third_party/libjpeg-turbo/jpegtran.1

.TH JPEGTRAN 1 "26 October 2020"
.SH NAME
jpegtran \- lossless transformation of JPEG files
.SH SYNOPSIS
.B jpegtran
[
.I options
]
[
.I filename
]
.LP
.SH DESCRIPTION
.LP
.B 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.
.PP
For EXIF files and JPEG files containing Exif data, you may prefer to use
.B exiftran
instead.
.PP
.B 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
.B djpeg
followed by
.B cjpeg
to accomplish the same conversion.  But by the same token,
.B 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
.B \-copy
option for specifics.
.PP
.B 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.
.SH OPTIONS
All switch names may be abbreviated; for example,
.B \-optimize
may be written
.B \-opt
or
.BR \-o .
Upper and lower case are equivalent.
British spellings are also accepted (e.g.,
.BR \-optimise ),
though for brevity these are not mentioned below.
.PP
To specify the coded JPEG representation used in the output file,
.B jpegtran
accepts a subset of the switches recognized by
.BR cjpeg :
.TP
.B \-optimize
Perform optimization of entropy encoding parameters.
.TP
.B \-progressive
Create progressive JPEG file.
.TP
.BI \-restart " N"
Emit a JPEG restart marker every N MCU rows, or every N MCU blocks if "B" is
attached to the number.
.TP
.B \-arithmetic
Use arithmetic coding.
.TP
.BI \-scans " file"
Use the scan script given in the specified text file.
.PP
See
.BR 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.
.PP
The image can be losslessly transformed by giving one of these switches:
.TP
.B \-flip horizontal
Mirror image horizontally (left-right).
.TP
.B \-flip vertical
Mirror image vertically (top-bottom).
.TP
.B \-rotate 90
Rotate image 90 degrees clockwise.
.TP
.B \-rotate 180
Rotate image 180 degrees.
.TP
.B \-rotate 270
Rotate image 270 degrees clockwise (or 90 ccw).
.TP
.B \-transpose
Transpose image (across UL-to-LR axis).
.TP
.B \-transverse
Transverse transpose (across UR-to-LL axis).
.PP
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.
.PP
.BR 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.
.PP
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
.B \-trim
switch:
.TP
.B \-trim
Drop non-transformable edge blocks.
.IP
Obviously, a transformation with
.B \-trim
is not reversible, so strictly speaking
.B jpegtran
with this switch is not lossless.  Also, the expected mathematical
equivalences between the transformations no longer hold.  For example,
.B \-rot 270 -trim
trims only the bottom edge, but
.B \-rot 90 -trim
followed by
.B \-rot 180 -trim
trims both edges.
.TP
.B \-perfect
If you are only interested in perfect transformations, add the
.B \-perfect
switch.  This causes
.B jpegtran
to fail with an error if the transformation is not perfect.
.IP
For example, you may want to do
.IP
.B (jpegtran \-rot 90 -perfect
.I foo.jpg
.B || djpeg
.I foo.jpg
.B | pnmflip \-r90 | cjpeg)
.IP
to do a perfect rotation, if available, or an approximated one if not.
.PP
This version of \fBjpegtran\fR 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:
.TP
.B \-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.)
.PP
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.
.PP
A complementary lossless wipe option is provided to discard (gray out) data
inside a given image region while losslessly preserving what is outside:
.TP
.B \-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.
.PP
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.
.PP
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:
.TP
.B \-drop +X+Y filename
Drop (insert) another image at point X,Y
.PP
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.
.PP
Other not-strictly-lossless transformation switches are:
.TP
.B \-grayscale
Force grayscale output.
.IP
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.)
.PP
.B jpegtran
also recognizes these switches that control what to do with "extra" markers,
such as comment blocks:
.TP
.B \-copy none
Copy no extra markers from source file.  This setting suppresses all
comments and other metadata in the source file.
.TP
.B \-copy comments
Copy only comment markers.  This setting copies comments from the source file
but discards any other metadata.
.TP
.B \-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.
.PP
The default behavior is \fB-copy comments\fR.  (Note: in IJG releases v6 and
v6a, \fBjpegtran\fR always did the equivalent of \fB-copy none\fR.)
.PP
Additional switches recognized by jpegtran are:
.TP
.BI \-icc " file"
Embed ICC color management profile contained in the specified file.  Note that
this will cause \fBjpegtran\fR to ignore any APP2 markers in the input file,
even if \fB-copy all\fR is specified.
.TP
.BI \-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,
.B \-max 4m
selects 4000000 bytes.  If more space is needed, an error will occur.
.TP
.BI \-maxscans " N"
Abort if the input image contains more than
.I 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.
.TP
.BI \-outfile " name"
Send output image to the named file, not to standard output.
.TP
.BI \-report
Report transformation progress.
.TP
.BI \-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.
.TP
.B \-verbose
Enable debug printout.  More
.BR \-v 's
give more output.  Also, version information is printed at startup.
.TP
.B \-debug
Same as
.BR \-verbose .
.TP
.B \-version
Print version information and exit.
.SH EXAMPLES
.LP
This example converts a baseline JPEG file to progressive form:
.IP
.B jpegtran \-progressive
.I foo.jpg
.B >
.I fooprog.jpg
.PP
This example rotates an image 90 degrees clockwise, discarding any
unrotatable edge pixels:
.IP
.B jpegtran \-rot 90 -trim
.I foo.jpg
.B >
.I foo90.jpg
.SH ENVIRONMENT
.TP
.B JPEGMEM
If this environment variable is set, its value is the default memory limit.
The value is specified as described for the
.B \-maxmemory
switch.
.B JPEGMEM
overrides the default value specified when the program was compiled, and
itself is overridden by an explicit
.BR \-maxmemory .
.SH SEE ALSO
.BR cjpeg (1),
.BR djpeg (1),
.BR rdjpgcom (1),
.BR wrjpgcom (1)
.br
Wallace, Gregory K.  "The JPEG Still Picture Compression Standard",
Communications of the ACM, April 1991 (vol. 34, no. 4), pp. 30-44.
.SH AUTHOR
Independent JPEG Group
.PP
This file was modified by The libjpeg-turbo Project to include only information
relevant to libjpeg-turbo and to wordsmith certain sections.
.SH BUGS
The transform options can't transform odd-size images perfectly.  Use
.B \-trim
or
.B \-perfect
if you don't like the results.
.PP
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.