JPEGTRAN(1)						      General Commands Manual						       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 transforma-
       tions 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.

       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  opera-
       tions;  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.

       -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 out-
	      side  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:

       -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,  conver-
	      sion,  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.

       -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, 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  sec-
       tions.

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.

								  1 January 2013						       JPEGTRAN(1)