PGM Format Specification(5) File Formats Manual PGM Format Specification(5)
NAME
pgm - Netpbm grayscale image format
DESCRIPTION
This program is part of Netpbm(1).
The PGM format is a lowest common denominator grayscale file format. It
is designed to be extremely easy to learn and write programs for. (It's
so simple that most people will simply reverse engineer it because it's
easier than reading this specification).
A PGM image represents a grayscale graphic image. There are many
pseudo-PGM formats in use where everything is as specified herein except
for the meaning of individual pixel values. For most purposes, a PGM
image can just be thought of an array of arbitrary integers, and all the
programs in the world that think they're processing a grayscale image
can easily be tricked into processing something else.
The name "PGM" is an acronym derived from "Portable Gray Map."
One official variant of PGM is the transparency mask. A transparency
mask in Netpbm is represented by a PGM image, except that in place of
pixel intensities, there are opaqueness values. See below.
THE FORMAT
The format definition is as follows. You can use the libnetpbm(1) C
subroutine library to conveniently and accurately read and interpret the
format.
A PGM file consists of a sequence of one or more PGM images. There are
no data, delimiters, or padding before, after, or between images.
Each PGM image consists of the following:
• A "magic number" for identifying the file type. A pgm image's
magic number is the two characters "P5".
• Whitespace (blanks, TABs, CRs, LFs).
• A width, formatted as ASCII characters in decimal.
• Whitespace.
• A height, again in ASCII decimal.
• Whitespace.
• The maximum gray value (Maxval), again in ASCII decimal. Must be
less than 65536, and more than zero.
• A single whitespace character (usually a newline).
• A raster of Height rows, in order from top to bottom. Each row
consists of Width gray values, in order from left to right. Each
gray value is a number from 0 through Maxval, with 0 being black
and Maxval being white. Each gray value is represented in pure
binary by either 1 or 2 bytes. If the Maxval is less than 256,
it is 1 byte. Otherwise, it is 2 bytes. The most significant
byte is first.
A row of an image is horizontal. A column is vertical. The pix-
els in the image are square and contiguous.
Each gray value is a number proportional to the intensity of the
pixel, adjusted by the ITU-R Recommendation BT.709 gamma transfer
function. (That transfer function specifies a gamma number of
2.2 and has a linear section for small intensities). A value of
zero is therefore black. A value of Maxval represents CIE D65
white and the most intense value in the image and any other image
to which the image might be compared.
BT.709's range of channel values (16-240) is irrelevant to PGM.
Note that a common variation from the PGM format is to have the
gray value be "linear," i.e. as specified above except without
the gamma adjustment. pnmgamma takes such a PGM variant as input
and produces a true PGM as output.
Another popular variation from PGM is to substitute the newer
sRGB transfer function for the BT.709 one. You can use pnmgamma
to convert between this variation and true PGM.
In the transparency mask variation from PGM, the value represents
opaqueness. It is proportional to the fraction of intensity of a
pixel that would show in place of an underlying pixel. So what
normally means white represents total opaqueness and what nor-
mally means black represents total transparency. In between, you
would compute the intensity of a composite pixel of an "under"
and "over" pixel as under * (1-(alpha/alpha_maxval)) + over *
(alpha/alpha_maxval). Note that there is no gamma transfer func-
tion in the transparency mask.
Strings starting with "#" may be comments, the same as with PBM(1).
Note that you can use pamdepth to convert between the format with 1 byte
per gray value and the one with 2 bytes per gray value.
All characters referred to herein are encoded in ASCII. "newline"
refers to the character known in ASCII as Line Feed or LF. A "white
space" character is space, CR, LF, TAB, VT, or FF (I.e. what the ANSI
standard C isspace() function calls white space).
Plain PGM
There is actually another version of the PGM format that is fairly rare:
"plain" PGM format. The format above, which generally considered the
normal one, is known as the "raw" PGM format. See pbm(1) for some com-
mentary on how plain and raw formats relate to one another and how to
use them.
The difference in the plain format is:
• There is exactly one image in a file.
• The magic number is P2 instead of P5.
• Each pixel in the raster is represented as an ASCII decimal num-
ber (of arbitrary size).
• Each pixel in the raster has white space before and after it.
There must be at least one character of white space between any
two pixels, but there is no maximum.
• No line should be longer than 70 characters.
Here is an example of a small image in the plain PGM format.
P2
# feep.pgm
24 7
15
0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0
0 3 3 3 3 0 0 7 7 7 7 0 0 11 11 11 11 0 0 15 15 15 15 0
0 3 0 0 0 0 0 7 0 0 0 0 0 11 0 0 0 0 0 15 0 0 15 0
0 3 3 3 0 0 0 7 7 7 0 0 0 11 11 11 0 0 0 15 15 15 15 0
0 3 0 0 0 0 0 7 0 0 0 0 0 11 0 0 0 0 0 15 0 0 0 0
0 3 0 0 0 0 0 7 7 7 7 0 0 11 11 11 11 0 0 15 0 0 0 0
0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0
There is a newline character at the end of each of these lines.
Programs that read this format should be as lenient as possible, accept-
ing anything that looks remotely like a PGM image.
INTERNET MEDIA TYPE
No Internet Media Type (aka MIME type, content type) for PGM has been
registered with IANA, but the value image/x-portable-graymap is conven-
tional.
Note that the PNM Internet Media Type image/x-portable-anymap also ap-
plies.
FILE NAME
There are no requirements on the name of a PGM file, but the convention
is to use the suffix ".pgm". "pnm" is also conventional, for cases
where distinguishing between the particular subformats of PNM is not
convenient.
COMPATIBILITY
Before April 2000, a raw format PGM file could not have a maxval greater
than 255. Hence, it could not have more than one byte per sample. Old
programs may depend on this.
Before July 2000, there could be at most one image in a PGM file. As a
result, most tools to process PGM files ignore (and don't read) any data
after the first image.
SEE ALSO
pnm(1), pbm(1), ppm(1), pam(1), libnetpbm(1), programs that process
PGM(1)
AUTHOR
Copyright (C) 1989, 1991 by Jef Poskanzer.
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/pgm.html
netpbm documentation 09 October 2016 PGM Format Specification(5)
Generated by dwww version 1.16 on Tue Dec 16 07:01:58 CET 2025.