JPEGTRAN(1) User Commands JPEGTRAN(1)

NAME

jpegtran - lossless transformation of JPEG files

SYNOPSIS

jpegtran [ options ] [ filename ]

DESCRIPTION

jpegtran performs various useful transformations of lossy (DCT-based)
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.

-arithmetic
Use arithmetic coding.

-restart N
Emit a JPEG restart marker every N MCU rows, or every N MCUs
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-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.

30 August 2024 JPEGTRAN(1)