PGM Format Specification(5) PGM Format Specification(5)NAMEpgm - Netpbm grayscale image format
DESCRIPTION
This program is part of Netpbm(1)
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
pseudo-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.
The name "PGM" is an acronym derived from "Portable Gray Map."
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. You can use the libnetpbm(1)
C subroutine library to conveniently and accurately read and interpret
the format.
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 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, and more than zero.
· A single whitespace character (usually a newline).
· A raster of Height rows, in order from top to bottom. Each row
consists of Width gray values, in order from left to right.
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 signifi‐
cant byte is first.
A row of an image is horizontal. A column is vertical. The
pixels in the image are square and contiguous.
Each gray value is a number proportional to the intensity of the
pixel, adjusted by the ITU-R Recommendation BT.709 gamma trans‐
fer 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 image 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 proportional to the fraction of intensity of
a pixel that would show in place of an underlying pixel. So
what normally means white represents total opaqueness and what
normally means black represents 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 * (alpha/alpha_maxval). Note that there is no gamma trans‐
fer function in the transparency mask.
Strings starting with '#' may be comments, the same as with PBM(1)
Note that you can use pamdepth 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(1)
for some commentary on how plain and raw formats relate to one another
and how to use them.
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 num‐
ber (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 image in the plain PGM 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
There is a newline character at the end of each of these lines.
Programs that read this format should be as lenient as possible,
accepting anything that looks remotely like a PGM.
All characters referred to herein are encoded in ASCII. 'newline'
refers the the character known in ASCII as Line Feed or LF. A 'white
space' character is space, CR, LF, TAB, VT, or FF (I.e. what the ANSI
standard C isspace() function calls white space).
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 sam‐
ple. 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 ALSOpnm(1) , pbm(1) , ppm(1) , pam(1) , libnetpbm(1) , programsthatpro‐
cessPGM(1) ,
AUTHOR
Copyright (C) 1989, 1991 by Jef Poskanzer.
netpbm documentation 03 October 2003 PGM Format Specification(5)
