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Jpegtopnm User Manual(0) Jpegtopnm User Manual(0)
NAME
jpegtopnm - convert JPEG/JFIF file to PPM or PGM image
SYNOPSIS
jpegtopnm [-dct {int|fast|float}] [-nosmooth] [-maxmemory N] [{-adobe|-notadobe}] [-com-
ments] [-dumpexif] [-exif=filespec] [-multiple] [-repair] [-verbose] [-tracelevel N]
[filename]
Minimum unique abbreviation of option is acceptable. You may use double hyphens instead
of single hyphen to denote options. You may use white space in place of the equals sign
to separate an option name from its value.
DESCRIPTION
This program is part of Netpbm(1).
jpegtopnm converts JFIF images to PPM or PGM images.
By default, jpegtopnm expects the input stream to contain one JFIF image and produces one
PGM or PPM image as output. It fails if the input stream is empty.
But with the -multiple option, jpegtopnm reads JFIF images sequentially from the input
stream and writes one PPM or PGM image to the output stream for each JFIF input. If the
input stream is empty, so is the output.
The input stream is the filename you specify or, if you don't specify filename, Standard
Input. The output stream is Standard Output.
If a JFIF input image is of the grayscale variety, jpegtopnm generates a PGM image. Oth-
erwise, it generates a PPM image.
Before Netpbm 10.11 (October 2002), jpegtopnm did not have the multiple image stream capa-
bility. From 10.11 through 10.22, Netpbm always behaved as if you specified -multiple.
Starting with Netpbm 10.23 (July 2004), Netpbm's default behavior went back to the
pre-10.11 behavior and the new -multiple option selected the 10.12 behavior. The reason
for the reversion was that there were discovered in the world files that contain JFIF
images followed by something other than another JFIF image. The producers of these files
expect them to work with any JFIF interpreter because most JFIF interpreters just stop
reading the file after the first JFIF image.
jpegtopnm uses the Independent JPEG Group's JPEG library to interpret the input file. See
http://www.ijg.org <http://www.ijg.org> for information on the library.
"JFIF" is the correct name for the image format commonly known as "JPEG." Strictly speak-
ing, JPEG is a method of compression. The image format using JPEG compression that is by
far the most common is JFIF. There is also a subformat of TIFF that uses JPEG compres-
sion.
EXIF is an image format that is a subformat of JFIF (to wit, a JFIF file that contains an
EXIF header as an APP1 marker). jpegtopnm handles EXIF.
JFIF 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 JFIF input has either 8 bits or 12 bits per sample. You can use pamdepth to
reduce a two-byte-per-sample file to a one-byte-per-sample file if you need to.
If the JFIF file uses the CMYK or YCCK color space, the input does not actually contain
enough information to know what color each pixel is. To know what color a pixel is, one
would have to know the properties of the inks to which the color space refers. jpegtopnm
interprets the colors using the common transformation which assumes all the inks are sim-
ply subtractive and linear.
See the jpegtopnm manual(1) for information on how images lose quality when you convert to
and from JFIF.
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.
-adobe
-notadobe
There are two variations on the CMYK (and likewise YCCK) color space that may be
used in the JFIF input. In the normal one, a zero value for a color components
indicates absence of ink. In the other, a zero value means the maximum ink cover-
age. The latter is used by Adobe Photoshop when it creates a bare JFIF output file
(but not when it creates JFIF output as part of Encapsulated Postscript output).
These options tell jpegtopnm which version of the CMYK or YCCK color space the
image uses. If you specify neither, jpegtopnm tries to figure it out on its own.
In the present version, it doesn't try very hard at all: It just assumes the Photo-
shop version, since Photoshop and its emulators seem to be the main source of CMYK
and YCCK images. But with experience of use, future versions might be more sophis-
ticated.
If the JFIF image does not indicate that it is CMYK or YCCK, these options have no
effect.
If you don't use the right one of these options, the symptom is output that looks
like a negative.
-dumpexif
Print the interpreted contents of any Exif header in the input file to the Standard
Error file. Similar to the program jhead (not part of the Netpbm package).
This option was added in Netpbm 9.19 (September 2001).
-exif=filespec
Extract the contents of the EXIF header from the input image and write it to the
file filespec. filespec=- means write it to Standard Output. When you write the
EXIF header to Standard Output, jpegtopnm does not output the converted image
(which is what normally would go to Standard Output) at all.
jpegtopnm writes the contents of the EXIF header byte-for-byte, starting with the
two byte length field (which length includes those two bytes).
You can use this file as input to pnmtojpeg to insert an identical EXIF header into
a new JFIF image.
If there is no EXIF header, jpegtopnm writes two bytes of binary zero and nothing
else.
An EXIF header takes the form of a JFIF APP1 marker. Only the first such marker
within the JFIF header counts.
This option was added in Netpbm 9.19 (September 2001).
-multiple
Read multiple JFIF images sequentially from the input stream. See Description
section <#description> for details.
This option was new in Netpbm 10.23 (July 2004).
-repair
If the JFIF input is invalid, try to salvage whatever information is there and pro-
duce a valid PNM image as output.
Without this option, some invalid input causes jpegtopnm to fail, and what output
it produces is undefined. With -repair such invalid input causes jpegtopnm to suc-
ceed instead.
But note that there are some forms of invalid input that always cause jpegtopnm to
fail and others that always cause it to salvage image information and succeed.
One particular case where -repair makes a difference is the common case that the
file is truncated somewhere after the JFIF header. Without -repair, that always
causes a failure; with -repair it always causes success. Because the image infor-
mation is laid out generally top to bottom in the JFIF stream, the image jpegtopnm
produces in this case has the proper image contents at the top, but the bottom is
padded with gray.
This option was new in Netpbm 10.38.0 (March 2007). Before that, jpegtopnm always
fails in the cases in question.
-comments
Print any comments in the input file to the Standard Error file.
-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 JFIF file foo.jpg to a PPM file named foo.ppm:
jpegtopnm foo.jpg >foo.ppm
HINTS
You can use pnmquant 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 cer-
tain other formats. ppmdither Does a more sophisticated quantization.
Use pamscale to change the dimensions of the resulting image.
Use ppmtopgm to convert a color JFIF file to a grayscale PGM file.
You can easily use these converters together. E.g.:
jpegtopnm foo.jpg | ppmtopgm | pamscale .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 avail-
able.
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(1), pgm(1), pnmtojpeg(1), pnmquant(1), pamscale(1), ppmtopgm(1), ppmdither(1),
pamdepth(1),
djpeg man page, cjpeg man page, jpegtran man page, rdjpgcom man page, wrjpgcom man page,
jhead man page
Wallace, Gregory K. "The JPEG Still Picture Compression Standard", Communications of the
ACM, April 1991 (vol. 34, no. 4), pp. 30-44.
AUTHOR
jpegtopnm and this manual were derived in large part from djpeg, by the Independent JPEG
Group. The program is otherwise by Bryan Henderson on March 19, 2000.
DOCUMENT SOURCE
This manual page was generated by the Netpbm tool 'makeman' from HTML source. The master
documentation is at
http://netpbm.sourceforge.net/doc/jpegtopnm.html
netpbm documentation 13 October 2002 Jpegtopnm User Manual(0)
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