jpegtopnm Man page

JPEGTOPNM(1) General Commands Manual JPEGTOPNM(1)


jpegtopnm – convert JPEG/JFIF file to portable pixmap or graymap


jpegtopnm [-dct {int|fast|float}] [-nosmooth] [-maxmemory N] [{-adobe|-notadobe}] [-comments] [-dumpexif] [-exif=filespec] [-ver‐
bose] [-tracelevel N] [ filename ]

All options may be abbreviated to their shortest unique prefix.


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 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 han‐
dles 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.


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 accu‐
rate than the other two.

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.


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 ver‐
sion, 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.

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).

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 identi‐
cal 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.

-tracelevel n
Turn on the JPEG library’s trace messages to the Standard Error
file. A higher value of n gets more trace information. -ver‐
bose implies a trace level of at least 1.

This example converts the color JFIF file foo.jpg to a PPM file named

jpegtopnm foo.jpg >foo.ppm

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

-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 signifi‐
cant in practice.

Another program, djpeg, is similar. djpeg is maintained by the Inde‐
pendent 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 com‐
monly 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.

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


ppm(5), pgm(5), ppmtojpeg, ppmquant, pnmscale, ppmtopgm,
ppmdither, pnmdepth,
djpeg(1), cjpeg(1), jpegtran(1), rdjpgcom(1), wrjpgcom(1), jhead
Wallace, Gregory K. “The JPEG Still Picture Compression Standard”,
Communications of the ACM, April 1991 (vol. 34, no. 4), pp. 30-44.

Arithmetic coding is not supported for legal reasons.

The program could be much faster.


jpegtopnm and this man page were derived in large part from djpeg, by
the Independent JPEG Group. The program is otherwise by Bryan Hender‐
son on March 19, 2000.

19 March 2000 JPEGTOPNM(1)

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