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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

       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.)

       If W or H is larger than the width/height of the input image, then the output image is expanded in  size,
       and  the  expanded region is filled in with zeros (neutral gray).  Attaching an 'f' character ("flatten")
       to the width number will cause each block in the expanded region to be filled in with the DC  coefficient
       of  the  nearest block in the input image rather than grayed out.  Attaching an 'r' character ("reflect")
       to the width number will cause the expanded region to be filled in with repeated reflections of the input
       image rather than grayed out.

       A  complementary  lossless wipe option is provided to discard (gray out) data inside a given image region
       while losslessly preserving what is outside:

       -wipe WxH+X+Y
              Wipe (gray out) a rectangular region of width W and height H from the  input  image,  starting  at
              point X,Y.

       Attaching  an  'f'  character ("flatten") to the width number will cause the region to be filled with the
       average of adjacent blocks rather than grayed out.  If the wipe region and the region  outside  the  wipe
       region,  when  adjusted  to  the  nearest  iMCU boundary, form two horizontally adjacent rectangles, then
       attaching an 'r' character ("reflect") to the width number will cause the wipe region to be  filled  with
       repeated reflections of the outside region rather than grayed out.

       A  lossless drop option is also provided, which allows another JPEG image to be inserted ("dropped") into
       the input image data at a given position, replacing the existing image data at that position:

       -drop +X+Y filename
              Drop (insert) another image at point X,Y

       Both the input image and the drop image must have the same subsampling level.  It is best  if  they  also
       have the same quantization (quality.)  Otherwise, the quantization of the output image will be adapted to
       accommodate the higher of the input image quality and the drop image quality.  The  trim  option  can  be
       used  with  the drop option to requantize the drop image to match the input image.  Note that a grayscale
       image can be dropped into a full-color image or vice versa, as  long  as  the  full-color  image  has  no
       vertical  subsampling.   If  the  input  image  is  grayscale  and the drop image is full-color, then the
       chrominance channels from the drop image will be discarded.

       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 icc
              Copy  only  ICC  profile  markers.   This  setting copies the ICC profile 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:

       -icc file
              Embed  ICC  color  management  profile contained in the specified file.  Note that this will cause
              jpegtran to ignore any APP2 markers in the  input  file,  even  if  -copy  all  or  -copy  icc  is
              specified.

       -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.

       -maxscans N
              Abort  if the input image contains more than N scans.  This feature demonstrates a method by which
              applications can guard against denial-of-service attacks instigated by specially-crafted malformed
              JPEG  images  containing  numerous  scans with missing image data or image data consisting only of
              "EOB runs" (a feature of progressive JPEG images that allows potentially hundreds of thousands  of
              adjoining  zero-value  pixels  to be represented using only a few bytes.)  Attempting to transform
              such malformed JPEG images can cause excessive CPU activity, since  the  decompressor  must  fully
              process each scan (even if the scan is corrupt) before it can proceed to the next scan.

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

       -report
              Report transformation progress.

       -strict
              Treat  all  warnings  as fatal.  This feature also demonstrates a method by which applications can
              guard against attacks instigated by specially-crafted malformed JPEG images.  Enabling this option
              will cause the decompressor to abort if the input image contains incomplete or corrupt image data.

       -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.

                                                  13 July 2021                                       JPEGTRAN(1)