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The manual page of the pgm (Portable Graymap Image) image format. The PGM format is a grayscale file format. It is designed to be extremely easy to learn and to write a variety of programs. It’s so simple that most people would rather simply decipher it than read the specification to it.
For example, the image format can be opened with NetPBM or GIMP on Linux and Adobe Photoshop on Windows.
Man page output
man pgmpgm(5) File Formats Manual pgm(5)
NAME
pgm - portable graymap file format
DESCRIPTION
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 inte‐
gers, 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 opaque‐
ness 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, delim‐
iters, or padding before, after, or between images.
Each PGM image consists of the following:
- A "magic number" for identifying the file type. A pgm image'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.
- Each gray value is a number proportional to the intensity of the pixel, adjusted by the
CIE Rec. 709 gamma transfer function. (That transfer function specifies a gamma number
of 2.2 and has a linear section for small intensities). A value of zero is therefore
black. A value of Maxval represents CIE D65 white and the most intense value in the im‐
age and any other image to which the image might be compared.
- Note that a common variation on the PGM format is to have the gray value be "linear,"
i.e. as specified above except without the gamma adjustment. pnmgamma takes such a PGM
variant as input and produces a true PGM as output.
- In the transparency mask variation on PGM, the value represents opaqueness. It is pro‐
portional to the fraction of intensity of a pixel that would show in place of an under‐
lying pixel, with the same gamma transfer function mentioned above applied. So what
normally means white represents total opaqueness and what normally means black repre‐
sents total transparency. In between, you would compute the intensity of a composite
pixel of an "under" and "over" pixel as under * (1-(alpha/alpha_maxval)) + over * (al‐
pha/alpha_maxval).<
- 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 for‐
mat. 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 an‐
other.
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.
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), pgmbent‐
ley(1), pgmcrater(1), pgmedge(1), pgmenhance(1), pgmhist(1), pgmnorm(1), pgmoil(1), pgm‐
ramp(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.
12 November 1991 pgm(5)
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