jpegtran
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 rota-
tion.
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 con-
version. But by the same token, jpegtran cannot perform lossy opera-
tions 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,
jpegtran 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.
-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
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 coef-
ficient 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 col-
umn 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 opera-
tions; 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.
Another not-strictly-lossless transformation switch is:
-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 recom-
pression. 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 sup-
presses 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 inessential data.
-copy all
Copy all extra markers. This setting preserves miscellaneous
markers found in the source file, such as JFIF thumbnails 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, -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
Arithmetic coding is not supported for legal reasons.
The transform options can’t transform odd-size images perfectly. Use
-trim if you don’t like the results without it.
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.
3 August 1997 JPEGTRAN(1)
