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Annotated edit history of djpeg(1) version 1, including all changes. View license author blame.
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1 perry 1 DJPEG
2 !!!DJPEG
3 NAME
4 SYNOPSIS
5 DESCRIPTION
6 OPTIONS
7 EXAMPLES
8 HINTS
9 ENVIRONMENT
10 SEE ALSO
11 AUTHOR
12 BUGS
13 ----
14 !!NAME
15
16
17 djpeg - decompress a JPEG file to an image file
18 !!SYNOPSIS
19
20
21 __djpeg__ [[ ''options'' ] [[ ''filename''
22 ]
23 !!DESCRIPTION
24
25
26 __djpeg__ decompresses the named JPEG file, or the
27 standard input if no file is named, and produces an image
28 file on the standard output. PBMPLUS (PPM/PGM), BMP, GIF,
29 Targa, or RLE (Utah Raster Toolkit) output format can be
30 selected. (RLE is supported only if the URT library is
31 available.)
32 !!OPTIONS
33
34
35 All switch names may be abbreviated; for example,
36 __-grayscale__ may be written __-gray__ or __-gr__.
37 Most of the
38 __-BMP__ is the same as __-bmp__). British
39 spellings are also accepted (e.g., __-greyscale__),
40 though for brevity these are not mentioned
41 below.
42
43
44 The basic switches are:
45
46
47 __-colors__ ''N''
48
49
50 Reduce image to at most N colors. This reduces the number of
51 colors used in the output image, so that it can be displayed
52 on a colormapped display or stored in a colormapped file
53 format. For example, if you have an 8-bit display, you'd
54 need to reduce to 256 or fewer colors.
55
56
57 __-quantize__ ''N''
58
59
60 Same as __-colors__. __-colors__ is the recommended
61 name, __-quantize__ is provided only for backwards
62 compatibility.
63
64
65 __-fast__
66
67
68 Select recommended processing options for fast, low quality
69 output. (The default options are chosen for highest quality
70 output.) Currently, this is equivalent to __-dct fast
71 -nosmooth -onepass -dither ordered__.
72
73
74 __-grayscale__
75
76
77 Force gray-scale output even if JPEG file is color. Useful
78 for viewing on monochrome displays; also, __djpeg__ runs
79 noticeably faster in this mode.
80
81
82 __-scale__ ''M/N''
83
84
85 Scale the output image by a factor M/N. Currently the scale
86 factor must be 1/1, 1/2, 1/4, or 1/8. Scaling is handy if
87 the image is larger than your screen; also, __djpeg__
88 runs much faster when scaling down the output.
89
90
91 __-bmp__
92
93
94 Select BMP output format (Windows flavor). 8-bit colormapped
95 format is emitted if __-colors__ or __-grayscale__ is
96 specified, or if the JPEG file is gray-scale; otherwise,
97 24-bit full-color format is emitted.
98
99
100 __-gif__
101
102
103 Select GIF output format. Since GIF does not support more
104 than 256 colors, __-colors 256__ is assumed (unless you
105 specify a smaller number of colors).
106
107
108 __-os2__
109
110
111 Select BMP output format (OS/2 1.x flavor). 8-bit
112 colormapped format is emitted if __-colors__ or
113 __-grayscale__ is specified, or if the JPEG file is
114 gray-scale; otherwise, 24-bit full-color format is
115 emitted.
116
117
118 __-pnm__
119
120
121 Select PBMPLUS (PPM/PGM) output format (this is the default
122 format). PGM is emitted if the JPEG file is gray-scale or if
123 __-grayscale__ is specified; otherwise PPM is
124 emitted.
125
126
127 __-rle__
128
129
130 Select RLE output format. (Requires URT
131 library.)
132
133
134 __-targa__
135
136
137 Select Targa output format. Gray-scale format is emitted if
138 the JPEG file is gray-scale or if __-grayscale__ is
139 specified; otherwise, colormapped format is emitted if
140 __-colors__ is specified; otherwise, 24-bit full-color
141 format is emitted.
142
143
144 Switches for advanced users:
145
146
147 __-dct int__
148
149
150 Use integer DCT method (default).
151
152
153 __-dct fast__
154
155
156 Use fast integer DCT (less accurate).
157
158
159 __-dct float__
160
161
162 Use floating-point DCT method. The float method is very
163 slightly more accurate than the int method, but is much
164 slower unless your machine has very fast floating-point
165 hardware. Also note that results of the floating-point
166 method may vary slightly across machines, while the integer
167 methods should give the same results everywhere. The fast
168 integer method is much less accurate than the other
169 two.
