Provided by: libjpeg-turbo-progs_2.1.5-2ubuntu1_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,  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
              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.

       -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  algorithm  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 generally 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 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 [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 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.

       -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
              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
              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 images 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.   Currently this option only works with the PBMPLUS (PPM/PGM), GIF, and
              Targa output formats.

       -strict
              Treat all warnings as fatal.  This feature also  demonstrates  a  method  by  which
              applications  can  guard  against attacks instigated by specially-crafted malformed
              JPEG images.  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
       default  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
       libjpeg.

                                         4 November 2020                                 DJPEG(1)