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       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

       -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 transformation, add the
              -perfect switch:

              Fails 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

       We also offer a lossless-crop option, which  discards  data  outside  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 this does not hold for the given crop parameters,
       we  silently  move  the upper left corner up and/or left to make it so,
       simultaneously increasing the region dimensions to keep the lower right
       crop  corner  unchanged.   (Thus,  the output image covers at least the
       requested region, but may cover more.)

       The image can be losslessly cropped by giving the switch:

       -crop WxH+X+Y
              Crop to a rectangular subarea of width W, height H  starting  at
              point X,Y.

       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.)

       -scale M/N
              Scale the output image by a factor M/N.

              Currently supported scale factors are M/N with all M from  1  to
              16,  where  N  is  the  source DCT size, which is 8 for baseline
              JPEG.  If the /N part is  omitted,  then  M  specifies  the  DCT
              scaled size to be applied on the given input.  For baseline JPEG
              this is equivalent to M/8 scaling, since the source DCT size for
              baseline  JPEG  is  8.   Caution:  An implementation of the JPEG
              SmartScale extension is required for this  feature.   SmartScale
              enabled  JPEG  is  not  yet widely implemented, so many decoders
              will be unable to view a SmartScale extended JPEG file at all.

       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

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

       -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

       -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 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  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.

                               28 December 2009