1DJPEG(1)                    General Commands Manual                   DJPEG(1)
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NAME

6       djpeg - decompress a JPEG file to an image file
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SYNOPSIS

9       djpeg [ options ] [ filename ]
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DESCRIPTION

12       djpeg  decompresses  the  named  JPEG file, or the standard input if no
13       file is named, and produces an image file on the standard output.  PBM‐
14       PLUS  (PPM/PGM),  BMP,  GIF, Targa, or RLE (Utah Raster Toolkit) output
15       format can be selected.  (RLE is supported only if the URT  library  is
16       available.)
17

OPTIONS

19       All  switch  names  may  be abbreviated; for example, -grayscale may be
20       written -gray or -gr.  Most of the "basic" switches can be  abbreviated
21       to  as little as one letter.  Upper and lower case are equivalent (thus
22       -BMP is the same as -bmp).  British spellings are also accepted  (e.g.,
23       -greyscale), though for brevity these are not mentioned below.
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25       The basic switches are:
26
27       -colors N
28              Reduce  image  to  at most N colors.  This reduces the number of
29              colors used in the output image, so that it can be displayed  on
30              a  colormapped  display  or stored in a colormapped file format.
31              For example, if you have an 8-bit display, you'd need to  reduce
32              to 256 or fewer colors.
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34       -quantize N
35              Same  as -colors.  -colors is the recommended name, -quantize is
36              provided only for backwards compatibility.
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38       -fast  Select recommended processing options for fast, low quality out‐
39              put.   (The  default options are chosen for highest quality out‐
40              put.)  Currently, this is  equivalent  to  -dct  fast  -nosmooth
41              -onepass -dither ordered.
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43       -grayscale
44              Force  gray-scale output even if JPEG file is color.  Useful for
45              viewing on monochrome  displays;  also,  djpeg  runs  noticeably
46              faster in this mode.
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48       -scale M/N
49              Scale  the  output  image  by a factor M/N.  Currently the scale
50              factor must be 1/1, 1/2, 1/4, or 1/8.  Scaling is handy  if  the
51              image  is  larger than your screen; also, djpeg runs much faster
52              when scaling down the output.
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54       -bmp   Select BMP output format (Windows  flavor).   8-bit  colormapped
55              format  is  emitted if -colors or -grayscale is specified, or if
56              the JPEG file is gray-scale; otherwise, 24-bit full-color format
57              is emitted.
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59       -gif   Select  GIF output format.  Since GIF does not support more than
60              256 colors, -colors 256 is assumed (unless you specify a smaller
61              number of colors).
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63       -os2   Select  BMP  output format (OS/2 1.x flavor).  8-bit colormapped
64              format is emitted if -colors or -grayscale is specified,  or  if
65              the JPEG file is gray-scale; otherwise, 24-bit full-color format
66              is emitted.
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68       -pnm   Select PBMPLUS (PPM/PGM) output format (this is the default for‐
69              mat).   PGM  is  emitted  if  the  JPEG file is gray-scale or if
70              -grayscale is specified; otherwise PPM is emitted.
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72       -rle   Select RLE output format.  (Requires URT library.)
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74       -targa Select Targa output format.  Gray-scale format is emitted if the
75              JPEG  file  is  gray-scale or if -grayscale is specified; other‐
76              wise, colormapped format is emitted  if  -colors  is  specified;
77              otherwise, 24-bit full-color format is emitted.
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79       Switches for advanced users:
80
81       -dct int
82              Use integer DCT method (default).
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84       -dct fast
85              Use fast integer DCT (less accurate).
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87       -dct float
88              Use  floating-point  DCT  method.   The  float  method  is  very
89              slightly more accurate than the int method, but is  much  slower
90              unless your machine has very fast floating-point hardware.  Also
91              note that results of the floating-point method may vary slightly
92              across  machines, while the integer methods should give the same
93              results everywhere.  The fast integer method is much less  accu‐
94              rate than the other two.
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96       -dither fs
97              Use Floyd-Steinberg dithering in color quantization.
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99       -dither ordered
100              Use ordered dithering in color quantization.
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102       -dither none
103              Do  not use dithering in color quantization.  By default, Floyd-
104              Steinberg dithering is applied when quantizing colors;  this  is
105              slow but usually produces the best results.  Ordered dither is a
106              compromise between speed and quality; no dithering is  fast  but
107              usually  looks  awful.   Note that these switches have no effect
108              unless color quantization is being done.  Ordered dither is only
109              available in -onepass mode.
110
111       -map file
112              Quantize  to  the colors used in the specified image file.  This
113              is useful for producing  multiple  files  with  identical  color
114              maps, or for forcing a predefined set of colors to be used.  The
115              file must be a GIF or PPM file. This  option  overrides  -colors
116              and -onepass.
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118       -nosmooth
119              Use a faster, lower-quality upsampling routine.
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121       -onepass
122              Use  one-pass  instead of two-pass color quantization.  The one-
123              pass method is faster and needs less memory, but it  produces  a
124              lower-quality  image.   -onepass  is ignored unless you also say
125              -colors N.  Also, the one-pass method is always used  for  gray-
126              scale output (the two-pass method is no improvement then).
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128       -maxmemory N
129              Set  limit  for  amount  of  memory  to  use in processing large
130              images.  Value is in thousands of bytes, or millions of bytes if
131              "M"  is  attached  to  the number.  For example, -max 4m selects
132              4000000 bytes.  If more space is needed, temporary files will be
133              used.
134
135       -outfile name
136              Send output image to the named file, not to standard output.
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138       -verbose
139              Enable  debug printout.  More -v's give more output.  Also, ver‐
140              sion information is printed at startup.
141
142       -debug Same as -verbose.
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EXAMPLES

