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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 men-
       tioned 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  compatibil-
              ity.

       -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 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 emit-
              ted.

       -pnm   Select PBMPLUS (PPM/PGM) output format (this is the default format).  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; 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 quan-
              tizing  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 mem-
              ory, 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, 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 set-
       tings.

       -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 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

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.



                                11 October 2010                       DJPEG(1)