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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 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
       available.)

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 out-
	      put.   (The  default options are chosen for highest quality out-
	      put.)  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.	 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 for-
	      mat).  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;	other-
	      wise, 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	decom-
	      pressing	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 qual-
	      ity=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.

       -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, temporary files will be
	      used.

       -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().)

       -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	dimen-
	      sions.

       -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.

       -verbose
	      Enable debug printout.  More -v's	give more output.  Also,  ver-
	      sion 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.

       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.

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	speci-
	      fied  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.

ISSUES
       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  any-
       way, 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.

			       18 February 2016			      DJPEG(1)

NAME | SYNOPSIS | DESCRIPTION | OPTIONS | EXAMPLES | HINTS | ENVIRONMENT | SEE ALSO | AUTHOR | ISSUES

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