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pgm - portable graymap file format
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 format 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
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 possible, accepting
anything that looks remotely like a graymap.
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.
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)
Copyright (C) 1989, 1991 by Jef Poskanzer.
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