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1 .TH JPEGTRAN 1 "3 August 1997"
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 .B jpegtran
23 works by rearranging the compressed data (DCT coefficients), without
24 ever fully decoding the image. Therefore, its transformations are lossless:
25 there is no image degradation at all, which would not be true if you used
26 .B djpeg
27 followed by
28 .B cjpeg
29 to accomplish the same conversion. But by the same token,
30 .B jpegtran
31 cannot perform lossy operations such as changing the image quality.
32 .PP
33 .B jpegtran
34 reads the named JPEG/JFIF file, or the standard input if no file is
35 named, and produces a JPEG/JFIF file on the standard output.
36 .SH OPTIONS
37 All switch names may be abbreviated; for example,
38 .B \-optimize
39 may be written
40 .B \-opt
41 or
42 .BR \-o .
43 Upper and lower case are equivalent.
44 British spellings are also accepted (e.g.,
45 .BR \-optimise ),
46 though for brevity these are not mentioned below.
47 .PP
48 To specify the coded JPEG representation used in the output file,
49 .B jpegtran
50 accepts a subset of the switches recognized by
51 .BR cjpeg :
52 .TP
53 .B \-optimize
54 Perform optimization of entropy encoding parameters.
55 .TP
56 .B \-progressive
57 Create progressive JPEG file.
58 .TP
59 .BI \-restart " N"
60 Emit a JPEG restart marker every N MCU rows, or every N MCU blocks if "B" is
61 attached to the number.
62 .TP
63 .BI \-scans " file"
64 Use the scan script given in the specified text file.
65 .PP
66 See
67 .BR cjpeg (1)
68 for more details about these switches.
69 If you specify none of these switches, you get a plain baseline-JPEG output
70 file. The quality setting and so forth are determined by the input file.
71 .PP
72 The image can be losslessly transformed by giving one of these switches:
73 .TP
74 .B \-flip horizontal
75 Mirror image horizontally (left-right).
76 .TP
77 .B \-flip vertical
78 Mirror image vertically (top-bottom).
79 .TP
80 .B \-rotate 90
81 Rotate image 90 degrees clockwise.
82 .TP
83 .B \-rotate 180
84 Rotate image 180 degrees.
85 .TP
86 .B \-rotate 270
87 Rotate image 270 degrees clockwise (or 90 ccw).
88 .TP
89 .B \-transpose
90 Transpose image (across UL-to-LR axis).
91 .TP
92 .B \-transverse
93 Transverse transpose (across UR-to-LL axis).
94 .PP
95 The transpose transformation has no restrictions regarding image dimensions.
96 The other transformations operate rather oddly if the image dimensions are not
97 a multiple of the iMCU size (usually 8 or 16 pixels), because they can only
98 transform complete blocks of DCT coefficient data in the desired way.
99 .PP
100 .BR jpegtran 's
101 default behavior when transforming an odd-size image is designed
102 to preserve exact reversibility and mathematical consistency of the
103 transformation set. As stated, transpose is able to flip the entire image
104 area. Horizontal mirroring leaves any partial iMCU column at the right edge
105 untouched, but is able to flip all rows of the image. Similarly, vertical
106 mirroring leaves any partial iMCU row at the bottom edge untouched, but is
107 able to flip all columns. The other transforms can be built up as sequences
108 of transpose and flip operations; for consistency, their actions on edge
109 pixels are defined to be the same as the end result of the corresponding
110 transpose-and-flip sequence.
111 .PP
112 For practical use, you may prefer to discard any untransformable edge pixels
113 rather than having a strange-looking strip along the right and/or bottom edges
114 of a transformed image. To do this, add the
115 .B \-trim
116 switch:
117 .TP
118 .B \-trim
119 Drop non-transformable edge blocks.
120 .PP
121 Obviously, a transformation with
122 .B \-trim
123 is not reversible, so strictly speaking
124 .B jpegtran
125 with this switch is not lossless. Also, the expected mathematical
126 equivalences between the transformations no longer hold. For example,
127 .B \-rot 270 -trim
128 trims only the bottom edge, but
129 .B \-rot 90 -trim
130 followed by
131 .B \-rot 180 -trim
132 trims both edges.
