Section: User Commands (1)
Updated: 1 January 2013Index
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jpegtran - lossless transformation of JPEG files
performs various useful transformations of JPEG files.
It can translate the coded representation from one variant of JPEG to another,
for example from baseline JPEG to progressive JPEG or vice versa. It can also
perform some rearrangements of the image data, for example turning an image
from landscape to portrait format by rotation.
works by rearranging the compressed data (DCT coefficients), without
ever fully decoding the image. Therefore, its transformations are lossless:
there is no image degradation at all, which would not be true if you used
to accomplish the same conversion. But by the same token,
cannot perform lossy operations such as changing the image quality.
reads the named JPEG/JFIF file, or the standard input if no file is
named, and produces a JPEG/JFIF file on the standard output.
All switch names may be abbreviated; for example,
may be written
Upper and lower case are equivalent.
British spellings are also accepted (e.g.,
though for brevity these are not mentioned below.
To specify the coded JPEG representation used in the output file,
accepts a subset of the switches recognized by
Perform optimization of entropy encoding parameters.
Create progressive JPEG file.
- -restart N
Emit a JPEG restart marker every N MCU rows, or every N MCU blocks if "B" is
attached to the number.
Use arithmetic coding.
- -scans file
Use the scan script given in the specified text file.
for more details about these switches.
If you specify none of these switches, you get a plain baseline-JPEG output
file. The quality setting and so forth are determined by the input file.
The image can be losslessly transformed by giving one of these switches:
- -flip horizontal
Mirror image horizontally (left-right).
- -flip vertical
Mirror image vertically (top-bottom).
- -rotate 90
Rotate image 90 degrees clockwise.
- -rotate 180
Rotate image 180 degrees.
- -rotate 270
Rotate image 270 degrees clockwise (or 90 ccw).
Transpose image (across UL-to-LR axis).
Transverse transpose (across UR-to-LL axis).
The transpose transformation has no restrictions regarding image dimensions.
The other transformations operate rather oddly if the image dimensions are not
a multiple of the iMCU size (usually 8 or 16 pixels), because they can only
transform complete blocks of DCT coefficient data in the desired way.
default behavior when transforming an odd-size image is designed
to preserve exact reversibility and mathematical consistency of the
transformation set. As stated, transpose is able to flip the entire image
area. Horizontal mirroring leaves any partial iMCU column at the right edge
untouched, but is able to flip all rows of the image. Similarly, vertical
mirroring leaves any partial iMCU row at the bottom edge untouched, but is
able to flip all columns. The other transforms can be built up as sequences
of transpose and flip operations; for consistency, their actions on edge
pixels are defined to be the same as the end result of the corresponding
For practical use, you may prefer to discard any untransformable edge pixels
rather than having a strange-looking strip along the right and/or bottom edges
of a transformed image. To do this, add the
Drop non-transformable edge blocks.
Obviously, a transformation with
is not reversible, so strictly speaking
with this switch is not lossless. Also, the expected mathematical
equivalences between the transformations no longer hold. For example,
-rot 270 -trim
trims only the bottom edge, but
-rot 90 -trim
-rot 180 -trim
trims both edges.
If you are only interested in perfect transformations, add the
switch. This causes
to fail with an error if the transformation is not perfect.
For example, you may want to do
(jpegtran -rot 90 -perfect
| pnmflip -r90 | cjpeg)
to do a perfect rotation, if available, or an approximated one if not.
- -crop WxH+X+Y
Crop the image to a rectangular region of width W and height H, starting at
point X,Y. The lossless crop feature discards data outside of a given image
region but losslessly preserves what is inside. Like the rotate and flip
transforms, lossless crop is restricted by the current JPEG format; the upper
left corner of the selected region must fall on an iMCU boundary. If it
doesn't, then it is silently moved up and/or left to the nearest iMCU boundary
(the lower right corner is unchanged.)
Other not-strictly-lossless transformation switches are:
Force grayscale output.
This option discards the chrominance channels if the input image is YCbCr
(ie, a standard color JPEG), resulting in a grayscale JPEG file. The
luminance channel is preserved exactly, so this is a better method of reducing
to grayscale than decompression, conversion, and recompression. This switch
is particularly handy for fixing a monochrome picture that was mistakenly
encoded as a color JPEG. (In such a case, the space savings from getting rid
of the near-empty chroma channels won't be large; but the decoding time for
a grayscale JPEG is substantially less than that for a color JPEG.)
also recognizes these switches that control what to do with "extra" markers,
such as comment blocks:
- -copy none
Copy no extra markers from source file. This setting suppresses all
comments and other excess baggage present in the source file.
- -copy comments
Copy only comment markers. This setting copies comments from the source file
but discards any other data that is inessential for image display.
- -copy all
Copy all extra markers. This setting preserves miscellaneous markers
found in the source file, such as JFIF thumbnails, Exif data, and Photoshop
settings. In some files, these extra markers can be sizable.
The default behavior is -copy comments. (Note: in IJG releases v6 and
v6a, jpegtran always did the equivalent of -copy none.)
Additional switches recognized by jpegtran are:
- -maxmemory N
Set limit for amount of memory to use in processing large images. Value is
in thousands of bytes, or millions of bytes if "M" is attached to the
number. For example,
selects 4000000 bytes. If more space is needed, temporary files will be used.
- -outfile name
Send output image to the named file, not to standard output.
Enable debug printout. More
give more output. Also, version information is printed at startup.
This example converts a baseline JPEG file to progressive form:
This example rotates an image 90 degrees clockwise, discarding any
unrotatable edge pixels:
jpegtran -rot 90 -trim
If this environment variable is set, its value is the default memory limit.
The value is specified as described for the
overrides the default value specified when the program was compiled, and
itself is overridden by an explicit
Wallace, Gregory K. "The JPEG Still Picture Compression Standard",
Communications of the ACM, April 1991 (vol. 34, no. 4), pp. 30-44.
Independent JPEG Group
This file was modified by The libjpeg-turbo Project to include only information
relevant to libjpeg-turbo and to wordsmith certain sections.
The transform options can't transform odd-size images perfectly. Use
if you don't like the results.
The entire image is read into memory and then written out again, even in
cases where this isn't really necessary. Expect swapping on large images,
especially when using the more complex transform options.
- SEE ALSO