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 jpegtran works by rearranging the compressed data (DCT coefficients),
19 without ever fully decoding the image. Therefore, its transformations
20 are lossless: there is no image degradation at all, which would not be
21 true if you used djpeg followed by cjpeg to accomplish the same conver‐
22 sion. But by the same token, jpegtran cannot perform lossy operations
23 such as changing the image quality.
24 jpegtran reads the named JPEG/JFIF file, or the standard input if no
26 file is named, and produces a JPEG/JFIF file on the standard output.
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29 All switch names may be abbreviated; for example, -optimize may be
30 written -opt or -o. Upper and lower case are equivalent. British
31 spellings are also accepted (e.g., -optimise), though for brevity these
32 are not mentioned below.
33 To specify the coded JPEG representation used in the output file, jpeg‐
35 tran accepts a subset of the switches recognized by cjpeg:
36 -optimize
38 Perform optimization of entropy encoding parameters.
39 -progressive
41 Create progressive JPEG file.
42 -restart N
44 Emit a JPEG restart marker every N MCU rows, or every N MCU
45 blocks if "B" is attached to the number.
46 -arithmetic
48 Use arithmetic coding.
49 -scans file
51 Use the scan script given in the specified text file.
52 See cjpeg(1) for more details about these switches. If you specify
54 none of these switches, you get a plain baseline-JPEG output file. The
55 quality setting and so forth are determined by the input file.
56 The image can be losslessly transformed by giving one of these
58 switches:
59 -flip horizontal
61 Mirror image horizontally (left-right).
62 -flip vertical
64 Mirror image vertically (top-bottom).
65 -rotate 90
67 Rotate image 90 degrees clockwise.
68 -rotate 180
70 Rotate image 180 degrees.
71 -rotate 270
73 Rotate image 270 degrees clockwise (or 90 ccw).
74 -transpose
76 Transpose image (across UL-to-LR axis).
77 -transverse
79 Transverse transpose (across UR-to-LL axis).
80 The transpose transformation has no restrictions regarding image dimen‐
82 sions. The other transformations operate rather oddly if the image
83 dimensions are not a multiple of the iMCU size (usually 8 or 16 pix‐
84 els), because they can only transform complete blocks of DCT coeffi‐
85 cient data in the desired way.
86 jpegtran's default behavior when transforming an odd-size image is
88 designed to preserve exact reversibility and mathematical consistency
89 of the transformation set. As stated, transpose is able to flip the
90 entire image area. Horizontal mirroring leaves any partial iMCU column
91 at the right edge untouched, but is able to flip all rows of the image.
92 Similarly, vertical mirroring leaves any partial iMCU row at the bottom
93 edge untouched, but is able to flip all columns. The other transforms
94 can be built up as sequences of transpose and flip operations; for con‐
95 sistency, their actions on edge pixels are defined to be the same as
96 the end result of the corresponding transpose-and-flip sequence.
97 For practical use, you may prefer to discard any untransformable edge
99 pixels rather than having a strange-looking strip along the right
100 and/or bottom edges of a transformed image. To do this, add the -trim
101 switch:
102 -trim Drop non-transformable edge blocks.
104 Obviously, a transformation with -trim is not reversible, so
106 strictly speaking jpegtran with this switch is not lossless.
107 Also, the expected mathematical equivalences between the trans‐
108 formations no longer hold. For example, -rot 270 -trim trims
109 only the bottom edge, but -rot 90 -trim followed by -rot 180
110 -trim trims both edges.
111 -perfect
113 If you are only interested in perfect transformations, add the
114 -perfect switch. This causes jpegtran to fail with an error if
115 the transformation is not perfect.
116 For example, you may want to do
118 (jpegtran -rot 90 -perfect foo.jpg || djpeg foo.jpg | pnmflip
120 -r90 | cjpeg)
121 to do a perfect rotation, if available, or an approximated one
123 if not.
124 -crop WxH+X+Y
126 Crop the image to a rectangular region of width W and height H,
127 starting at point X,Y. The lossless crop feature discards data
128 outside of a given image region but losslessly preserves what is
129 inside. Like the rotate and flip transforms, lossless crop is
130 restricted by the current JPEG format; the upper left corner of
131 the selected region must fall on an iMCU boundary. If it
132 doesn't, then it is silently moved up and/or left to the nearest
133 iMCU boundary (the lower right corner is unchanged.)
134 Other not-strictly-lossless transformation switches are:
136 -grayscale
138 Force grayscale output.
139 This option discards the chrominance channels if the input image
141 is YCbCr (ie, a standard color JPEG), resulting in a grayscale
142 JPEG file. The luminance channel is preserved exactly, so this
143 is a better method of reducing to grayscale than decompression,
144 conversion, and recompression. This switch is particularly
145 handy for fixing a monochrome picture that was mistakenly
146 encoded as a color JPEG. (In such a case, the space savings
147 from getting rid of the near-empty chroma channels won't be
148 large; but the decoding time for a grayscale JPEG is substan‐
149 tially less than that for a color JPEG.)
150 jpegtran also recognizes these switches that control what to do with
152 "extra" markers, such as comment blocks:
153 -copy none
155 Copy no extra markers from source file. This setting suppresses
156 all comments and other excess baggage present in the source
157 file.
158 -copy comments
160 Copy only comment markers. This setting copies comments from
161 the source file but discards any other data which is inessential
162 for image display.
163 -copy all
165 Copy all extra markers. This setting preserves miscellaneous
166 markers found in the source file, such as JFIF thumbnails, Exif
167 data, and Photoshop settings. In some files, these extra mark‐
168 ers can be sizable.
169 The default behavior is -copy comments. (Note: in IJG releases v6 and
171 v6a, jpegtran always did the equivalent of -copy none.)
172 Additional switches recognized by jpegtran are:
174 -maxmemory N
176 Set limit for amount of memory to use in processing large
177 images. Value is in thousands of bytes, or millions of bytes if
178 "M" is attached to the number. For example, -max 4m selects
179 4000000 bytes. If more space is needed, temporary files will be
180 used.
181 -outfile name
183 Send output image to the named file, not to standard output.
184 -verbose
186 Enable debug printout. More -v's give more output. Also, ver‐
187 sion information is printed at startup.
188 -debug Same as -verbose.
190
192 This example converts a baseline JPEG file to progressive form:
193 jpegtran -progressive foo.jpg > fooprog.jpg
195 This example rotates an image 90 degrees clockwise, discarding any
197 unrotatable edge pixels:
198 jpegtran -rot 90 -trim foo.jpg > foo90.jpg
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202 JPEGMEM
203 If this environment variable is set, its value is the default
204 memory limit. The value is specified as described for the
205 -maxmemory switch. JPEGMEM overrides the default value speci‐
206 fied when the program was compiled, and itself is overridden by
207 an explicit -maxmemory.
208
210 cjpeg(1), djpeg(1), rdjpgcom(1), wrjpgcom(1)
211 Wallace, Gregory K. "The JPEG Still Picture Compression Standard",
212 Communications of the ACM, April 1991 (vol. 34, no. 4), pp. 30-44.
213
215 Independent JPEG Group
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218 The transform options can't transform odd-size images perfectly. Use
219 -trim or -perfect if you don't like the results.
220 The entire image is read into memory and then written out again, even
222 in cases where this isn't really necessary. Expect swapping on large
223 images, especially when using the more complex transform options.
224 11 October 2010 JPEGTRAN(1)