Provided by: libjpeg-turbo-progs_2.0.1-0ubuntu2_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.

       -scale M/N
              Scale the output image by a factor M/N.  Currently the scale factor  must  be  M/8,
              where  M  is an integer between 1 and 16 inclusive, or any reduced fraction thereof
              (such as 1/2, 3/4, etc.)  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).   In  libjpeg-turbo,  the  fast  method  is
              generally  about  5-15%  faster  than the int method when using the x86/x86-64 SIMD
              extensions (results may  vary  with  other  SIMD  implementations,  or  when  using
              libjpeg-turbo  without  SIMD extensions.)  If the JPEG image was compressed using a
              quality level of 85 or below,  then  there  should  be  little  or  no  perceptible
              difference  between  the  two  algorithms.   When  decompressing  images  that were
              compressed using quality levels above 85, however, the difference between the  fast
              and  int methods becomes more pronounced.  With images compressed using quality=97,
              for instance, the fast method incurs generally  about  a  4-6  dB  loss  (in  PSNR)
              relative  to  the  int  method, but this can be larger for some images.  If you can
              avoid it, do not use the fast method when decompressing images that were compressed
              using quality levels above 97.  The algorithm often degenerates for such images and
              can actually produce a more lossy output image than if  the  JPEG  image  had  been
              compressed using lower quality levels.

       -dct float
              Use  floating-point  DCT  method.  The float method is mainly a legacy feature.  It
              does not produce significantly more accurate results than the int method, and it is
              much  slower.   The  float  method  may  also  give  different results on different
              machines due to varying roundoff behavior, whereas the integer methods should  give
              the same results on all machines.

       -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.

       -icc file
              Extract ICC color management profile to the specified file.

       -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
              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, an error will

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

              Load input file into memory before decompressing.   This  feature  was  implemented
              mainly as a way of testing the in-memory source manager (jpeg_mem_src().)

       -skip Y0,Y1
              Decompress  all  rows of the JPEG image except those between Y0 and Y1 (inclusive.)
              Note that if decompression scaling is being used, then Y0 and Y1  are  relative  to
              the scaled image dimensions.

       -crop WxH+X+Y
              Decompress  only  a  rectangular subregion of the image, starting at point X,Y with
              width W and height H.  If necessary, X will be shifted left  to  the  nearest  iMCU
              boundary,  and the width will be increased accordingly.  Note that if decompression
              scaling is being used, then X, Y, W,  and  H  are  relative  to  the  scaled  image
              dimensions.   Currently this option only works with the PBMPLUS (PPM/PGM), GIF, and
              Targa output formats.

              Enable debug printout.  More -v's give more output.  Also, version  information  is
              printed at startup.

       -debug Same as -verbose.

              Print version information and exit.


       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

       This file was modified by The libjpeg-turbo Project to include only  information  relevant
       to  libjpeg-turbo,  to wordsmith certain sections, and to describe features not present in


       Support for compressed GIF output files was removed in djpeg v6b due to concerns over  the
       Unisys LZW patent.  Although this patent expired in 2006, djpeg still lacks compressed GIF
       support, for these historical reasons.  (Conversion of JPEG files to GIF is usually a  bad
       idea  anyway,  since  GIF  is  a 256-color format.)  The uncompressed GIF files that djpeg
       generates are larger than they should be, but they are readable by standard GIF decoders.

                                         13 November 2017                                DJPEG(1)