Provided by: libjpeg-progs_9b-2_amd64 bug


       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.  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 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.

              Force  grayscale  output  even  if  JPEG  file  is  color.   Useful  for viewing on
              monochrome displays; also, djpeg runs noticeably faster in this mode.

       -rgb   Force RGB output even if JPEG file is  grayscale.   This  is  provided  to  support
              applications that don't want to cope with grayscale as a separate case.

       -scale M/N
              Scale  the output image by a factor M/N.  Currently supported scale factors are M/N
              with all M from 1 to 16, where N is the source DCT size, which is  8  for  baseline
              JPEG.   If  the  /N  part  is  omitted,  then M specifies the DCT scaled size to be
              applied on the given input.  For baseline JPEG this is equivalent to  M/8  scaling,
              since the source DCT size for baseline JPEG is 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 grayscale; 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  grayscale;  otherwise,
              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 grayscale or if -grayscale is specified; otherwise  PPM
              is emitted.

       -rle   Select RLE output format.  (Requires URT library.)

       -targa Select  Targa  output  format.   Grayscale  format  is  emitted if the JPEG file is
              grayscale 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.

              Don't use high-quality upsampling.

              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
              grayscale 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.

              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 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 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 overrides the
              default value specified 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


       To avoid the Unisys LZW patent (now  expired),  djpeg  produces  uncompressed  GIF  files.
       These are larger than they should be, but are readable by standard GIF decoders.

                                           26 July 2015                                  DJPEG(1)