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ca48cf7 @tom-gall Initial commit based on 1.1.0 release from
tom-gall authored Jul 5, 2011
1 .TH DJPEG 1 "11 October 2010"
3 djpeg \- decompress a JPEG file to an image file
5 .B djpeg
6 [
7 .I options
8 ]
9 [
10 .I filename
11 ]
12 .LP
14 .LP
15 .B djpeg
16 decompresses the named JPEG file, or the standard input if no file is named,
17 and produces an image file on the standard output. PBMPLUS (PPM/PGM), BMP,
18 GIF, Targa, or RLE (Utah Raster Toolkit) output format can be selected.
19 (RLE is supported only if the URT library is available.)
21 All switch names may be abbreviated; for example,
22 .B \-grayscale
23 may be written
24 .B \-gray
25 or
26 .BR \-gr .
27 Most of the "basic" switches can be abbreviated to as little as one letter.
28 Upper and lower case are equivalent (thus
29 .B \-BMP
30 is the same as
31 .BR \-bmp ).
32 British spellings are also accepted (e.g.,
33 .BR \-greyscale ),
34 though for brevity these are not mentioned below.
35 .PP
36 The basic switches are:
37 .TP
38 .BI \-colors " N"
39 Reduce image to at most N colors. This reduces the number of colors used in
40 the output image, so that it can be displayed on a colormapped display or
41 stored in a colormapped file format. For example, if you have an 8-bit
42 display, you'd need to reduce to 256 or fewer colors.
43 .TP
44 .BI \-quantize " N"
45 Same as
46 .BR \-colors .
47 .B \-colors
48 is the recommended name,
49 .B \-quantize
50 is provided only for backwards compatibility.
51 .TP
52 .B \-fast
53 Select recommended processing options for fast, low quality output. (The
54 default options are chosen for highest quality output.) Currently, this is
55 equivalent to \fB\-dct fast \-nosmooth \-onepass \-dither ordered\fR.
56 .TP
57 .B \-grayscale
58 Force gray-scale output even if JPEG file is color. Useful for viewing on
59 monochrome displays; also,
60 .B djpeg
61 runs noticeably faster in this mode.
62 .TP
63 .BI \-scale " M/N"
64 Scale the output image by a factor M/N. Currently the scale factor must be
65 1/1, 1/2, 1/4, or 1/8. Scaling is handy if the image is larger than your
66 screen; also,
67 .B djpeg
68 runs much faster when scaling down the output.
69 .TP
70 .B \-bmp
71 Select BMP output format (Windows flavor). 8-bit colormapped format is
72 emitted if
73 .B \-colors
74 or
75 .B \-grayscale
76 is specified, or if the JPEG file is gray-scale; otherwise, 24-bit full-color
77 format is emitted.
78 .TP
79 .B \-gif
80 Select GIF output format. Since GIF does not support more than 256 colors,
81 .B \-colors 256
82 is assumed (unless you specify a smaller number of colors).
83 .TP
84 .B \-os2
85 Select BMP output format (OS/2 1.x flavor). 8-bit colormapped format is
86 emitted if
87 .B \-colors
88 or
89 .B \-grayscale
90 is specified, or if the JPEG file is gray-scale; otherwise, 24-bit full-color
91 format is emitted.
92 .TP
93 .B \-pnm
94 Select PBMPLUS (PPM/PGM) output format (this is the default format).
95 PGM is emitted if the JPEG file is gray-scale or if
96 .B \-grayscale
97 is specified; otherwise PPM is emitted.
98 .TP
99 .B \-rle
100 Select RLE output format. (Requires URT library.)
101 .TP
102 .B \-targa
103 Select Targa output format. Gray-scale format is emitted if the JPEG file is
104 gray-scale or if
105 .B \-grayscale
106 is specified; otherwise, colormapped format is emitted if
107 .B \-colors
108 is specified; otherwise, 24-bit full-color format is emitted.
109 .PP
110 Switches for advanced users:
111 .TP
112 .B \-dct int
113 Use integer DCT method (default).
114 .TP
115 .B \-dct fast
116 Use fast integer DCT (less accurate).
117 .TP
118 .B \-dct float
119 Use floating-point DCT method.
120 The float method is very slightly more accurate than the int method, but is
121 much slower unless your machine has very fast floating-point hardware. Also
122 note that results of the floating-point method may vary slightly across
123 machines, while the integer methods should give the same results everywhere.
