1JPEGTRAN(1) General Commands Manual JPEGTRAN(1)
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6 jpegtran - lossless transformation of JPEG files
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9 jpegtran [ options ] [ filename ]
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12 jpegtran performs various useful transformations of JPEG files. It can
13 translate the coded representation from one variant of JPEG to another,
14 for example from baseline JPEG to progressive JPEG or vice versa. It
15 can also perform some rearrangements of the image data, for example
16 turning an image from landscape to portrait format by rotation.
17 For EXIF files and JPEG files containing Exif data, you may prefer to
19 use exiftran instead.
20 jpegtran works by rearranging the compressed data (DCT coefficients),
22 without ever fully decoding the image. Therefore, its transformations
23 are lossless: there is no image degradation at all, which would not be
24 true if you used djpeg followed by cjpeg to accomplish the same conver‐
25 sion. But by the same token, jpegtran cannot perform lossy operations
26 such as changing the image quality. However, while the image data is
27 losslessly transformed, metadata can be removed. See the -copy option
28 for specifics.
29 jpegtran reads the named JPEG/JFIF file, or the standard input if no
31 file is named, and produces a JPEG/JFIF file on the standard output.
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34 All switch names may be abbreviated; for example, -optimize may be
35 written -opt or -o. Upper and lower case are equivalent. British
36 spellings are also accepted (e.g., -optimise), though for brevity these
37 are not mentioned below.
38 To specify the coded JPEG representation used in the output file, jpeg‐
40 tran accepts a subset of the switches recognized by cjpeg:
41 -optimize
43 Perform optimization of entropy encoding parameters.
44 -progressive
46 Create progressive JPEG file.
47 -restart N
49 Emit a JPEG restart marker every N MCU rows, or every N MCU
50 blocks if "B" is attached to the number.
51 -arithmetic
53 Use arithmetic coding.
54 -scans file
56 Use the scan script given in the specified text file.
57 See cjpeg(1) for more details about these switches. If you specify
59 none of these switches, you get a plain baseline-JPEG output file. The
60 quality setting and so forth are determined by the input file.
61 The image can be losslessly transformed by giving one of these
63 switches:
64 -flip horizontal
66 Mirror image horizontally (left-right).
67 -flip vertical
69 Mirror image vertically (top-bottom).
70 -rotate 90
72 Rotate image 90 degrees clockwise.
73 -rotate 180
75 Rotate image 180 degrees.
76 -rotate 270
78 Rotate image 270 degrees clockwise (or 90 ccw).
79 -transpose
81 Transpose image (across UL-to-LR axis).
82 -transverse
84 Transverse transpose (across UR-to-LL axis).
85 The transpose transformation has no restrictions regarding image dimen‐
87 sions. The other transformations operate rather oddly if the image
88 dimensions are not a multiple of the iMCU size (usually 8 or 16 pix‐
89 els), because they can only transform complete blocks of DCT coeffi‐
90 cient data in the desired way.
91 jpegtran's default behavior when transforming an odd-size image is
93 designed to preserve exact reversibility and mathematical consistency
94 of the transformation set. As stated, transpose is able to flip the
95 entire image area. Horizontal mirroring leaves any partial iMCU column
96 at the right edge untouched, but is able to flip all rows of the image.
97 Similarly, vertical mirroring leaves any partial iMCU row at the bottom
98 edge untouched, but is able to flip all columns. The other transforms
99 can be built up as sequences of transpose and flip operations; for con‐
100 sistency, their actions on edge pixels are defined to be the same as
101 the end result of the corresponding transpose-and-flip sequence.
102 For practical use, you may prefer to discard any untransformable edge
104 pixels rather than having a strange-looking strip along the right
105 and/or bottom edges of a transformed image. To do this, add the -trim
106 switch:
107 -trim Drop non-transformable edge blocks.
109 Obviously, a transformation with -trim is not reversible, so
111 strictly speaking jpegtran with this switch is not lossless.
112 Also, the expected mathematical equivalences between the trans‐
113 formations no longer hold. For example, -rot 270 -trim trims
114 only the bottom edge, but -rot 90 -trim followed by -rot 180
115 -trim trims both edges.
116 -perfect
118 If you are only interested in perfect transformations, add the
119 -perfect switch. This causes jpegtran to fail with an error if
120 the transformation is not perfect.
121 For example, you may want to do
123 (jpegtran -rot 90 -perfect foo.jpg || djpeg foo.jpg | pnmflip
125 -r90 | cjpeg)
126 to do a perfect rotation, if available, or an approximated one
128 if not.
