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jpegtran.man
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1995-09-05
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JPEGTRAN(1) JPEGTRAN(1)
NAME
jpegtran - lossless transcoding of JPEG files
SYNOPSIS
jpegtran
[
options
] [
filename
]
DESCRIPTION
jpegtran
translates JPEG files from one variant of JPEG to
another, for example from baseline JPEG to progressive
JPEG. The transformation is lossless: no image degrada
tion occurs, which would not be true if you used
djpeg
followed by
cjpeg
. However, you cannot alter the image
quality, because that would not be a lossless operation.
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
jpegtran
accepts a subset of the switches recognized by
cjpeg
. If you specify no switches, you get a plain base
line JPEG output file.
All switch names may be abbreviated; for example,
-opti
mize
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.
The basic switches are:
-optimize
Perform optimization of entropy encoding parame
ters. Without this, default encoding parameters
are used.
-optimize
usually makes the JPEG file a
little smaller, but at the price of slower compres
sion. Note that
-progressive
implies
-optimize
.
-progressive
Create progressive JPEG file (see below).
The
-progressive
switch creates a "progressive JPEG" file.
In this type of JPEG file, the data is stored in multiple
scans of increasing quality. If the file is being trans
mitted over a slow communications link, the decoder can
use the first scan to display a low-quality image very
quickly, and can then improve the display with each subse
quent scan. The final image is exactly equivalent to a
standard JPEG file of the same quality setting, and the
total file size is about the same --- often a little
smaller.
Caution:
progressive JPEG is not yet widely
implemented, so many decoders will be unable to view a
progressive JPEG file at all.
15 June 1995 1
JPEGTRAN(1) JPEGTRAN(1)
Switches for advanced users:
-restart
N
Emit a JPEG restart marker every N MCU rows, or
every N MCU blocks if "B" is attached to the num
ber.
-restart
0
(the default) means no restart
markers.
-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 stan
dard output.
-verbose
Enable debug printout. More
-v
's give more output.
Also, version information is printed at startup.
-debug
Same as
-verbose
.
The
-restart
option inserts extra markers that allow a
JPEG decoder to resynchronize after a transmission error.
Without restart markers, any damage to a compressed file
will usually ruin the image from the point of the error to
the end of the image; with restart markers, the damage is
usually confined to the portion of the image up to the
next restart marker. Of course, the restart markers
occupy extra space. We recommend
-restart
1
for images
that will be transmitted across unreliable networks such
as Usenet.
Switches for wizards:
-scans
file
Use the scan script given in the specified text
file.
The "wizard" switches are intended for experimentation
with JPEG. If you don't know what you are doing,
don't
use
them
. These switches are documented further in the
file wizard.doc.
EXAMPLES
This example converts a baseline JPEG file to progressive
form:
jpegtran
-progressive
foo.jpg
>
fooprog.jpg
15 June 1995 2
JPEGTRAN(1) JPEGTRAN(1)
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 pro
gram 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
Arithmetic coding is not supported for legal reasons.
Still not as fast as we'd like.
15 June 1995 3