170
171
172 __-dither fs__
173
174
175 Use Floyd-Steinberg dithering in color
176 quantization.
177
178
179 __-dither ordered__
180
181
182 Use ordered dithering in color quantization.
183
184
185 __-dither none__
186
187
188 Do not use dithering in color quantization. By default,
189 Floyd-Steinberg dithering is applied when quantizing colors;
190 this is slow but usually produces the best results. Ordered
191 dither is a compromise between speed and quality; no
192 dithering is fast but usually looks awful. Note that these
193 switches have no effect unless color quantization is being
194 done. Ordered dither is only available in __-onepass__
195 mode.
196
197
198 __-map__ ''file''
199
200
201 Quantize to the colors used in the specified image file.
202 This is useful for producing multiple files with identical
203 color maps, or for forcing a predefined set of colors to be
204 used. The ''file'' must be a GIF or PPM file. This option
205 overrides __-colors__ and __-onepass__.
206
207
208 __-nosmooth__
209
210
211 Use a faster, lower-quality upsampling routine.
212
213
214 __-onepass__
215
216
217 Use one-pass instead of two-pass color quantization. The
218 one-pass method is faster and needs less memory, but it
219 produces a lower-quality image. __-onepass__ is ignored
220 unless you also say __-colors__ ''N''. Also, the
221 one-pass method is always used for gray-scale output (the
222 two-pass method is no improvement then).
223
224
225 __-maxmemory__ ''N''
226
227
228 Set limit for amount of memory to use in processing large
229 images. Value is in thousands of bytes, or millions of bytes
230 if
231 -max 4m__ selects 4000000 bytes. If more space is
232 needed, temporary files will be used.
233
234
235 __-outfile__ ''name''
236
237
238 Send output image to the named file, not to standard
239 output.
240
241
242 __-verbose__
243
244
245 Enable debug printout. More __-v__'s give more output.
246 Also, version information is printed at
247 startup.
248
249
250 __-debug__
251
252
253 Same as __-verbose__.
254 !!EXAMPLES
255
256
257 This example decompresses the JPEG file foo.jpg, quantizes
258 it to 256 colors, and saves the output in 8-bit BMP format
259 in foo.bmp:
260
261
262 __djpeg -colors 256 -bmp__ ''foo.jpg'' ____
263 ''foo.bmp''
264 !!HINTS
265
266
267 To get a quick preview of an image, use the
268 __-grayscale__ and/or __-scale__ switches.
269 __-grayscale -scale 1/8__ is the fastest
270 case.
271
272
273 Several options are available that trade off image quality
274 to gain speed. __-fast__ turns on the recommended
275 settings.
276
277
278 __-dct fast__ and/or __-nosmooth__ gain speed at a
279 small sacrifice in quality. When producing a color-quantized
280 image, __-onepass -dither ordered__ is fast but much
281 lower quality than the default behavior. __-dither none__
282 may give acceptable results in two-pass mode, but is seldom
283 tolerable in one-pass mode.
284
285
286 If you are fortunate enough to have very fast floating point
287 hardware, __-dct float__ may be even faster than __-dct
288 fast__. But on most machines __-dct float__ is slower
289 than __-dct int__; in this case it is not worth using,
290 because its theoretical accuracy advantage is too small to
291 be significant in practice.
292 !!ENVIRONMENT
293
294
295 __JPEGMEM__
296
297
298 If this environment variable is set, its value is the
299 default memory limit. The value is specified as described
300 for the __-maxmemory__ switch. __JPEGMEM__ overrides
301 the default value specified when the program was compiled,
302 and itself is overridden by an explicit
303 __-maxmemory__.
304 !!SEE ALSO
305
306
307 cjpeg(1), jpegtran(1), rdjpgcom(1),
308 wrjpgcom(1)__
309 ppm__(5), pgm(5)
310 Wallace, Gregory K.
311 !!AUTHOR
312
313
314 Independent JPEG Group
315 !!BUGS
316
317
318 Arithmetic coding is not supported for legal
319 reasons.
320
321
322 To avoid the Unisys LZW patent, __djpeg__ produces
323 uncompressed GIF files. These are larger than they should
324 be, but are readable by standard GIF decoders.
325
326
327 Still not as fast as we'd like.
328 ----
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