Provided by: netpbm_11.01.00-2build1_amd64 
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 pixels 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 normally 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 function 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 a 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 commentary 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 number (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, accepting anything that
looks remotely like a PGM.
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 conventional.
Note that the PNM Internet Media Type image/x-portable-anymap also applies.
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