jpegtran - lossless transformation of JPEG files
jpegtran [ options ] [ filename ]
jpegtran 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.
jpegtran 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 djpeg followed by cjpeg to accomplish the same
conversion. But by the same token, jpegtran cannot perform lossy
operations such as changing the image quality.
jpegtran 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, -optimize may be
written -opt or -o. Upper and lower case are equivalent. British
spellings are also accepted (e.g., -optimise), though for brevity these
are not mentioned below.
To specify the coded JPEG representation used in the output file,
jpegtran accepts a subset of the switches recognized by cjpeg:
Perform optimization of entropy encoding parameters.
Create progressive JPEG file.
Emit a JPEG restart marker every N MCU rows, or every N MCU
blocks if "B" is attached to the number.
Use arithmetic coding.
Use the scan script given in the specified text file.
See cjpeg(1) 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
Mirror image horizontally (left-right).
Mirror image vertically (top-bottom).
Rotate image 90 degrees clockwise.
Rotate image 180 degrees.
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.
jpegtran’s 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 transpose-and-flip sequence.
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 -trim switch:
-trim Drop non-transformable edge blocks.
Obviously, a transformation with -trim is not reversible, so
strictly speaking jpegtran 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 followed by -rot
180 -trim trims both edges.
If you are only interested in perfect transformation, add the
Fails with an error if the transformation is not perfect.
For example you may want to do
(jpegtran -rot 90 -perfect foo.jpg || djpeg foo.jpg | pnmflip
-r90 | cjpeg)
to do a perfect rotation if available or an approximated one if
We also offer a lossless-crop option, which discards data outside 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 this does not hold for the given crop parameters,
we silently move the upper left corner up and/or left to make it so,
simultaneously increasing the region dimensions to keep the lower right
crop corner unchanged. (Thus, the output image covers at least the
requested region, but may cover more.)
The image can be losslessly cropped by giving the switch:
Crop to a rectangular subarea of width W, height H starting at
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.)
Scale the output image by a factor M/N.
Currently supported scale factors are M/N with all M from 1 to
16, where N is the source DCT size, which is 8 for baseline
JPEG. If the /N part is omitted, then M specifies the DCT
scaled size to be applied on the given input. For baseline JPEG
this is equivalent to M/8 scaling, since the source DCT size for
baseline JPEG is 8. Caution: An implementation of the JPEG
SmartScale extension is required for this feature. SmartScale
enabled JPEG is not yet widely implemented, so many decoders
will be unable to view a SmartScale extended JPEG file at all.
jpegtran also recognizes these switches that control what to do with
"extra" markers, such as comment blocks:
Copy no extra markers from source file. This setting suppresses
all comments and other excess baggage present in the source
Copy only comment markers. This setting copies comments from
the source file, but discards any other inessential (for image
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:
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, -max 4m selects
4000000 bytes. If more space is needed, temporary files will be
Send output image to the named file, not to standard output.
Enable debug printout. More -v’s give more output. Also,
version information is printed at startup.
-debug Same as -verbose.
This example converts a baseline JPEG file to progressive form:
jpegtran -progressive foo.jpg > fooprog.jpg
This example rotates an image 90 degrees clockwise, discarding any
unrotatable edge pixels:
jpegtran -rot 90 -trim foo.jpg > foo90.jpg
If this environment variable is set, its value is the default
memory limit. The value is specified as described for the
-maxmemory switch. JPEGMEM overrides the default value
specified when the program was compiled, and itself is
overridden by an explicit -maxmemory.
cjpeg(1), djpeg(1), rdjpgcom(1), wrjpgcom(1)
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
The transform options can’t transform odd-size images perfectly. Use
-trim or -perfect 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.
28 December 2009