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.
- -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 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 -perfect switch:
- -perfect
- 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 not.
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 WxH+X+Y
- Crop to a rectangular subarea of width W, height H starting
at point X,Y.
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 substantially less
than that for a color JPEG.)
- -scale M/N
- 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 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 (for image
display) data.
- -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 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, 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¶
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.