129 This version of jpegtran also offers a lossless crop option, which dis‐
131 cards data outside of a given image region but losslessly preserves
132 what is inside. Like the rotate and flip transforms, lossless crop is
133 restricted by the current JPEG format; the upper left corner of the
134 selected region must fall on an iMCU boundary. If it doesn't, then it
135 is silently moved up and/or left to the nearest iMCU boundary (the
136 lower right corner is unchanged.) Thus, the output image covers at
137 least the requested region, but it may cover more. The adjustment of
138 the region dimensions may be optionally disabled by attaching an 'f'
139 character ("force") to the width or height number.
140 The image can be losslessly cropped by giving the switch:
142 -crop WxH+X+Y
144 Crop the image to a rectangular region of width W and height H,
145 starting at point X,Y. The lossless crop feature discards data
146 outside of a given image region but losslessly preserves what is
147 inside. Like the rotate and flip transforms, lossless crop is
148 restricted by the current JPEG format; the upper left corner of
149 the selected region must fall on an iMCU boundary. If it
150 doesn't, then it is silently moved up and/or left to the nearest
151 iMCU boundary (the lower right corner is unchanged.)
152 Other not-strictly-lossless transformation switches are:
154 -grayscale
156 Force grayscale output.
157 This option discards the chrominance channels if the input image
159 is YCbCr (ie, a standard color JPEG), resulting in a grayscale
160 JPEG file. The luminance channel is preserved exactly, so this
161 is a better method of reducing to grayscale than decompression,
162 conversion, and recompression. This switch is particularly
163 handy for fixing a monochrome picture that was mistakenly
164 encoded as a color JPEG. (In such a case, the space savings
165 from getting rid of the near-empty chroma channels won't be
166 large; but the decoding time for a grayscale JPEG is substan‐
167 tially less than that for a color JPEG.)
168 jpegtran also recognizes these switches that control what to do with
170 "extra" markers, such as comment blocks:
171 -copy none
173 Copy no extra markers from source file. This setting suppresses
174 all comments and other metadata in the source file.
175 -copy comments
177 Copy only comment markers. This setting copies comments from
178 the source file but discards any other metadata.
179 -copy all
181 Copy all extra markers. This setting preserves miscellaneous
182 markers found in the source file, such as JFIF thumbnails, Exif
183 data, and Photoshop settings. In some files, these extra mark‐
184 ers can be sizable. Note that this option will copy thumbnails
185 as-is; they will not be transformed.
186 The default behavior is -copy comments. (Note: in IJG releases v6 and
188 v6a, jpegtran always did the equivalent of -copy none.)
189 Additional switches recognized by jpegtran are:
191 -maxmemory N
193 Set limit for amount of memory to use in processing large
194 images. Value is in thousands of bytes, or millions of bytes if
195 "M" is attached to the number. For example, -max 4m selects
196 4000000 bytes. If more space is needed, an error will occur.
197 -outfile name
199 Send output image to the named file, not to standard output.
200 -verbose
202 Enable debug printout. More -v's give more output. Also, ver‐
203 sion information is printed at startup.
204 -debug Same as -verbose.
206 -version
208 Print version information and exit.
209
211 This example converts a baseline JPEG file to progressive form:
212 jpegtran -progressive foo.jpg > fooprog.jpg
214 This example rotates an image 90 degrees clockwise, discarding any
216 unrotatable edge pixels:
217 jpegtran -rot 90 -trim foo.jpg > foo90.jpg
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221 JPEGMEM
222 If this environment variable is set, its value is the default
223 memory limit. The value is specified as described for the
224 -maxmemory switch. JPEGMEM overrides the default value speci‐
225 fied when the program was compiled, and itself is overridden by
226 an explicit -maxmemory.
227
229 cjpeg(1), djpeg(1), rdjpgcom(1), wrjpgcom(1)
230 Wallace, Gregory K. "The JPEG Still Picture Compression Standard",
231 Communications of the ACM, April 1991 (vol. 34, no. 4), pp. 30-44.
232
234 Independent JPEG Group
235 This file was modified by The libjpeg-turbo Project to include only
237 information relevant to libjpeg-turbo and to wordsmith certain sec‐
238 tions.
239
241 The transform options can't transform odd-size images perfectly. Use
242 -trim or -perfect if you don't like the results.
243 The entire image is read into memory and then written out again, even
245 in cases where this isn't really necessary. Expect swapping on large
246 images, especially when using the more complex transform options.
247 18 March 2017 JPEGTRAN(1)