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djpeg (1)
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    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 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; other-
              wise, 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; other-
              wise, 24-bit full-color format is emitted.
    
         -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 emit-
              ted  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 for-
              mat 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 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 mil-
              lions 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 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 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 com-
              piled, and itself is overridden by  an  explicit  -max-
              memory.
    
    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.
    
    
    
    


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