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 standard output.  PBM-
       PLUS  (PPM/PGM),  BMP,  GIF, Targa, or RLE (Utah Raster Toolkit) output
       format can be selected.  (RLE is supported only if the URT  library  is

       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 out-
              put.   (The  default options are chosen for highest quality out-
              put.)  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 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 emitted.

       -pnm   Select PBMPLUS (PPM/PGM) output format (this is the default for-
              mat).   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; other-
              wise, 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  accu-
              rate 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 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

       -outfile name
              Send output image to the named file, not to standard output.

              Enable  debug printout.  More -v's give more output.  Also, ver-
              sion 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  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  signifi-
       cant 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 speci-
              fied 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

       Still not as fast as we'd like.

                                22 August 1997                        DJPEG(1)