Provided by: libjpeg-progs_9b-1ubuntu1_amd64 
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.
For EXIF files and JPEG files containing Exif data, you may prefer to use exiftran
instead.
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. However, while the image data is
losslessly transformed, metadata can be removed. See the -copy option for specifics.
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
adjustment of the region dimensions may be optionally disabled by attaching an 'f'
character ("force") to the width or height number.
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.
A complementary lossless-wipe option is provided to discard (gray out) data inside a given
image region while losslessly preserving what is outside:
-wipe WxH+X+Y
Wipe (gray out) 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 metadata in the source file.
-copy comments
Copy only comment markers. This setting copies comments from the source file, but
discards any other metadata.
-copy all
Copy all extra markers. This setting preserves metadata found in the source file,
such as JFIF thumbnails, Exif data, and Photoshop settings. In some files these
extra markers can be sizable. Note that this option will copy thumbnails as-is;
they will not be transformed.
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.
20 September 2015 JPEGTRAN(1)