Provided by: libjpeg-progs_9d-1_amd64 bug

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, which it isn't on most non-Unix systems.)

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  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 (LZW compressed).  Since GIF does not  support  more  than
              256 colors, -colors 256 is assumed (unless you specify a smaller number of colors).
              If you specify -fast, the default number of colors is 216.

       -gif0  Select GIF output format (uncompressed).  Since GIF does not support more than  256
              colors, -colors 256 is assumed (unless you specify a smaller number of colors).  If
              you specify -fast, the default number of colors is 216.

       -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.

       -nosmooth
              Don't use high-quality upsampling.

       -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
              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.

       -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 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

                                          28 April 2019                                  DJPEG(1)