source: trunk/libjpeg/jpegtran.1 @ 283

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JPEG plugin: libjpeg updated to version 7

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1.TH JPEGTRAN 1 "28 March 2009"
2.SH NAME
3jpegtran \- 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
16performs various useful transformations of JPEG files.
17It can translate the coded representation from one variant of JPEG to another,
18for example from baseline JPEG to progressive JPEG or vice versa.  It can also
19perform some rearrangements of the image data, for example turning an image
20from landscape to portrait format by rotation.
21.PP
22.B jpegtran
23works by rearranging the compressed data (DCT coefficients), without
24ever fully decoding the image.  Therefore, its transformations are lossless:
25there is no image degradation at all, which would not be true if you used
26.B djpeg
27followed by
28.B cjpeg
29to accomplish the same conversion.  But by the same token,
30.B jpegtran
31cannot perform lossy operations such as changing the image quality.
32.PP
33.B jpegtran
34reads the named JPEG/JFIF file, or the standard input if no file is
35named, and produces a JPEG/JFIF file on the standard output.
36.SH OPTIONS
37All switch names may be abbreviated; for example,
38.B \-optimize
39may be written
40.B \-opt
41or
42.BR \-o .
43Upper and lower case are equivalent.
44British spellings are also accepted (e.g.,
45.BR \-optimise ),
46though for brevity these are not mentioned below.
47.PP
48To specify the coded JPEG representation used in the output file,
49.B jpegtran
50accepts a subset of the switches recognized by
51.BR cjpeg :
52.TP
53.B \-optimize
54Perform optimization of entropy encoding parameters.
55.TP
56.B \-progressive
57Create progressive JPEG file.
58.TP
59.BI \-restart " N"
60Emit a JPEG restart marker every N MCU rows, or every N MCU blocks if "B" is
61attached to the number.
62.TP
63.B \-arithmetic
64Use arithmetic coding.
65.TP
66.BI \-scans " file"
67Use the scan script given in the specified text file.
68.PP
69See
70.BR cjpeg (1)
71for more details about these switches.
72If you specify none of these switches, you get a plain baseline-JPEG output
73file.  The quality setting and so forth are determined by the input file.
74.PP
75The image can be losslessly transformed by giving one of these switches:
76.TP
77.B \-flip horizontal
78Mirror image horizontally (left-right).
79.TP
80.B \-flip vertical
81Mirror image vertically (top-bottom).
82.TP
83.B \-rotate 90
84Rotate image 90 degrees clockwise.
85.TP
86.B \-rotate 180
87Rotate image 180 degrees.
88.TP
89.B \-rotate 270
90Rotate image 270 degrees clockwise (or 90 ccw).
91.TP
92.B \-transpose
93Transpose image (across UL-to-LR axis).
94.TP
95.B \-transverse
96Transverse transpose (across UR-to-LL axis).
97.IP
98The transpose transformation has no restrictions regarding image dimensions.
99The other transformations operate rather oddly if the image dimensions are not
100a multiple of the iMCU size (usually 8 or 16 pixels), because they can only
101transform complete blocks of DCT coefficient data in the desired way.
102.IP
103.BR jpegtran 's
104default behavior when transforming an odd-size image is designed
105to preserve exact reversibility and mathematical consistency of the
106transformation set.  As stated, transpose is able to flip the entire image
107area.  Horizontal mirroring leaves any partial iMCU column at the right edge
108untouched, but is able to flip all rows of the image.  Similarly, vertical
109mirroring leaves any partial iMCU row at the bottom edge untouched, but is
110able to flip all columns.  The other transforms can be built up as sequences
111of transpose and flip operations; for consistency, their actions on edge
112pixels are defined to be the same as the end result of the corresponding
113transpose-and-flip sequence.
114.IP
115For practical use, you may prefer to discard any untransformable edge pixels
116rather than having a strange-looking strip along the right and/or bottom edges
117of a transformed image.  To do this, add the
118.B \-trim
119switch:
120.TP
121.B \-trim
122Drop non-transformable edge blocks.
123.IP
124Obviously, a transformation with
125.B \-trim
126is not reversible, so strictly speaking
127.B jpegtran
128with this switch is not lossless.  Also, the expected mathematical
129equivalences between the transformations no longer hold.  For example,
130.B \-rot 270 -trim
131trims only the bottom edge, but
132.B \-rot 90 -trim
133followed by
134.B \-rot 180 -trim
135trims both edges.
136.IP
137If you are only interested in perfect transformation, add the
138.B \-perfect
139switch:
140.TP
141.B \-perfect
142Fails with an error if the transformation is not perfect.
