JPEGTRAN(1) UNIX System V (3 August 1997) 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
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.
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-
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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 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:
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JPEGTRAN(1) UNIX System V (3 August 1997) JPEGTRAN(1)-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 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.)
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
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:
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JPEGTRAN(1) UNIX System V (3 August 1997) JPEGTRAN(1)-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, 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,
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 specified 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
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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.
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