Unix/Linux Go Back    

RedHat 9 (Linux i386) - man page for jpegtran (redhat section 1)

Linux & Unix Commands - Search Man Pages
Man Page or Keyword Search:   man
Select Man Page Set:       apropos Keyword Search (sections above)

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.

       -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  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:

	      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 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.

	      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

       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)
Unix & Linux Commands & Man Pages : ©2000 - 2018 Unix and Linux Forums

All times are GMT -4. The time now is 02:58 PM.