Provided by: libjpeg-turbo-progs_1.1.90+svn733-0ubuntu4_amd64 bug


       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  conversion.   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, -optimize 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 below.

       To  specify  the  coded  JPEG  representation  used in the output file, jpegtran accepts a
       subset of the switches recognized by cjpeg:

              Perform optimization of entropy encoding parameters.

              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 number.

              Use arithmetic coding.

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

       See  cjpeg(1)  for  more  details  about  these  switches.   If  you specify none of these
       switches, you get a plain baseline-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 consistency, 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 untransformable 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.

              If  you  are  only  interested in perfect transformations, add the -perfect switch.
              This causes jpegtran to fail with an error if the transformation is not perfect.

              For example, you may want to do

              (jpegtran -rot 90 -perfect foo.jpg || djpeg foo.jpg | pnmflip -r90 | cjpeg)

              to do a perfect rotation, if available, or an approximated one if not.

       -crop WxH+X+Y
              Crop the image to a rectangular region of width W and height H, starting  at  point
              X,Y.   The  lossless crop feature discards data outside of a given image region but
              losslessly preserves what is inside.  Like the rotate and flip transforms, lossless
              crop  is  restricted  by  the  current  JPEG  format;  the upper left corner of the
              selected region must fall on an iMCU boundary.  If it doesn't, then it is  silently
              moved  up  and/or  left  to  the  nearest  iMCU boundary (the lower right corner is

       Other not-strictly-lossless transformation switches are:

              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  recompression.  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 setting suppresses all  comments  and
              other excess baggage present in the source file.

       -copy comments
              Copy  only  comment markers.  This setting copies comments from the source file but
              discards any other data which is inessential for image display.

       -copy all
              Copy all extra markers.  This setting preserves miscellaneous markers found in  the
              source  file,  such as JFIF thumbnails, Exif data, 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 standard 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 form:

              jpegtran -progressive foo.jpg > fooprog.jpg

       This  example  rotates  an  image  90  degrees  clockwise, discarding any unrotatable edge

              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 program 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


       The transform options can't transform odd-size images perfectly.  Use -trim or -perfect if
       you don't like the results.

       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.

                                         11 October 2010                              JPEGTRAN(1)