djpeg man page on Ultrix

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DJPEG(1)							      DJPEG(1)

NAME
       djpeg - decompress a JPEG file to an image file

SYNOPSIS
       djpeg [ options ] [ filename ]

DESCRIPTION
       djpeg  decompresses  the	 named	JPEG file, or the standard input if no
       file is named, and produces an image file on the standard output.  PBM‐
       PLUS  (PPM/PGM),	 BMP,  GIF, Targa, or RLE (Utah Raster Toolkit) output
       format can be selected.	(RLE is supported only if the URT  library  is
       available.)

OPTIONS
       All  switch  names  may	be abbreviated; for example, -grayscale may be
       written -gray or -gr.  Most of the "basic" switches can be  abbreviated
       to  as little as one letter.  Upper and lower case are equivalent (thus
       -BMP is the same as -bmp).  British spellings are also accepted	(e.g.,
       -greyscale), though for brevity these are not mentioned below.

       The basic switches are:

       -colors N
	      Reduce  image  to	 at most N colors.  This reduces the number of
	      colors used in the output image, so that it can be displayed  on
	      a	 colormapped  display  or stored in a colormapped file format.
	      For example, if you have an 8-bit display, you'd need to	reduce
	      to 256 or fewer colors.

       -quantize N
	      Same  as -colors.	 -colors is the recommended name, -quantize is
	      provided only for backwards compatibility.

       -fast  Select recommended processing options for fast, low quality out‐
	      put.   (The  default options are chosen for highest quality out‐
	      put.)  Currently, this is	 equivalent  to	 -dct  fast  -nosmooth
	      -onepass -dither ordered.

       -grayscale
	      Force  gray-scale output even if JPEG file is color.  Useful for
	      viewing on monochrome  displays;	also,  djpeg  runs  noticeably
	      faster in this mode.

       -scale M/N
	      Scale  the  output  image	 by a factor M/N.  Currently the scale
	      factor must be 1/1, 1/2, 1/4, or 1/8.  Scaling is handy  if  the
	      image  is	 larger than your screen; also, djpeg runs much faster
	      when scaling down the output.

       -bmp   Select BMP output format (Windows	 flavor).   8-bit  colormapped
	      format  is  emitted if -colors or -grayscale is specified, or if
	      the JPEG file is gray-scale; otherwise, 24-bit full-color format
	      is emitted.

       -gif   Select  GIF output format.  Since GIF does not support more than
	      256 colors, -colors 256 is assumed (unless you specify a smaller
	      number of colors).

       -os2   Select  BMP  output format (OS/2 1.x flavor).  8-bit colormapped
	      format is emitted if -colors or -grayscale is specified,	or  if
	      the JPEG file is gray-scale; otherwise, 24-bit full-color format
	      is emitted.

       -pnm   Select PBMPLUS (PPM/PGM) output format (this is the default for‐
	      mat).   PGM  is  emitted	if  the	 JPEG file is gray-scale or if
	      -grayscale is specified; otherwise PPM is emitted.

       -rle   Select RLE output format.	 (Requires URT library.)

       -targa Select Targa output format.  Gray-scale format is emitted if the
	      JPEG  file  is  gray-scale or if -grayscale is specified; other‐
	      wise, colormapped format is emitted  if  -colors	is  specified;
	      otherwise, 24-bit full-color format is emitted.

       Switches for advanced users:

       -dct int
	      Use integer DCT method (default).

       -dct fast
	      Use fast integer DCT (less accurate).

       -dct float
	      Use  floating-point  DCT	method.	  The  float  method  is  very
	      slightly more accurate than the int method, but is  much	slower
	      unless your machine has very fast floating-point hardware.  Also
	      note that results of the floating-point method may vary slightly
	      across  machines, while the integer methods should give the same
	      results everywhere.  The fast integer method is much less	 accu‐
	      rate than the other two.

       -dither fs
	      Use Floyd-Steinberg dithering in color quantization.

       -dither ordered
	      Use ordered dithering in color quantization.

       -dither none
	      Do  not use dithering in color quantization.  By default, Floyd-
	      Steinberg dithering is applied when quantizing colors;  this  is
	      slow but usually produces the best results.  Ordered dither is a
	      compromise between speed and quality; no dithering is  fast  but
	      usually  looks  awful.   Note that these switches have no effect
	      unless color quantization is being done.	Ordered dither is only
	      available in -onepass mode.

       -map file
	      Quantize	to  the colors used in the specified image file.  This
	      is useful for producing  multiple	 files	with  identical	 color
	      maps, or for forcing a predefined set of colors to be used.  The
	      file must be a GIF or PPM file. This  option  overrides  -colors
	      and -onepass.

       -nosmooth
	      Use a faster, lower-quality upsampling routine.

       -onepass
	      Use  one-pass  instead of two-pass color quantization.  The one-
	      pass method is faster and needs less memory, but it  produces  a
	      lower-quality  image.   -onepass	is ignored unless you also say
	      -colors N.  Also, the one-pass method is always used  for	 gray-
	      scale output (the two-pass method is no improvement then).

       -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 decompresses the JPEG file foo.jpg, quantizes  it  to  256
       colors, and saves the output in 8-bit BMP format in foo.bmp:

	      djpeg -colors 256 -bmp foo.jpg > foo.bmp

HINTS
       To  get	a  quick preview of an image, use the -grayscale and/or -scale
       switches.  -grayscale -scale 1/8 is the fastest case.

       Several options are available that trade	 off  image  quality  to  gain
       speed.  -fast turns on the recommended settings.

       -dct  fast and/or -nosmooth gain speed at a small sacrifice in quality.
       When producing a color-quantized image,	-onepass  -dither  ordered  is
       fast  but  much	lower quality than the default behavior.  -dither none
       may give acceptable results in two-pass mode, but is  seldom  tolerable
       in one-pass mode.

       If  you are fortunate enough to have very fast floating point hardware,
       -dct float may be even faster than -dct fast.   But  on	most  machines
       -dct float is slower than -dct int; in this case it is not worth using,
       because its theoretical accuracy advantage is too small to be  signifi‐
       cant in practice.

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), jpegtran(1), rdjpgcom(1), wrjpgcom(1)
       ppm(5), pgm(5)
       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.

       To avoid the Unisys LZW patent, djpeg produces uncompressed GIF	files.
       These  are larger than they should be, but are readable by standard GIF
       decoders.

       Still not as fast as we'd like.

				22 August 1997			      DJPEG(1)
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