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Issue 1939823002: Update to libjpeg_turbo 1.4.90 (Closed) Base URL: https://chromium.googlesource.com/chromium/deps/libjpeg_turbo.git@master
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1 .TH JPEGTRAN 1 "18 February 2016"
2 .SH NAME
3 jpegtran \- lossless transformation of JPEG files
4 .SH SYNOPSIS
5 .B jpegtran
6 [
7 .I options
8 ]
9 [
10 .I filename
11 ]
12 .LP
13 .SH DESCRIPTION
14 .LP
15 .B jpegtran
16 performs various useful transformations of JPEG files.
17 It can translate the coded representation from one variant of JPEG to another,
18 for example from baseline JPEG to progressive JPEG or vice versa. It can also
19 perform some rearrangements of the image data, for example turning an image
20 from landscape to portrait format by rotation.
21 .PP
22 For EXIF files and JPEG files containing Exif data, you may prefer to use
23 .B exiftran
24 instead.
25 .PP
26 .B jpegtran
27 works by rearranging the compressed data (DCT coefficients), without
28 ever fully decoding the image. Therefore, its transformations are lossless:
29 there is no image degradation at all, which would not be true if you used
30 .B djpeg
31 followed by
32 .B cjpeg
33 to accomplish the same conversion. But by the same token,
34 .B jpegtran
35 cannot perform lossy operations such as changing the image quality. However,
36 while the image data is losslessly transformed, metadata can be removed. See
37 the
38 .B \-copy
39 option for specifics.
40 .PP
41 .B jpegtran
42 reads the named JPEG/JFIF file, or the standard input if no file is
43 named, and produces a JPEG/JFIF file on the standard output.
44 .SH OPTIONS
45 All switch names may be abbreviated; for example,
46 .B \-optimize
47 may be written
48 .B \-opt
49 or
50 .BR \-o .
51 Upper and lower case are equivalent.
52 British spellings are also accepted (e.g.,
53 .BR \-optimise ),
54 though for brevity these are not mentioned below.
55 .PP
56 To specify the coded JPEG representation used in the output file,
57 .B jpegtran
58 accepts a subset of the switches recognized by
59 .BR cjpeg :
60 .TP
61 .B \-optimize
62 Perform optimization of entropy encoding parameters.
63 .TP
64 .B \-progressive
65 Create progressive JPEG file.
66 .TP
67 .BI \-restart " N"
68 Emit a JPEG restart marker every N MCU rows, or every N MCU blocks if "B" is
69 attached to the number.
70 .TP
71 .B \-arithmetic
72 Use arithmetic coding.
73 .TP
74 .BI \-scans " file"
75 Use the scan script given in the specified text file.
76 .PP
77 See
78 .BR cjpeg (1)
79 for more details about these switches.
80 If you specify none of these switches, you get a plain baseline-JPEG output
81 file. The quality setting and so forth are determined by the input file.
82 .PP
83 The image can be losslessly transformed by giving one of these switches:
84 .TP
85 .B \-flip horizontal
86 Mirror image horizontally (left-right).
87 .TP
88 .B \-flip vertical
89 Mirror image vertically (top-bottom).
90 .TP
91 .B \-rotate 90
92 Rotate image 90 degrees clockwise.
93 .TP
94 .B \-rotate 180
95 Rotate image 180 degrees.
96 .TP
97 .B \-rotate 270
98 Rotate image 270 degrees clockwise (or 90 ccw).
99 .TP
100 .B \-transpose
101 Transpose image (across UL-to-LR axis).
102 .TP
103 .B \-transverse
104 Transverse transpose (across UR-to-LL axis).
105 .PP
106 The transpose transformation has no restrictions regarding image dimensions.
107 The other transformations operate rather oddly if the image dimensions are not
108 a multiple of the iMCU size (usually 8 or 16 pixels), because they can only
109 transform complete blocks of DCT coefficient data in the desired way.
110 .PP
111 .BR jpegtran 's
112 default behavior when transforming an odd-size image is designed
113 to preserve exact reversibility and mathematical consistency of the
114 transformation set. As stated, transpose is able to flip the entire image
115 area. Horizontal mirroring leaves any partial iMCU column at the right edge
116 untouched, but is able to flip all rows of the image. Similarly, vertical
117 mirroring leaves any partial iMCU row at the bottom edge untouched, but is
118 able to flip all columns. The other transforms can be built up as sequences
119 of transpose and flip operations; for consistency, their actions on edge
120 pixels are defined to be the same as the end result of the corresponding
121 transpose-and-flip sequence.
122 .PP
123 For practical use, you may prefer to discard any untransformable edge pixels
124 rather than having a strange-looking strip along the right and/or bottom edges
125 of a transformed image. To do this, add the
126 .B \-trim
127 switch:
128 .TP
129 .B \-trim
130 Drop non-transformable edge blocks.
131 .IP
132 Obviously, a transformation with
133 .B \-trim
134 is not reversible, so strictly speaking
135 .B jpegtran
136 with this switch is not lossless. Also, the expected mathematical
137 equivalences between the transformations no longer hold. For example,
138 .B \-rot 270 -trim
139 trims only the bottom edge, but
140 .B \-rot 90 -trim
141 followed by
142 .B \-rot 180 -trim
143 trims both edges.
144 .TP
145 .B \-perfect
146 If you are only interested in perfect transformations, add the
147 .B \-perfect
148 switch. This causes
149 .B jpegtran
150 to fail with an error if the transformation is not perfect.
