SunOS man pages : pgm (5)
Standards, Environments, and Macros pgm(5)
NAME
pgm - portable graymap file format
DESCRIPTION
The portable graymap format is a lowest common denominator
grayscale file format.
The format definition is as follows.
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
file'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.
- Newline or other single whitespace character.
- A raster of Width * Height gray values, proceeding through
the image in normal English reading order. 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.
- Characters from a "#" to the next end-of-line, before the
maxval line, are comments and are ignored.
Note that you can use pnmdepth To convert between a the for-
mat with 1 byte per gray value and the one with 2 bytes per
gray value.
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(5) for some commentary on how plain and
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Standards, Environments, and Macros pgm(5)
raw formats relate to one another.
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 graymap in this 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
Programs that read this format should be as lenient as pos-
sible, accepting anything that looks remotely like a gray-
map.
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
fitstopgm(1), fstopgm(1), hipstopgm(1), lispmtopgm(1), psidtopgm(1)
, rawtopgm(1), pgmbentley(1), pgmcrater(1),
pgmedge(1), pgmenhance(1), pgmhist(1), pgmnorm(1),
pgmoil(1), pgmramp(1), pgmtexture(1), pgmtofits(1),
pgmtofs(1), pgmtolispm(1), pgmtopbm(1), pnm(5), pbm(5),
ppm(5)
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Standards, Environments, and Macros pgm(5)
AUTHOR
Copyright (C) 1989, 1991 by Jef Poskanzer.
SunOS 5.8 Last change: 12 November 1991 3
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