JPEGTOPNM
Section: User Commands (1)
Updated: 19 March 2000
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NAME
jpegtopnm - convert JPEG/JFIF file to portable pixmap or graymap
SYNOPSIS
jpegtopnm
[
options
]
[
filename
]
DESCRIPTION
jpegtopnm
converts the named JPEG/JFIF file, or the standard input if no file is
named to a PPM or PGM image file on the standard output. If the JPEG
file is of the grayscale variety,
jpegtopnm
generates a PGM (Portable Graymap) file. Otherwise, it generates a
PPM (Portable Pixmap) file.
jpegtopnm
uses the Independent JPEG Group's JPEG library to interpret the input file.
See
http://www.ijg.org
for information on the library.
JPEG files can have either 8 bits per sample or 12 bits per sample.
The 8 bit variety is by far the most common. There are two versions
of the IJG JPEG library. One reads only 8 bit files and the other
reads only 12 bit files. You must link the appropriate one of these
libraries with
jpegtopnm.
Ordinarily, this means the library is in your shared library search
path when you run
jpegtopnm.
jpegtopnm
generates output with either one byte or two bytes per sample depending on
whether the JPEG input has either 8 bits or 12 bits per sample. You can
use
pnmdepth
to reduce a two-byte-per-sample file to a one-byte-per-sample file if
you need to.
OPTIONS
The options are only for advanced users:
- --dct int
-
Use integer DCT method (default).
- --dct fast
-
Use fast integer DCT (less accurate).
- --dct float
-
Use floating-point DCT method.
The float method is very slightly more accurate than the int method, but is
much slower unless your machine has very fast floating-point hardware. Also
note that results of the floating-point method may vary slightly across
machines, while the integer methods should give the same results everywhere.
The fast integer method is much less accurate than the other two.
- --nosmooth
-
Use a faster, lower-quality upsampling routine.
- --maxmemory N
-
Set limit on the amount of memory
jpegtopnm
uses in processing large images. Value is in thousands of bytes, or
millions of bytes if "M" is suffixed to the number. For example,
--maxmemory 4m
selects 4000000 bytes. If
jpegtopnm
needs more space, it uses temporary files.
- --verbose
-
Print details about the conversion to the Standard Error file.
- --tracelevel n
-
Turn on the JPEG library's trace messages to the Standard Error file.
A higher value of
n
gets more trace information.
--verbose
implies a trace level of at least 1.
EXAMPLES
This example converts the color JPEG/JFIF file foo.jpg to a PPM file
named foo.ppm:
-
jpegtopnm foo.jpg >foo.ppm
HINTS
You can use
ppmquant
to color quantize the result, i.e. to reduce the number of distinct colors
in the image. In fact, you may have to if you want to convert the PPM
file to certain other formats.
ppmdither
Does a more sophisticated quantization.
Use
pnmscale
to change the dimensions of the resulting image.
Use
ppmtopgm
to convert a color JPEG file to a grayscale PGM file.
You can easily use these converters together. E.g.:
-
jpegtopnm foo.jpg | ppmtopgm | pnmscale .25
>foo.pgm
--dct fast
and/or
--nosmooth
gain speed at a small sacrifice in quality.
If you are fortunate enough to have very fast floating point hardware,
--dct float may be even faster than --dct fast. But
on most machines --dct float is slower than --dct int;
in this case it is not worth using, because its theoretical accuracy
advantage is too small to be significant in practice.
Another program,
djpeg,
is similar.
djpeg
is maintained by the Independent JPEG Group and packaged with the
JPEG library which
jpegtopnm
uses for all its JPEG work. Because of that, you may expect it to exploit
more current JPEG features. Also, since you have to have the library to
run
jpegtopnm,
but not vice versa,
cjpeg
may be more commonly available.
On the other hand,
djpeg
does not use the NetPBM libraries to generate its output, as all the NetPBM
tools such as
jpegtopnm
do. This means it is less likely to be consistent with all the other programs
that deal with the NetPBM formats. Also, the command syntax of
jpegtopnm
is consistent with that of the other Netpbm tools, unlike
djpeg.
ENVIRONMENT
- JPEGMEM
-
If this environment variable is set, its value is the default memory limit.
The value is specified as described for the
--maxmemory
option. An explicit
--maxmemory
option overrides any
JPEGMEM.
SEE ALSO
ppm(5),
pgm(5),
ppmtojpeg(1),
ppmquant(1),
pnmscale(1),
ppmtopgm(1),
ppmdither(1),
pnmdepth(1),
djpeg(1),
cjpeg(1),
jpegtran(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.
LIMITATIONS
Arithmetic coding is not supported for legal reasons.
The program could be much faster.
AUTHOR
jpegtopnm
and this man page were derived in large part from
djpeg,
by the Independent JPEG Group.
The program is otherwise by Bryan Henderson on March 19, 2000.
Index
- NAME
-
- SYNOPSIS
-
- DESCRIPTION
-
- OPTIONS
-
- EXAMPLES
-
- HINTS
-
- ENVIRONMENT
-
- SEE ALSO
-
- LIMITATIONS
-
- AUTHOR
-
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