libjpegtwrp/jpegtran.1

.TH JPEGTRAN 1 "3 August 1997"
.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
.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.
.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
.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.
.PP
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.
.PP
Another not-strictly-lossless transformation switch is:
.TP
.B \-grayscale
Force grayscale output.
.PP
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 excess baggage present in the source file.
.TP
.B \-copy comments
Copy only comment markers.  This setting copies comments from the source file,
but discards any other inessential data.
.TP
.B \-copy all
Copy all extra markers.  This setting preserves miscellaneous markers
found in the source file, such as JFIF thumbnails and Photoshop settings.
In some files these extra markers can be sizable.
.PP
The default behavior is
.BR "\-copy comments" .
(Note: in IJG releases v6 and v6a,
.B jpegtran
always did the equivalent of
.BR "\-copy none" .)
.PP
Additional switches recognized by jpegtran are:
.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, temporary files will be used.
.TP
.BI \-outfile " name"
Send output image to the named file, not to standard output.
.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 .
.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
.SH BUGS
Arithmetic coding is not supported for legal reasons.
.PP
The transform options can't transform odd-size images perfectly.  Use
.B \-trim
if you don't like the results without it.
.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.