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libjpeg - A library for manipulating JPEG image format files.…  more info»


DJPEG(1)                                                    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  stan‐
       dard  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

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

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

              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

       -bmp   Select BMP output format (Windows flavor).  8-bit col‐
              ormapped 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; oth‐
              erwise, 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
              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  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.

              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 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

              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

              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 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.

              If this environment variable is set, its value is  the
              default  memory  limit.   The  value  is  specified as
              described for the -maxmemory  switch.   JPEGMEM  over‐
              rides the default value specified when the program was
              compiled, and itself  is  overridden  by  an  explicit

       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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