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1 perry 1 JPEGTRAN
2 !!!JPEGTRAN
3 NAME
4 SYNOPSIS
5 DESCRIPTION
6 OPTIONS
7 EXAMPLES
8 ENVIRONMENT
9 SEE ALSO
10 AUTHOR
11 BUGS
12 ----
13 !!NAME
14
15
16 jpegtran - lossless transformation of JPEG files
17 !!SYNOPSIS
18
19
20 __jpegtran__ [[ ''options'' ] [[ ''filename''
21 ]
22 !!DESCRIPTION
23
24
25 __jpegtran__ performs various useful transformations of
26 JPEG files. It can translate the coded representation from
27 one variant of JPEG to another, for example from baseline
28 JPEG to progressive JPEG or vice versa. It can also perform
29 some rearrangements of the image data, for example turning
30 an image from landscape to portrait format by
31 rotation.
32
33
34 __jpegtran__ works by rearranging the compressed data
35 (DCT coefficients), without ever fully decoding the image.
36 Therefore, its transformations are lossless: there is no
37 image degradation at all, which would not be true if you
38 used __djpeg__ followed by __cjpeg__ to accomplish the
39 same conversion. But by the same token, __jpegtran__
40 cannot perform lossy operations such as changing the image
41 quality.
42
43
44 __jpegtran__ reads the named JPEG/JFIF file, or the
45 standard input if no file is named, and produces a JPEG/JFIF
46 file on the standard output.
47 !!OPTIONS
48
49
50 All switch names may be abbreviated; for example,
51 __-optimize__ may be written __-opt__ or __-o__.
52 Upper and lower case are equivalent. British spellings are
53 also accepted (e.g., __-optimise__), though for brevity
54 these are not mentioned below.
55
56
57 To specify the coded JPEG representation used in the output
58 file, __jpegtran__ accepts a subset of the switches
59 recognized by __cjpeg__:
60
61
62 __-optimize__
63
64
65 Perform optimization of entropy encoding
66 parameters.
67
68
69 __-progressive__
70
71
72 Create progressive JPEG file.
73
74
75 __-restart__ ''N''
76
77
78 Emit a JPEG restart marker every N MCU rows, or every N MCU
79 blocks if
80
81
82 __-scans__ ''file''
83
84
85 Use the scan script given in the specified text
86 file.
87
88
89 See cjpeg(1) for more details about these switches.
90 If you specify none of these switches, you get a plain
91 baseline-JPEG output file. The quality setting and so forth
92 are determined by the input file.
93
94
95 The image can be losslessly transformed by giving one of
96 these switches:
97
98
99 __-flip horizontal__
100
101
102 Mirror image horizontally (left-right).
103
104
105 __-flip vertical__
106
107
108 Mirror image vertically (top-bottom).
109
110
111 __-rotate 90__
112
113
114 Rotate image 90 degrees clockwise.
115
116
117 __-rotate 180__
118
119
120 Rotate image 180 degrees.
121
122
123 __-rotate 270__
124
125
126 Rotate image 270 degrees clockwise (or 90 ccw).
127
128
129 __-transpose__
130
131
132 Transpose image (across UL-to-LR axis).
133
134
135 __-transverse__
136
137
138 Transverse transpose (across UR-to-LL axis).
139
140
141 The transpose transformation has no restrictions regarding
142 image dimensions. The other transformations operate rather
143 oddly if the image dimensions are not a multiple of the iMCU
144 size (usually 8 or 16 pixels), because they can only
145 transform complete blocks of DCT coefficient data in the
146 desired way.
147
148
149 __jpegtran__'s default behavior when transforming an
150 odd-size image is designed to preserve exact reversibility
151 and mathematical consistency of the transformation set. As
152 stated, transpose is able to flip the entire image area.
153 Horizontal mirroring leaves any partial iMCU column at the
154 right edge untouched, but is able to flip all rows of the
155 image. Similarly, vertical mirroring leaves any partial iMCU
156 row at the bottom edge untouched, but is able to flip all
157 columns. The other transforms can be built up as sequences
158 of transpose and flip operations; for consistency, their
159 actions on edge pixels are defined to be the same as the end
160 result of the corresponding transpose-and-flip
161 sequence.
162
163
164 For practical use, you may prefer to discard any
165 untransformable edge pixels rather than having a
166 strange-looking strip along the right and/or bottom edges of
167 a transformed image. To do this, add the __-trim__
168 switch:
169
170
171 __-trim__
172
173
174 Drop non-transformable edge blocks.
175
176
177 Obviously, a transformation with __-trim__ is not
178 reversible, so strictly speaking __jpegtran__ with this
179 switch is not lossless. Also, the expected mathematical
180 equivalences between the transformations no longer hold. For
181 example, __-rot 270 -trim__ trims only the bottom edge,
182 but __-rot 90 -trim__ followed by __-rot 180 -trim__
183 trims both edges.
