djpeg man page on Archlinux

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DJPEG(1)							      DJPEG(1)

       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.  PBM‐
       PLUS  (PPM/PGM),	 BMP,  GIF, Targa, or RLE (Utah Raster Toolkit) output
       format can be selected.	(RLE is supported only if the URT  library  is

       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 out‐
	      put.   (The  default options are chosen for highest quality out‐
	      put.)  Currently, this is	 equivalent  to	 -dct  fast  -nosmooth
	      -onepass -dither ordered.

	      Force  gray-scale output even if JPEG file is color.  Useful for
	      viewing on monochrome  displays;	also,  djpeg  runs  noticeably
	      faster in this mode.

       -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 gray-scale; 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 gray-scale; otherwise, 24-bit full-color format
	      is emitted.

       -pnm   Select PBMPLUS (PPM/PGM) output format (this is the default for‐
	      mat).  PGM is emitted if the  JPEG  file	is  gray-scale	or  if
	      -grayscale is specified; otherwise PPM is emitted.

       -rle   Select RLE output format.	 (Requires URT library.)

       -targa Select Targa output format.  Gray-scale format is emitted if the
	      JPEG file is gray-scale or if -grayscale	is  specified;	other‐
	      wise,  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 accu‐
	      rate 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.

	      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 gray-
	      scale 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

       -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().)

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

       -debug Same as -verbose.

       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 signifi‐
       cant 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	speci‐
	      fied  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 libjpeg.

       To  avoid the Unisys LZW patent, djpeg produces uncompressed GIF files.
       These are larger than they should be, but are readable by standard  GIF

				18 January 2013			      DJPEG(1)

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