151 lines
4.4 KiB
Groff
151 lines
4.4 KiB
Groff
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.TH JPEGTRAN 1 "15 June 1995"
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.SH NAME
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jpegtran \- lossless transcoding of JPEG files
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.SH SYNOPSIS
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.B jpegtran
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[
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.I options
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]
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[
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.I filename
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]
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.LP
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.SH DESCRIPTION
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.LP
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.B jpegtran
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translates JPEG files from one variant of JPEG to another, for example
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from baseline JPEG to progressive JPEG. The transformation is lossless:
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no image degradation occurs, which would not be true if you used
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.B djpeg
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followed by
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.BR cjpeg .
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However, you cannot alter the image quality, because that would not be
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a lossless operation.
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.B jpegtran
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reads the named JPEG/JFIF file, or the standard input if no file is
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named, and produces a JPEG/JFIF file on the standard output.
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.SH OPTIONS
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.B jpegtran
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accepts a subset of the switches recognized by
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.BR cjpeg .
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If you specify no switches, you get a plain baseline JPEG output file.
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.PP
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All switch names may be abbreviated; for example,
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.B \-optimize
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may be written
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.B \-opt
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or
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.BR \-o .
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Upper and lower case are equivalent.
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British spellings are also accepted (e.g.,
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.BR \-optimise ),
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though for brevity these are not mentioned below.
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.PP
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The basic switches are:
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.TP
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.B \-optimize
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Perform optimization of entropy encoding parameters. Without this, default
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encoding parameters are used.
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.B \-optimize
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usually makes the JPEG file a little smaller, but at the price of slower
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compression. Note that
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.B \-progressive
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implies
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.BR \-optimize .
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.TP
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.B \-progressive
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Create progressive JPEG file (see below).
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.PP
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The
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.B \-progressive
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switch creates a "progressive JPEG" file. In this type of JPEG file, the data
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is stored in multiple scans of increasing quality. If the file is being
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transmitted over a slow communications link, the decoder can use the first
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scan to display a low-quality image very quickly, and can then improve the
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display with each subsequent scan. The final image is exactly equivalent to a
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standard JPEG file of the same quality setting, and the total file size is
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about the same --- often a little smaller.
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.B Caution:
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progressive JPEG is not yet widely implemented, so many decoders will be
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unable to view a progressive JPEG file at all.
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.PP
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Switches for advanced users:
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.TP
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.BI \-restart " N"
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Emit a JPEG restart marker every N MCU rows, or every N MCU blocks if "B" is
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attached to the number.
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.B \-restart 0
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(the default) means no restart markers.
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.TP
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.BI \-maxmemory " N"
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Set limit for amount of memory to use in processing large images. Value is
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in thousands of bytes, or millions of bytes if "M" is attached to the
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number. For example,
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.B \-max 4m
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selects 4000000 bytes. If more space is needed, temporary files will be used.
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.TP
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.BI \-outfile " name"
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Send output image to the named file, not to standard output.
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.TP
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.B \-verbose
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Enable debug printout. More
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.BR \-v 's
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give more output. Also, version information is printed at startup.
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.TP
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.B \-debug
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Same as
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.BR \-verbose .
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.PP
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The
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.B \-restart
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option inserts extra markers that allow a JPEG decoder to resynchronize after
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a transmission error. Without restart markers, any damage to a compressed
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file will usually ruin the image from the point of the error to the end of the
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image; with restart markers, the damage is usually confined to the portion of
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the image up to the next restart marker. Of course, the restart markers
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occupy extra space. We recommend
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.B \-restart 1
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for images that will be transmitted across unreliable networks such as Usenet.
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.PP
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Switches for wizards:
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.TP
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.BI \-scans " file"
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Use the scan script given in the specified text file.
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.PP
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The "wizard" switches are intended for experimentation with JPEG. If you
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don't know what you are doing, \fBdon't use them\fR. These switches are
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documented further in the file wizard.doc.
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.SH EXAMPLES
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.LP
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This example converts a baseline JPEG file to progressive form:
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.IP
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.B jpegtran \-progressive
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.I foo.jpg
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.B >
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.I fooprog.jpg
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.SH ENVIRONMENT
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.TP
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.B JPEGMEM
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If this environment variable is set, its value is the default memory limit.
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The value is specified as described for the
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.B \-maxmemory
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switch.
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.B JPEGMEM
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overrides the default value specified when the program was compiled, and
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itself is overridden by an explicit
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.BR \-maxmemory .
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.SH SEE ALSO
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.BR cjpeg (1),
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.BR djpeg (1),
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.BR rdjpgcom (1),
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.BR wrjpgcom (1)
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.br
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Wallace, Gregory K. "The JPEG Still Picture Compression Standard",
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Communications of the ACM, April 1991 (vol. 34, no. 4), pp. 30-44.
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.SH AUTHOR
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Independent JPEG Group
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.SH BUGS
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Arithmetic coding is not supported for legal reasons.
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.PP
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Still not as fast as we'd like.
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