jpegtopnm
convert JPEG/JFIF file to portable pixmap or graymap
see also :
ppmtojpeg - ppmquant - pnmscale - ppmtopgm - ppmdither - pnmdepth - djpeg - cjpeg - jpegtran - rdjpgcom - wrjpgcom
Synopsis
jpegtopnm
[-dct {int|fast|float}]
[-nosmooth] [-maxmemory N]
[{-adobe|-notadobe}] [-comments]
[-dumpexif] [-exif=filespec]
[-verbose] [-tracelevel N] [
filename ]
All options may
be abbreviated to their shortest unique prefix.
add an example, a script, a trick and tips
examples
This example converts the color JFIF file foo.jpg to a PPM file
named foo.ppm:
jpegtopnm foo.jpg >foo.ppm
description
jpegtopnm
converts the named JFIF file, or the standard input if no
file is named to a PPM or PGM image file on the standard
output. If the JFIF 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.
"JFIF"
is the correct name for the image format commonly known as
"JPEG." Strictly speaking, 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 compression.
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 pnmdepth 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 simply
subtractive and linear.
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.
-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 coverage. 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 Photoshop 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 sophisticated.
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).
-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. In this case, jpegtopnm does not
output the converted image 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 ppmtojpeg 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.
-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.
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.
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 JFIF 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.
limitations
Arithmetic coding is not supported for legal reasons.
The program could be much faster.
see also
ppm,
pgm, ppmtojpeg , ppmquant ,
pnmscale , ppmtopgm , ppmdither ,
pnmdepth ,
djpeg , cjpeg , jpegtran ,
rdjpgcom , wrjpgcom , jhead
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 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.