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

DJPEG(1)			     General Commands Manual				 DJPEG(1)

       djpeg - decompress a JPEG file to an image file

       djpeg [ options ] [ filename ]

       djpeg  decompresses  the  named	JPEG file, or the standard input if no file is named, and
       produces an image file on the standard output.  PBMPLUS (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.)

       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 output.  (The default
	      options are chosen for highest quality output.)  Currently, this is  equivalent  to
	      -dct fast -nosmooth -onepass -dither ordered.

	      Force  gray-scale  output  even if JPEG file is color.  Useful for viewing on mono-
	      chrome 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 format).  PGM is emit-
	      ted if the JPEG file is gray-scale or if -grayscale is specified; otherwise PPM  is

       -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; otherwise, 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 accurate 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 pro-
	      ducing 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 -col-
	      ors and -onepass.

	      Use a faster, lower-quality upsampling routine.

	      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.

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

       -debug Same as -verbose.

       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

       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  sel-
       dom 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 significant in practice.

	      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), 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.

       Independent JPEG Group

       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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