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 con-
version. 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, -opti-
mize 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 out-
put file, jpegtran accepts a subset of the switches recog-
nized by cjpeg:
-optimize
Perform optimization of entropy encoding parame-
ters.
-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 num-
ber.
-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 base-
line-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 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 consis-
tency, 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 untrans-
formable 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 recompres-
sion. 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.)
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 set-
ting suppresses all comments and other excess bag-
gage 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 stan-
dard output.
-verbose
Enable debug printout. More -v's give more output.
Also, version 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, dis-
carding 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 specified when the pro-
gram 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 per-
fectly. 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.