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)