jpegtran



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 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 conver-
       sion.   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  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, jpeg-
       tran 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 dimen-
       sions.  The other transformations operate rather  oddly  if  the  image
       dimensions  are  not  a multiple of the iMCU size (usually 8 or 16 pix-
       els), because they can only transform complete blocks  of  DCT  coeffi-
       cient 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 con-
       sistency,  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.

       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 recompres-
       sion.   This  switch is particularly handy for fixing a monochrome pic-
       ture 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  comments  from
              the source file, but discards any other inessential data.

       -copy all
              Copy  all  extra  markers.  This setting preserves miscellaneous
              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 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
              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, ver-
              sion 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,  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 speci-
              fied 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  perfectly.   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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