jpegtran.1

Jpeg source code,jpeg编码解码示范

.TH JPEGTRAN 1 "15 June 1995"
.SH NAME
jpegtran \- lossless transcoding of JPEG files
.SH SYNOPSIS
.B jpegtran
[
.I options
]
[
.I filename
]
.LP
.SH DESCRIPTION
.LP
.B 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 degradation occurs, which would not be true if you used
.B djpeg
followed by
.BR cjpeg .
However, you cannot alter the image quality, because that would not be
a lossless operation.
.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
.B jpegtran
accepts a subset of the switches recognized by
.BR cjpeg .
If you specify no switches, you get a plain baseline JPEG output file.
.PP
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
The basic switches are:
.TP
.B \-optimize
Perform optimization of entropy encoding parameters.  Without this, default
encoding parameters are used.
.B \-optimize
usually makes the JPEG file a little smaller, but at the price of slower
compression.  Note that
.B \-progressive
implies
.BR \-optimize .
.TP
.B \-progressive
Create progressive JPEG file (see below).
.PP
The
.B \-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
transmitted 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 subsequent 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.
.B Caution:
progressive JPEG is not yet widely implemented, so many decoders will be
unable to view a progressive JPEG file at all.
.PP
Switches for advanced users:
.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.
.B \-restart 0
(the default) means no restart markers.
.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 .
.PP
The
.B \-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
.B \-restart 1
for images that will be transmitted across unreliable networks such as Usenet.
.PP
Switches for wizards:
.TP
.BI \-scans " file"
Use the scan script given in the specified text file.
.PP
The "wizard" switches are intended for experimentation with JPEG.  If you
don't know what you are doing, \fBdon't use them\fR.  These switches are
documented further in the file wizard.doc.
.SH EXAMPLES
.LP
This example converts a baseline JPEG file to progressive form:
.IP
.B jpegtran \-progressive
.I foo.jpg
.B >
.I fooprog.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
Still not as fast as we'd like.