143.IP
144For example you may want to do
145.IP
146.B (jpegtran \-rot 90 -perfect
147.I foo.jpg
148.B || djpeg
149.I foo.jpg
150.B | pnmflip \-r90 | cjpeg)
151.IP
152to do a perfect rotation if available or an approximated one if not.
153.PP
154We also offer a lossless-crop option, which discards data outside a given
155image region but losslessly preserves what is inside.  Like the rotate and
156flip transforms, lossless crop is restricted by the current JPEG format: the
157upper left corner of the selected region must fall on an iMCU boundary.  If
158this does not hold for the given crop parameters, we silently move the upper
159left corner up and/or left to make it so, simultaneously increasing the region
160dimensions to keep the lower right crop corner unchanged.  (Thus, the output
161image covers at least the requested region, but may cover more.)
162
163The image can be losslessly cropped by giving the switch:
164.TP
165.B \-crop WxH+X+Y
166Crop to a rectangular subarea of width W, height H starting at point X,Y.
167.PP
168Another not-strictly-lossless transformation switch is:
169.TP
170.B \-grayscale
171Force grayscale output.
172.IP
173This option discards the chrominance channels if the input image is YCbCr
174(ie, a standard color JPEG), resulting in a grayscale JPEG file.  The
175luminance channel is preserved exactly, so this is a better method of reducing
176to grayscale than decompression, conversion, and recompression.  This switch
177is particularly handy for fixing a monochrome picture that was mistakenly
178encoded as a color JPEG.  (In such a case, the space savings from getting rid
179of the near-empty chroma channels won't be large; but the decoding time for
180a grayscale JPEG is substantially less than that for a color JPEG.)
181.PP
182.B jpegtran
183also recognizes these switches that control what to do with "extra" markers,
184such as comment blocks:
185.TP
186.B \-copy none
187Copy no extra markers from source file.  This setting suppresses all
188comments and other excess baggage present in the source file.
189.TP
190.B \-copy comments
191Copy only comment markers.  This setting copies comments from the source file,
192but discards any other inessential (for image display) data.
193.TP
194.B \-copy all
195Copy all extra markers.  This setting preserves miscellaneous markers
196found in the source file, such as JFIF thumbnails, Exif data, and Photoshop
197settings.  In some files these extra markers can be sizable.
198.IP
199The default behavior is
200.BR "\-copy comments" .
201(Note: in IJG releases v6 and v6a,
202.B jpegtran
203always did the equivalent of
204.BR "\-copy none" .)
205.PP
206Additional switches recognized by jpegtran are:
207.TP
208.BI \-maxmemory " N"
209Set limit for amount of memory to use in processing large images.  Value is
210in thousands of bytes, or millions of bytes if "M" is attached to the
211number.  For example,
212.B \-max 4m
213selects 4000000 bytes.  If more space is needed, temporary files will be used.
214.TP
215.BI \-outfile " name"
216Send output image to the named file, not to standard output.
217.TP
218.B \-verbose
219Enable debug printout.  More
220.BR \-v 's
221give more output.  Also, version information is printed at startup.
222.TP
223.B \-debug
224Same as
225.BR \-verbose .
226.SH EXAMPLES
227.LP
228This example converts a baseline JPEG file to progressive form:
229.IP
230.B jpegtran \-progressive
231.I foo.jpg
232.B >
233.I fooprog.jpg
234.PP
235This example rotates an image 90 degrees clockwise, discarding any
236unrotatable edge pixels:
237.IP
238.B jpegtran \-rot 90 -trim
239.I foo.jpg
240.B >
241.I foo90.jpg
242.SH ENVIRONMENT
243.TP
244.B JPEGMEM
245If this environment variable is set, its value is the default memory limit.
246The value is specified as described for the
247.B \-maxmemory
248switch.
249.B JPEGMEM
250overrides the default value specified when the program was compiled, and
251itself is overridden by an explicit
252.BR \-maxmemory .
253.SH SEE ALSO
254.BR cjpeg (1),
255.BR djpeg (1),
256.BR rdjpgcom (1),
257.BR wrjpgcom (1)
258.br
259Wallace, Gregory K.  "The JPEG Still Picture Compression Standard",
260Communications of the ACM, April 1991 (vol. 34, no. 4), pp. 30-44.
261.SH AUTHOR
262Independent JPEG Group
263.SH BUGS
264The transform options can't transform odd-size images perfectly.  Use
265.B \-trim
266or
267.B \-perfect
268if you don't like the results.
269.PP
270The entire image is read into memory and then written out again, even in
271cases where this isn't really necessary.  Expect swapping on large images,
272especially when using the more complex transform options.
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