pgm(5)

NAME
pgm - portable graymap file format

DESCRIPTION
The portable graymap format is a lowest common denominator grayscale file format. The definition is as follows:

-   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:

-   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.

COMPATIBILITY
Before April 2000, a raw format PBM 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.

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)

AUTHOR
Copyright (C) 1989, 1991 by Jef Poskanzer.