.TL
PGM Format Specification
.SH 1
pgm
.LP
Updated: 03 October 2003
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Table Of Contents
.SH 2
NAME
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pgm - Netpbm grayscale image format
.SH 2
DESCRIPTION
.LP
.LP
This program is part of Netpbm.
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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).
.LP
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.
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The name "PGM" is an acronym derived from "Portable Gray Map."
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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.
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The format definition is as follows. You can use the libnetpbm C subroutine library to conveniently
and accurately read and interpret the format.
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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.
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Each PGM image consists of the following:
.IP 1
A "magic number" for identifying the file type.
A pgm image's magic number is the two characters "P5".
.IP 2
Whitespace (blanks, TABs, CRs, LFs).
.IP 3
A width, formatted as ASCII characters in decimal.
.IP 4
Whitespace.
.IP 5
A height, again in ASCII decimal.
.IP 6
Whitespace.
.IP 7
The maximum gray value (Maxval), again in ASCII decimal. Must be less
than 65536, and more than zero.
.IP 8
Newline or other single whitespace character.
.IP 9
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 significant byte is first.
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A row of an image is horizontal. A column is vertical. The pixels
in the image are square and contiguous.
.IP 10
Each gray value is a number proportional to the intensity of the
pixel, adjusted by the ITU-R Recommendation BT.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 image and any other image to which the image
might be compared.
.IP 11
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. \fBpnmgamma\fR takes such a PGM
variant as input and produces a true PGM as output.
.IP 12
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 transfer function
in the transparency mask.
.IP 13
Characters from a "#" to
the next end-of-line, before the maxval line, are comments and are
ignored.
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Note that you can use \fBpamdepth\fR to convert between a the
format with 1 byte per gray value and the one with 2 bytes per gray
value.
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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 \fBpbm\fR for some commentary on how plain
and raw formats relate to one another and how to use them.
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The difference in the plain format is:
.RS
.IP "-"
There is exactly one image in a file.
.IP "-"
The magic number is P2 instead of P5.
.IP "-"
Each pixel in the raster is represented as an ASCII decimal number
(of arbitrary size).
.IP "-"
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.
.IP "-"
No line should be longer than 70 characters.
.RE
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Here is an example of a small image in the plain PGM format.
.DS L
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
.DE
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There is a newline character at the end of each of these lines.
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Programs that read this format should be as lenient as possible,
accepting anything that looks remotely like a PGM.
.SH 2
COMPATIBILITY
.LP
.LP
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.
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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.
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.SH 2
SEE ALSO
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pnm,
pbm,
ppm,
pam,
libnetpbm,
programs that process PGM,
.SH 2
AUTHOR
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Copyright (C) 1989, 1991 by Jef Poskanzer.
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\l'5i'
.SH 2
Table Of Contents
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NAME
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DESCRIPTION
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COMPATIBILITY
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SEE ALSO
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AUTHOR
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