jpegtran - lossless transformation of JPEG files

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JPEGTRAN(1) JPEGTRAN(1)

NAME
jpegtran - lossless transformation of JPEG files

SYNOPSIS
jpegtran [ options ] [ filename ]

DESCRIPTION
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 conver‐
sion. 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.

OPTIONS
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, jpeg‐
tran accepts a subset of the switches recognized by cjpeg:

-optimize
Perform optimization of entropy encoding parameters.

-progressive
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.

-arithmetic
Use arithmetic coding.

-scans file
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
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
Transpose image (across UL-to-LR axis).

-transverse
Transverse transpose (across UR-to-LL axis).

The transpose transformation has no restrictions regarding image dimen‐
sions. The other transformations operate rather oddly if the image
dimensions are not a multiple of the iMCU size (usually 8 or 16 pix‐
els), because they can only transform complete blocks of DCT coeffi‐
cient 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 con‐
sistency, 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 trans‐
formations 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.

-perfect
If you are only interested in perfect transformations, add the
-perfect switch. This causes jpegtran to fail 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 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:

-grayscale
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 substan‐
tially less than that for a color JPEG.)

jpegtran 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 which 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 mark‐
ers 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, -max 4m 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.

-verbose
Enable debug printout. More -v's give more output. Also, ver‐
sion information is printed at startup.

-debug Same as -verbose.

EXAMPLES
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

ENVIRONMENT
JPEGMEM
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 speci‐
fied when the program was compiled, and itself is overridden by
an explicit -maxmemory.

SEE ALSO
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.

AUTHOR
Independent JPEG Group

BUGS
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.

11 October 2010 JPEGTRAN(1)