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NAME

       jpegtran - lossless transformation of JPEG files

SYNOPSIS

       jpegtran [ options ] [ filename ]

DESCRIPTION

       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.

OPTIONS

       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:

       -optimize
              Perform optimization of entropy encoding parameters.

       -progressive
              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.

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

       -transverse
              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.

       -perfect
              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:

       -grayscale
              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, an error will occur.

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

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

       -debug Same as -verbose.

       -version
              Print version information and exit.

EXAMPLES

       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

ENVIRONMENT

       JPEGMEM
              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.

SEE ALSO

       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.

AUTHOR

       Independent JPEG Group

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

BUGS

       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 March 2017                                      JPEGTRAN(1)