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CentOS 7.0 - man page for jpegtran (centos section 1)

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 degra-
       dation 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.

       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.,  -opti-
       mise), though for brevity these are not mentioned below.

       To  specify the coded JPEG representation used in the output file, jpegtran accepts a sub-
       set 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 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 image (across UL-to-LR axis).

	      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.  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 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 equiva-
	      lences  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

	      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

       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  decom-
	      pression,  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 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 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.

	      Enable debug printout.  More -v's give more output.  Also, version  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 pix-

	      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 specified 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 sections.

       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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