jpegtran man page on Archlinux

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JPEGTRAN(1)							   JPEGTRAN(1)

       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.

       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.

       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:

	      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 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

       -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 trans‐
	      formations 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.

       -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 substan‐
	      tially 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

       -copy comments
	      Copy  only  comment  markers.  This setting copies comments from
	      the source file but discards any other data that is  inessential
	      for image display.

       -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 mark‐
	      ers 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

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

	      Enable  debug printout.  More -v's give more output.  Also, ver‐
	      sion information is printed at startup.

       -debug Same as -verbose.

       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 speci‐
	      fied 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 sec‐

       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.

				1 January 2013			   JPEGTRAN(1)

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