184
185
186 We also offer a lossless-crop option, which discards data
187 outside a given image region but losslessly preserves what
188 is inside. Like the rotate and flip transforms, lossless
189 crop is restricted by the JPEG format: the upper left corner
190 of the selected region must fall on an iMCU boundary. If
191 this does not hold for the given crop parameters, we
192 silently move the upper left corner up and/or left to make
193 it so, simultaneously increasing the region dimensions to
194 keep the lower right crop corner unchanged. (Thus, the
195 output image covers at least the requested region, but may
196 cover more.)
197
198
199 Note: lossless-crop is an enhancement from
200 http://sylvana.net/jpegcrop/ that may not be available on
201 non-Debian systems.
202
203
204 The image can be losslessly cropped by giving the
205 switch:
206
207
208 __-crop WxH+X+Y__
209
210
211 Crop to a rectangular subarea of width W, height H starting
212 at point X,Y.
213
214
215 Another not-strictly-lossless transformation switch
216 is:
217
218
219 __-grayscale__
220
221
222 Force grayscale output.
223
224
225 This option discards the chrominance channels if the input
226 image is YCbCr (ie, a standard color JPEG), resulting in a
227 grayscale JPEG file. The luminance channel is preserved
228 exactly, so this is a better method of reducing to grayscale
229 than decompression, conversion, and recompression. This
230 switch is particularly handy for fixing a monochrome picture
231 that was mistakenly encoded as a color JPEG. (In such a
232 case, the space savings from getting rid of the near-empty
233 chroma channels won't be large; but the decoding time for a
234 grayscale JPEG is substantially less than that for a color
235 JPEG.)
236
237
238 __jpegtran__ also recognizes these switches that control
239 what to do with
240 __
241
242
243 __-copy none__
244
245
246 Copy no extra markers from source file. This setting
247 suppresses all comments and other excess baggage present in
248 the source file.
249
250
251 __-copy comments__
252
253
254 Copy only comment markers. This setting copies comments from
255 the source file, but discards any other inessential
256 data.
257
258
259 __-copy all__
260
261
262 Copy all extra markers. This setting preserves miscellaneous
263 markers found in the source file, such as JFIF thumbnails
264 and Photoshop settings. In some files these extra markers
265 can be sizable.
266
267
268 The default behavior is __-copy comments__. (Note: in IJG
269 releases v6 and v6a, __jpegtran__ always did the
270 equivalent of __-copy none__.)
271
272
273 Additional switches recognized by jpegtran are:
274
275
276 __-maxmemory__ ''N''
277
278
279 Set limit for amount of memory to use in processing large
280 images. Value is in thousands of bytes, or millions of bytes
281 if
282 -max 4m__ selects 4000000 bytes. If more space is
283 needed, temporary files will be used.
284
285
286 __-outfile__ ''name''
287
288
289 Send output image to the named file, not to standard
290 output.
291
292
293 __-verbose__
294
295
296 Enable debug printout. More __-v__'s give more output.
297 Also, version information is printed at
298 startup.
299
300
301 __-debug__
302
303
304 Same as __-verbose__.
305 !!EXAMPLES
306
307
308 This example converts a baseline JPEG file to progressive
309 form:
310
311
312 __jpegtran -progressive__ ''foo.jpg'' ____
313 ''fooprog.jpg''
314
315
316 This example rotates an image 90 degrees clockwise,
317 discarding any unrotatable edge pixels:
318
319
320 __jpegtran -rot 90 -trim__ ''foo.jpg'' ____
321 ''foo90.jpg''
322 !!ENVIRONMENT
323
324
325 __JPEGMEM__
326
327
328 If this environment variable is set, its value is the
329 default memory limit. The value is specified as described
330 for the __-maxmemory__ switch. __JPEGMEM__ overrides
331 the default value specified when the program was compiled,
332 and itself is overridden by an explicit
333 __-maxmemory__.
334 !!SEE ALSO
335
336
337 cjpeg(1), djpeg(1), rdjpgcom(1),
338 wrjpgcom(1)
339 Wallace, Gregory K.
340 !!AUTHOR
341
342
343 Independent JPEG Group
344 !!BUGS
345
346
347 Arithmetic coding is not supported for legal
348 reasons.
349
350
351 The transform options can't transform odd-size images
352 perfectly. Use __-trim__ if you don't like the results
353 without it.
354
355
356 The entire image is read into memory and then written out
357 again, even in cases where this isn't really necessary.
358 Expect swapping on large images, especially when using the
359 more complex transform options.
360 ----
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