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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.  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.)

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 output.  (The default
	      options are chosen for highest quality output.)  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 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  M/8,
	      where  M	is an integer between 1 and 16 inclusive, or any reduced fraction thereof
	      (such as 1/2, 3/4, etc.)	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
	      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; 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.

       -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.

       -memsrc
	      Load  input  file  into  memory before decompressing.  This feature was implemented
	      mainly as a way of testing the in-memory source manager (jpeg_mem_src().)

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

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), 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

       This file was modified by The libjpeg-turbo Project to include only  information  relevant
       to  libjpeg-turbo,  to wordsmith certain sections, and to describe features not present in
       libjpeg.

BUGS
       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.

					 18 January 2013				 DJPEG(1)
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