151 .IP
152 For example, you may want to do
153 .IP
154 .B (jpegtran \-rot 90 -perfect
155 .I foo.jpg
156 .B || djpeg
157 .I foo.jpg
158 .B | pnmflip \-r90 | cjpeg)
159 .IP
160 to do a perfect rotation, if available, or an approximated one if not.
161 .PP
162 This version of \fBjpegtran\fR also offers a lossless crop option, which
163 discards data outside of a given image region but losslessly preserves what is
164 inside. Like the rotate and flip transforms, lossless crop is restricted by the
165 current JPEG format; the upper left corner of the selected region must fall on
166 an iMCU boundary. If it doesn't, then it is silently moved up and/or left to
167 the nearest iMCU boundary (the lower right corner is unchanged.) Thus, the
168 output image covers at least the requested region, but it may cover more. The
169 adjustment of the region dimensions may be optionally disabled by attaching
170 an 'f' character ("force") to the width or height number.
171
172 The image can be losslessly cropped by giving the switch:
173 .TP
174 .B \-crop WxH+X+Y
175 Crop the image to a rectangular region of width W and height H, starting at
176 point X,Y. The lossless crop feature discards data outside of a given image
177 region but losslessly preserves what is inside. Like the rotate and flip
178 transforms, lossless crop is restricted by the current JPEG format; the upper
179 left corner of the selected region must fall on an iMCU boundary. If it
180 doesn't, then it is silently moved up and/or left to the nearest iMCU boundary
181 (the lower right corner is unchanged.)
182 .PP
183 Other not-strictly-lossless transformation switches are:
184 .TP
185 .B \-grayscale
186 Force grayscale output.
187 .IP
188 This option discards the chrominance channels if the input image is YCbCr
189 (ie, a standard color JPEG), resulting in a grayscale JPEG file. The
190 luminance channel is preserved exactly, so this is a better method of reducing
191 to grayscale than decompression, conversion, and recompression. This switch
192 is particularly handy for fixing a monochrome picture that was mistakenly
193 encoded as a color JPEG. (In such a case, the space savings from getting rid
194 of the near-empty chroma channels won't be large; but the decoding time for
195 a grayscale JPEG is substantially less than that for a color JPEG.)
196 .PP
197 .B jpegtran
198 also recognizes these switches that control what to do with "extra" markers,
199 such as comment blocks:
200 .TP
201 .B \-copy none
202 Copy no extra markers from source file. This setting suppresses all
203 comments and other metadata in the source file.
204 .TP
205 .B \-copy comments
206 Copy only comment markers. This setting copies comments from the source file
207 but discards any other metadata.
208 .TP
209 .B \-copy all
210 Copy all extra markers. This setting preserves miscellaneous markers
211 found in the source file, such as JFIF thumbnails, Exif data, and Photoshop
212 settings. In some files, these extra markers can be sizable. Note that this
213 option will copy thumbnails as-is; they will not be transformed.
214 .PP
215 The default behavior is \fB-copy comments\fR. (Note: in IJG releases v6 and
216 v6a, \fBjpegtran\fR always did the equivalent of \fB-copy none\fR.)
217 .PP
218 Additional switches recognized by jpegtran are:
219 .TP
220 .BI \-maxmemory " N"
221 Set limit for amount of memory to use in processing large images. Value is
222 in thousands of bytes, or millions of bytes if "M" is attached to the
223 number. For example,
224 .B \-max 4m
225 selects 4000000 bytes. If more space is needed, temporary files will be used.
226 .TP
227 .BI \-outfile " name"
228 Send output image to the named file, not to standard output.
229 .TP
230 .B \-verbose
231 Enable debug printout. More
232 .BR \-v 's
233 give more output. Also, version information is printed at startup.
234 .TP
235 .B \-debug
236 Same as
237 .BR \-verbose .
238 .TP
239 .B \-version
240 Print version information and exit.
241 .SH EXAMPLES
242 .LP
243 This example converts a baseline JPEG file to progressive form:
244 .IP
245 .B jpegtran \-progressive
246 .I foo.jpg
247 .B >
248 .I fooprog.jpg
249 .PP
250 This example rotates an image 90 degrees clockwise, discarding any
251 unrotatable edge pixels:
252 .IP
253 .B jpegtran \-rot 90 -trim
254 .I foo.jpg
255 .B >
256 .I foo90.jpg
257 .SH ENVIRONMENT
258 .TP
259 .B JPEGMEM
260 If this environment variable is set, its value is the default memory limit.
261 The value is specified as described for the
262 .B \-maxmemory
263 switch.
264 .B JPEGMEM
265 overrides the default value specified when the program was compiled, and
266 itself is overridden by an explicit
267 .BR \-maxmemory .
268 .SH SEE ALSO
269 .BR cjpeg (1),
270 .BR djpeg (1),
271 .BR rdjpgcom (1),
272 .BR wrjpgcom (1)
273 .br
274 Wallace, Gregory K. "The JPEG Still Picture Compression Standard",
275 Communications of the ACM, April 1991 (vol. 34, no. 4), pp. 30-44.
276 .SH AUTHOR
277 Independent JPEG Group
278 .PP
279 This file was modified by The libjpeg-turbo Project to include only information
280 relevant to libjpeg-turbo and to wordsmith certain sections.
281 .SH BUGS
282 The transform options can't transform odd-size images perfectly. Use
283 .B \-trim
284 or
285 .B \-perfect
286 if you don't like the results.
287 .PP
288 The entire image is read into memory and then written out again, even in
289 cases where this isn't really necessary. Expect swapping on large images,
290 especially when using the more complex transform options.
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