− convert JPEG/JFIF file to portable pixmap or
[-nosmooth] [-maxmemory N]
[-verbose] [-tracelevel N] [
All options may
be abbreviated to their shortest unique prefix.
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.
uses the Independent JPEG Group’s JPEG library to
interpret the input file. See http://www.ijg.org for
information on the library.
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.
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.
The options are
only for advanced users:
Use integer DCT method
Use fast integer DCT (less
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.
Use a faster, lower-quality
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
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
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
If the JFIF
image does not indicate that it is CMYK or YCCK, these
options have no effect.
don’t use the right one of these options, the symptom
is output that looks like a negative.
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
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.
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.
Print any comments in the input
file to the Standard Error file.
Print details about the
conversion to the Standard Error file.
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
converts the color JFIF file foo.jpg to a PPM file named
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
pnmscale to change the dimensions of the resulting
ppmtopgm to convert a color JFIF file to a grayscale
You can easily
use these converters together. E.g.:
foo.jpg | ppmtopgm | pnmscale .25
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.
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
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
pgm(5), ppmtojpeg(1), ppmquant(1),
pnmscale(1), ppmtopgm(1), ppmdither(1),
djpeg(1), cjpeg(1), jpegtran(1),
rdjpgcom(1), wrjpgcom(1), jhead(1)
Wallace, Gregory K. "The JPEG Still Picture Compression
Standard", Communications of the ACM, April 1991 (vol.
34, no. 4), pp. 30-44.
coding is not supported for legal reasons.
could be much faster.
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.