.TH pgm 5 "12 November 1991"
.SH NAME
pgm - portable graymap file format
.SH DESCRIPTION
The portable graymap format is a lowest common denominator grayscale
file format.
.IX "PGM file format"
The definition is as follows:
.IP - 2
A "magic number" for identifying the file type.
A pgm file's magic number is the two characters "P2".
.IX "magic numbers"
.IP - 2
Whitespace (blanks, TABs, CRs, LFs).
.IP - 2
A width, formatted as ASCII characters in decimal.
.IP - 2
Whitespace.
.IP - 2
A height, again in ASCII decimal.
.IP - 2
Whitespace.
.IP - 2
The maximum gray value, again in ASCII decimal.
.IP - 2
Whitespace.
.IP - 2
Width * height gray values, each in ASCII decimal, between 0 and the specified
maximum value, separated by whitespace, starting at the top-left
corner of the graymap, proceeding in normal English reading order.
A value of 0 means black, and the maximum value means white.
.IP - 2
Characters from a "#" to the next end-of-line are ignored (comments).
.IP - 2
No line should be longer than 70 characters.
.PP
Here is an example of a small graymap in this format:
.nf
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
.fi
.PP
Programs that read this format should be as lenient as possible,
accepting anything that looks remotely like a graymap.
.PP
There is also a "rawbits" variant on the format.
This variant is different in the following ways:
.IP - 2
The "magic number" is "P5" instead of "P2".
.IP - 2
The gray values are stored as plain bytes, instead of ASCII decimal.
.IP - 2
No whitespace is allowed in the grays section, and only a single character
of whitespace, typically a newline, is allowed after the maxval.
(This last requirement is actually due to a botch in the design of
the file format.)
.IP - 2
The files are smaller and many times faster to read and write.
.PP
If maxval is between 256 and 65535, the gray values are stored as
two bytes, low byte first and high byte second.
Images with maxval higher than 65535 must be written in the slower
but more general plain format.
.SH "SEE ALSO"
pbmplus(1),
pbm(5), ppm(5), pnm(5)
.SH AUTHOR
Copyright (C) 1989, 1991, 1999 by Jef Poskanzer.
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