133 .PP
134 Another not-strictly-lossless transformation switch is:
135 .TP
136 .B \-grayscale
137 Force grayscale output.
138 .PP
139 This option discards the chrominance channels if the input image is YCbCr
140 (ie, a standard color JPEG), resulting in a grayscale JPEG file. The
141 luminance channel is preserved exactly, so this is a better method of reducing
142 to grayscale than decompression, conversion, and recompression. This switch
143 is particularly handy for fixing a monochrome picture that was mistakenly
144 encoded as a color JPEG. (In such a case, the space savings from getting rid
145 of the near-empty chroma channels won't be large; but the decoding time for
146 a grayscale JPEG is substantially less than that for a color JPEG.)
147 .PP
148 .B jpegtran
149 also recognizes these switches that control what to do with "extra" markers,
150 such as comment blocks:
151 .TP
152 .B \-copy none
153 Copy no extra markers from source file. This setting suppresses all
154 comments and other excess baggage present in the source file.
155 .TP
156 .B \-copy comments
157 Copy only comment markers. This setting copies comments from the source file,
158 but discards any other inessential data.
159 .TP
160 .B \-copy all
161 Copy all extra markers. This setting preserves miscellaneous markers
162 found in the source file, such as JFIF thumbnails and Photoshop settings.
163 In some files these extra markers can be sizable.
164 .PP
165 The default behavior is
166 .BR "\-copy comments" .
167 (Note: in IJG releases v6 and v6a,
168 .B jpegtran
169 always did the equivalent of
170 .BR "\-copy none" .)
171 .PP
172 Additional switches recognized by jpegtran are:
173 .TP
174 .BI \-maxmemory " N"
175 Set limit for amount of memory to use in processing large images. Value is
176 in thousands of bytes, or millions of bytes if "M" is attached to the
177 number. For example,
178 .B \-max 4m
179 selects 4000000 bytes. If more space is needed, temporary files will be used.
180 .TP
181 .BI \-outfile " name"
182 Send output image to the named file, not to standard output.
183 .TP
184 .B \-verbose
185 Enable debug printout. More
186 .BR \-v 's
187 give more output. Also, version information is printed at startup.
188 .TP
189 .B \-debug
190 Same as
191 .BR \-verbose .
192 .SH EXAMPLES
193 .LP
194 This example converts a baseline JPEG file to progressive form:
195 .IP
196 .B jpegtran \-progressive
197 .I foo.jpg
198 .B >
199 .I fooprog.jpg
200 .PP
201 This example rotates an image 90 degrees clockwise, discarding any
202 unrotatable edge pixels:
203 .IP
204 .B jpegtran \-rot 90 -trim
205 .I foo.jpg
206 .B >
207 .I foo90.jpg
208 .SH ENVIRONMENT
209 .TP
210 .B JPEGMEM
211 If this environment variable is set, its value is the default memory limit.
212 The value is specified as described for the
213 .B \-maxmemory
214 switch.
215 .B JPEGMEM
216 overrides the default value specified when the program was compiled, and
217 itself is overridden by an explicit
218 .BR \-maxmemory .
219 .SH SEE ALSO
220 .BR cjpeg (1),
221 .BR djpeg (1),
222 .BR rdjpgcom (1),
223 .BR wrjpgcom (1)
224 .br
225 Wallace, Gregory K. "The JPEG Still Picture Compression Standard",
226 Communications of the ACM, April 1991 (vol. 34, no. 4), pp. 30-44.
227 .SH AUTHOR
228 Independent JPEG Group
229 .SH BUGS
230 Arithmetic coding is not supported for legal reasons.
231 .PP
232 The transform options can't transform odd-size images perfectly. Use
233 .B \-trim
234 if you don't like the results without it.
235 .PP
236 The entire image is read into memory and then written out again, even in
237 cases where this isn't really necessary. Expect swapping on large images,
238 especially when using the more complex transform options.