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.

   For  EXIF  files and JPEG files containing Exif data, you may prefer to
   use exiftran instead.

   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.  However, while the
   image data is losslessly transformed, metadata can be removed.  See the
   -copy option for specifics.

   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

   -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.

   This  version  of  jpegtran  also  offers a lossless crop option, which
   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  unchanged.)  Thus, the output image covers at
   least the requested region, but it may cover more.  The  adjustment  of
   the  region  dimensions  may be optionally disabled by attaching an 'f'
   character ("force") to the width or height number.

   The image can be losslessly cropped by giving the switch:

   -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 unchanged.)

   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 metadata in the source file.

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

   -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.   Note  that  this  option  will  copy
          thumbnails as-is; they will not be transformed.

   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.

          Print version information and exit.


   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

   This  file  was  modified  by The libjpeg-turbo Project to include only
   information  relevant  to  libjpeg-turbo  and  to   wordsmith   certain


   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.

                           18 February 2016                    JPEGTRAN(1)

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