145       This example decompresses the JPEG file foo.jpg, quantizes  it  to  256
146       colors, and saves the output in 8-bit BMP format in foo.bmp:
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148              djpeg -colors 256 -bmp foo.jpg > foo.bmp
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HINTS

151       To  get  a  quick preview of an image, use the -grayscale and/or -scale
152       switches.  -grayscale -scale 1/8 is the fastest case.
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154       Several options are available that trade  off  image  quality  to  gain
155       speed.  -fast turns on the recommended settings.
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157       -dct  fast and/or -nosmooth gain speed at a small sacrifice in quality.
158       When producing a color-quantized image,  -onepass  -dither  ordered  is
159       fast  but  much  lower quality than the default behavior.  -dither none
160       may give acceptable results in two-pass mode, but is  seldom  tolerable
161       in one-pass mode.
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163       If  you are fortunate enough to have very fast floating point hardware,
164       -dct float may be even faster than -dct fast.   But  on  most  machines
165       -dct float is slower than -dct int; in this case it is not worth using,
166       because its theoretical accuracy advantage is too small to be  signifi‐
167       cant in practice.
168

ENVIRONMENT

170       JPEGMEM
171              If  this  environment  variable is set, its value is the default
172              memory limit.  The value  is  specified  as  described  for  the
173              -maxmemory  switch.   JPEGMEM overrides the default value speci‐
174              fied when the program was compiled, and itself is overridden  by
175              an explicit -maxmemory.
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SEE ALSO

178       cjpeg(1), jpegtran(1), rdjpgcom(1), wrjpgcom(1)
179       ppm(5), pgm(5)
180       Wallace,  Gregory  K.   "The  JPEG Still Picture Compression Standard",
181       Communications of the ACM, April 1991 (vol. 34, no. 4), pp. 30-44.
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AUTHOR

184       Independent JPEG Group
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BUGS

187       To avoid the Unisys LZW patent, djpeg produces uncompressed GIF  files.
188       These  are larger than they should be, but are readable by standard GIF
189       decoders.
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193                                11 October 2010                       DJPEG(1)
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