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jpegtran.1.gz

JPEGTRAN(1)                                              JPEGTRAN(1)



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 re‐
       arrangements  of the image data, for example turning an image
       from landscape to portrait format by rotation.

       jpegtran works by rearranging the compressed data (DCT  coef‐
       ficients), without ever fully decoding the image.  Therefore,
       its transformations are lossless: there is no image  degrada‐
       tion  at  all, which would not be true if you used djpeg fol‐
       lowed 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.

OPTIONS
       All switch names may be abbreviated; for  example,  -optimize
       may  be written -opt or -o.  Upper and lower case are equiva‐
       lent.  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.

       -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 trans‐
       form 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  mathe‐
       matical  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.   Simi‐
       larly,  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  untrans‐
       formable  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.

       Another not-strictly-lossless transformation switch is:

       -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  excess  baggage
              present in the source file.

       -copy comments
              Copy  only  comment markers.  This setting copies com‐
              ments from the source file,  but  discards  any  other
              inessential data.

       -copy all
              Copy  all  extra markers.  This setting preserves mis‐
              cellaneous markers found in the source file,  such  as
              JFIF thumbnails 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 mil‐
              lions 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.

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

       -debug Same as -verbose.

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, discard‐
       ing 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  over‐
              rides 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

BUGS
       Arithmetic coding is not supported for legal reasons.

       The  transform  options  can't transform odd-size images per‐
       fectly.  Use -trim if you don't like the results without it.

       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.



                            3 August 1997                JPEGTRAN(1)
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