124 The fast integer method is much less accurate than the other two.
125 .TP
126 .B \-dither fs
127 Use Floyd-Steinberg dithering in color quantization.
128 .TP
129 .B \-dither ordered
130 Use ordered dithering in color quantization.
131 .TP
132 .B \-dither none
133 Do not use dithering in color quantization.
134 By default, Floyd-Steinberg dithering is applied when quantizing colors; this
135 is slow but usually produces the best results. Ordered dither is a compromise
136 between speed and quality; no dithering is fast but usually looks awful. Note
137 that these switches have no effect unless color quantization is being done.
138 Ordered dither is only available in
139 .B \-onepass
140 mode.
141 .TP
142 .BI \-map " file"
143 Quantize to the colors used in the specified image file. This is useful for
144 producing multiple files with identical color maps, or for forcing a
145 predefined set of colors to be used. The
146 .I file
147 must be a GIF or PPM file. This option overrides
148 .B \-colors
149 and
150 .BR \-onepass .
151 .TP
152 .B \-nosmooth
153 Use a faster, lower-quality upsampling routine.
154 .TP
155 .B \-onepass
156 Use one-pass instead of two-pass color quantization. The one-pass method is
157 faster and needs less memory, but it produces a lower-quality image.
158 .B \-onepass
159 is ignored unless you also say
160 .B \-colors
161 .IR N .
162 Also, the one-pass method is always used for gray-scale output (the two-pass
163 method is no improvement then).
164 .TP
165 .BI \-maxmemory " N"
166 Set limit for amount of memory to use in processing large images. Value is
167 in thousands of bytes, or millions of bytes if "M" is attached to the
168 number. For example,
169 .B \-max 4m
170 selects 4000000 bytes. If more space is needed, temporary files will be used.
171 .TP
172 .BI \-outfile " name"
173 Send output image to the named file, not to standard output.
174 .TP
175 .B \-verbose
176 Enable debug printout. More
177 .BR \-v 's
178 give more output. Also, version information is printed at startup.
179 .TP
180 .B \-debug
181 Same as
182 .BR \-verbose .
184 .LP
185 This example decompresses the JPEG file foo.jpg, quantizes it to
186 256 colors, and saves the output in 8-bit BMP format in foo.bmp:
187 .IP
188 .B djpeg \-colors 256 \-bmp
189 .I foo.jpg
190 .B >
191 .I foo.bmp
193 To get a quick preview of an image, use the
194 .B \-grayscale
195 and/or
196 .B \-scale
197 switches.
198 .B \-grayscale \-scale 1/8
199 is the fastest case.
200 .PP
201 Several options are available that trade off image quality to gain speed.
202 .B \-fast
203 turns on the recommended settings.
204 .PP
205 .B \-dct fast
206 and/or
207 .B \-nosmooth
208 gain speed at a small sacrifice in quality.
209 When producing a color-quantized image,
210 .B \-onepass \-dither ordered
211 is fast but much lower quality than the default behavior.
212 .B \-dither none
213 may give acceptable results in two-pass mode, but is seldom tolerable in
214 one-pass mode.
215 .PP
216 If you are fortunate enough to have very fast floating point hardware,
217 \fB\-dct float\fR may be even faster than \fB\-dct fast\fR. But on most
218 machines \fB\-dct float\fR is slower than \fB\-dct int\fR; in this case it is
219 not worth using, because its theoretical accuracy advantage is too small to be
220 significant in practice.
222 .TP
224 If this environment variable is set, its value is the default memory limit.
225 The value is specified as described for the
226 .B \-maxmemory
227 switch.
229 overrides the default value specified when the program was compiled, and
230 itself is overridden by an explicit
231 .BR \-maxmemory .
233 .BR cjpeg (1),
234 .BR jpegtran (1),
235 .BR rdjpgcom (1),
236 .BR wrjpgcom (1)
237 .br
238 .BR ppm (5),
239 .BR pgm (5)
240 .br
241 Wallace, Gregory K. "The JPEG Still Picture Compression Standard",
242 Communications of the ACM, April 1991 (vol. 34, no. 4), pp. 30-44.
244 Independent JPEG Group
245 .SH BUGS
246 To avoid the Unisys LZW patent,
247 .B djpeg
248 produces uncompressed GIF files. These are larger than they should be, but
249 are readable by standard GIF decoders.
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