizable.

       The  default  behavior  is  -copy comments.  (Note: in IJG
       releases v6 and v6a, jpegtran always did the equivalent of
       -copy none.)

       Additional switches recognized by jpegtran are:

       -maxmemory N
              Set limit for amount of memory to use in processing
              large images.  Value is in thousands of  bytes,  or
              millions of bytes if "M" is attached to the number.
              For example, -max 4m  selects  4000000  bytes.   If
              more space is needed, temporary files will be used.

       -outfile name
              Send output image to the named file, not  to  stan-
              dard output.

              Enable debug printout.  More -v's give more output.
              Also, version information is printed at startup.

       -debug Same as -verbose.

       This example converts a baseline JPEG file to  progressive

              jpegtran -progressive foo.jpg > fooprog.jpg

       This  example  rotates an image 90 degrees clockwise, dis-
       carding any unrotatable edge pixels:

              jpegtran -rot 90 -trim foo.jpg > foo90.jpg

              If this environment variable is set, its  value  is
              the  default  memory limit.  The value is specified
              as described for the  -maxmemory  switch.   JPEGMEM
              overrides the default value specified when the pro-
              gram was compiled, and itself is overridden  by  an
              explicit -maxmemory.

       cjpeg(1), djpeg(1), rdjpgcom(1), wrjpgcom(1)
       Wallace,  Gregory  K.  "The JPEG Still Picture Compression
       Standard", Communications of the ACM, April 1991 (vol. 34,
       no. 4), pp. 30-44.

       Independent JPEG Group

       Arithmetic coding is not supported for legal reasons.

       The transform options can't transform odd-size images per-
       fectly.  Use -trim if you don't like the  results  without

       The  entire image is read into memory and then written out
       again, even in cases where this  isn't  really  necessary.
       Expect swapping on large images, especially when using the
       more complex transform options.

                          3 August 1997                         1