pgm(5) | File Formats Manual | pgm(5) |
pgm - portable graymap file format
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 psueudo-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.
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 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:
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:
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.
12 November 1991 |