Provided by: libjpeg-turbo-progs_2.1.5-3ubuntu2_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.
-perfect
If you are only interested in perfect transformations, add the -perfect switch.
This causes jpegtran to fail 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.
This version of jpegtran also offers a lossless crop option, which discards data outside
of 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 it doesn't, then it is silently
moved up and/or left to the nearest iMCU boundary (the lower right corner is unchanged.)
Thus, the output image covers at least the requested region, but it 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 the image to a rectangular region of width W and height H, starting at point
X,Y. The lossless crop feature discards data outside of 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 it doesn't, then it is silently
moved up and/or left to the nearest iMCU boundary (the lower right corner is
unchanged.)
If W or H is larger than the width/height of the input image, then the output image is
expanded in size, and the expanded region is filled in with zeros (neutral gray).
Attaching an 'f' character ("flatten") to the width number will cause each block in the
expanded region to be filled in with the DC coefficient of the nearest block in the input
image rather than grayed out. Attaching an 'r' character ("reflect") to the width number
will cause the expanded region to be filled in with repeated reflections of the input
image rather than grayed out.
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 region of width W and height H from the input image,
starting at point X,Y.
Attaching an 'f' character ("flatten") to the width number will cause the region to be
filled with the average of adjacent blocks rather than grayed out. If the wipe region and
the region outside the wipe region, when adjusted to the nearest iMCU boundary, form two
horizontally adjacent rectangles, then attaching an 'r' character ("reflect") to the width
number will cause the wipe region to be filled with repeated reflections of the outside
region rather than grayed out.
A lossless drop option is also provided, which allows another JPEG image to be inserted
("dropped") into the input image data at a given position, replacing the existing image
data at that position:
-drop +X+Y filename
Drop (insert) another image at point X,Y
Both the input image and the drop image must have the same subsampling level. It is best
if they also have the same quantization (quality.) Otherwise, the quantization of the
output image will be adapted to accommodate the higher of the input image quality and the
drop image quality. The trim option can be used with the drop option to requantize the
drop image to match the input image. Note that a grayscale image can be dropped into a
full-color image or vice versa, as long as the full-color image has no vertical
subsampling. If the input image is grayscale and the drop image is full-color, then the
chrominance channels from the drop image will be discarded.
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.)
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 icc
Copy only ICC profile markers. This setting copies the ICC profile from the source
file but discards any other metadata.
-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. 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:
-icc file
Embed ICC color management profile contained in the specified file. Note that this
will cause jpegtran to ignore any APP2 markers in the input file, even if -copy all
or -copy icc is specified.
-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, an error will
occur.
-maxscans N
Abort if the input image contains more than N scans. This feature demonstrates a
method by which applications can guard against denial-of-service attacks instigated
by specially-crafted malformed JPEG images containing numerous scans with missing
image data or image data consisting only of "EOB runs" (a feature of progressive
JPEG images that allows potentially hundreds of thousands of adjoining zero-value
pixels to be represented using only a few bytes.) Attempting to transform such
malformed JPEG images can cause excessive CPU activity, since the decompressor must
fully process each scan (even if the scan is corrupt) before it can proceed to the
next scan.
-outfile name
Send output image to the named file, not to standard output.
-report
Report transformation progress.
-strict
Treat all warnings as fatal. This feature also demonstrates a method by which
applications can guard against attacks instigated by specially-crafted malformed
JPEG images. Enabling this option will cause the decompressor to abort if the
input image contains incomplete or corrupt image data.
-verbose
Enable debug printout. More -v's give more output. Also, version information is
printed at startup.
-debug Same as -verbose.
-version
Print version information and exit.
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
This file was modified by The libjpeg-turbo Project to include only information relevant
to libjpeg-turbo and to wordsmith certain sections.
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.
13 July 2021 JPEGTRAN(1)