man > DJPEG

DJPEG(1)                               General Commands Manual                              DJPEG(1)

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 pro‐
       duces an image file on the standard output.  PBMPLUS (PPM/PGM), BMP,  GIF,  or  Targa  output
       format can be selected.

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  out‐
              put  image,  so  that it can be displayed on a colormapped display or stored in a col‐
              ormapped 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 op‐
              tions 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.

       -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 (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.

       -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 accurate integer DCT method (default).

       -dct fast
              Use  less  accurate  integer  DCT  method [legacy feature].  When the Independent JPEG
              Group's software was first released in 1991, the decompression time for a  1-megapixel
              JPEG image on a mainstream PC was measured in minutes.  Thus, the fast integer DCT al‐
              gorithm  provided  noticeable  performance  benefits.  On modern CPUs running libjpeg-
              turbo, however, the decompression time for a 1-megapixel JPEG  image  is  measured  in
              milliseconds,  and  thus  the performance benefits of the fast algorithm are much less
              noticeable.  On modern x86/x86-64 CPUs that support AVX2 instructions,  the  fast  and
              int methods have similar performance.  On other types of CPUs, the fast method is gen‐
              erally about 5-15% faster than the int method.

              If  the  JPEG  image  was  compressed using a quality level of 85 or below, then there
              should be little or no perceptible quality  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 im‐
              ages.  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  [legacy feature].  The float method 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 ma‐
              chines.

       -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 quantiza‐
              tion 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.

       -nosmooth
              Use a faster, lower-quality upsampling routine.

       -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 un‐
              less 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 thou‐
              sands 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 occur.

       -maxscans N
              Abort  if  the  JPEG  image  contains  more than N scans.  This feature demonstrates a
              method by which applications can guard against denial-of-service attacks instigated by
              specially-crafted malformed JPEG images containing numerous scans with  missing  image
              data or image data consisting only of "EOB runs" (a feature of progressive JPEG images
              that  allows  potentially  hundreds  of thousands of adjoining zero-value pixels to be
              represented using only a few bytes.)  Attempting to decompress such malformed JPEG im‐
              ages can cause excessive CPU activity, since the decompressor must fully process  each
              scan (even if the scan is corrupt) before it can proceed to the next scan.

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

       -memsrc
              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().)

       -report
              Report decompression progress.

       -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.  Cur‐
              rently this option only works with the PBMPLUS (PPM/PGM), GIF, and Targa  output  for‐
              mats.

       -strict
              Treat  all warnings as fatal.  This feature also demonstrates a method by which appli‐
              cations can guard against attacks instigated by specially-crafted malformed  JPEG  im‐
              ages.   Enabling  this  option  will cause the decompressor to abort if the JPEG image
              contains incomplete or corrupt image data.

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

       -debug Same as -verbose.

       -version
              Print version information and exit.

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  de‐
       fault  behavior.   -dither  none  may give acceptable results in two-pass mode, but is seldom
       tolerable in one-pass mode.

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

       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 lib‐
       jpeg.

                                           4 November 2020                                  DJPEG(1)