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 -scans file
48 Use the scan script given in the specified text file.
49 See cjpeg(1) for more details about these switches. If you specify
51 none of these switches, you get a plain baseline-JPEG output file. The
52 quality setting and so forth are determined by the input file.
53 The image can be losslessly transformed by giving one of these
55 switches:
56 -flip horizontal
58 Mirror image horizontally (left-right).
59 -flip vertical
61 Mirror image vertically (top-bottom).
62 -rotate 90
64 Rotate image 90 degrees clockwise.
65 -rotate 180
67 Rotate image 180 degrees.
68 -rotate 270
70 Rotate image 270 degrees clockwise (or 90 ccw).
71 -transpose
73 Transpose image (across UL-to-LR axis).
74 -transverse
76 Transverse transpose (across UR-to-LL axis).
77 The transpose transformation has no restrictions regarding image dimen‐
79 sions. The other transformations operate rather oddly if the image
80 dimensions are not a multiple of the iMCU size (usually 8 or 16 pix‐
81 els), because they can only transform complete blocks of DCT coeffi‐
82 cient data in the desired way.
83 jpegtran's default behavior when transforming an odd-size image is
85 designed to preserve exact reversibility and mathematical consistency
86 of the transformation set. As stated, transpose is able to flip the
87 entire image area. Horizontal mirroring leaves any partial iMCU column
88 at the right edge untouched, but is able to flip all rows of the image.
89 Similarly, vertical mirroring leaves any partial iMCU row at the bottom
90 edge untouched, but is able to flip all columns. The other transforms
91 can be built up as sequences of transpose and flip operations; for con‐
92 sistency, their actions on edge pixels are defined to be the same as
93 the end result of the corresponding transpose-and-flip sequence.
94 For practical use, you may prefer to discard any untransformable edge
96 pixels rather than having a strange-looking strip along the right
97 and/or bottom edges of a transformed image. To do this, add the -trim
98 switch:
99 -trim Drop non-transformable edge blocks.
101 Obviously, a transformation with -trim is not reversible, so strictly
103 speaking jpegtran with this switch is not lossless. Also, the expected
104 mathematical equivalences between the transformations no longer hold.
105 For example, -rot 270 -trim trims only the bottom edge, but -rot 90
106 -trim followed by -rot 180 -trim trims both edges.
107 Another not-strictly-lossless transformation switch is:
109 -grayscale
111 Force grayscale output.
112 This option discards the chrominance channels if the input image is
114 YCbCr (ie, a standard color JPEG), resulting in a grayscale JPEG file.
115 The luminance channel is preserved exactly, so this is a better method
116 of reducing to grayscale than decompression, conversion, and recompres‐
117 sion. This switch is particularly handy for fixing a monochrome pic‐
118 ture that was mistakenly encoded as a color JPEG. (In such a case, the
119 space savings from getting rid of the near-empty chroma channels won't
120 be large; but the decoding time for a grayscale JPEG is substantially
121 less than that for a color JPEG.)
122 jpegtran also recognizes these switches that control what to do with
124 "extra" markers, such as comment blocks:
125 -copy none
127 Copy no extra markers from source file. This setting suppresses
128 all comments and other excess baggage present in the source
129 file.
130 -copy comments
132 Copy only comment markers. This setting copies comments from
133 the source file, but discards any other inessential data.
134 -copy all
136 Copy all extra markers. This setting preserves miscellaneous
137 markers found in the source file, such as JFIF thumbnails and
138 Photoshop settings. In some files these extra markers can be
139 sizable.
140 The default behavior is -copy comments. (Note: in IJG releases v6 and
142 v6a, jpegtran always did the equivalent of -copy none.)
143 Additional switches recognized by jpegtran are:
145 -maxmemory N
147 Set limit for amount of memory to use in processing large
148 images. Value is in thousands of bytes, or millions of bytes if
149 "M" is attached to the number. For example, -max 4m selects
150 4000000 bytes. If more space is needed, temporary files will be
151 used.
152 -outfile name
154 Send output image to the named file, not to standard output.
155 -verbose
157 Enable debug printout. More -v's give more output. Also, ver‐
158 sion information is printed at startup.
159 -debug Same as -verbose.
161
163 This example converts a baseline JPEG file to progressive form:
164 jpegtran -progressive foo.jpg > fooprog.jpg
166 This example rotates an image 90 degrees clockwise, discarding any
168 unrotatable edge pixels:
169 jpegtran -rot 90 -trim foo.jpg > foo90.jpg
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173 JPEGMEM
174 If this environment variable is set, its value is the default
175 memory limit. The value is specified as described for the
176 -maxmemory switch. JPEGMEM overrides the default value speci‐
177 fied when the program was compiled, and itself is overridden by
178 an explicit -maxmemory.
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181 cjpeg(1), djpeg(1), rdjpgcom(1), wrjpgcom(1)
182 Wallace, Gregory K. "The JPEG Still Picture Compression Standard",
183 Communications of the ACM, April 1991 (vol. 34, no. 4), pp. 30-44.
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186 Independent JPEG Group
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189 Arithmetic coding is not supported for legal reasons.
190 The transform options can't transform odd-size images perfectly. Use
192 -trim if you don't like the results without it.
193 The entire image is read into memory and then written out again, even
195 in cases where this isn't really necessary. Expect swapping on large
196 images, especially when using the more complex transform options.
197 3 August 1997 JPEGTRAN(1)