pgm
Updated: 03 October 2003
Table Of Contents
NAMEpgm ‐ Netpbm grayscale image format
DESCRIPTION
This program is part of Netpbm.
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 be‐
cause 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 pro‐
grams in the
world that think they’re processing a grayscale image can eas‐
ily 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 trans‐
parency 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
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:
1. A "magic number" for identifying the file type. A pgm im‐
age’s magic
number is the two characters "P5".
2. Whitespace (blanks, TABs, CRs, LFs).
3. A width, formatted as ASCII characters in decimal.
4. Whitespace.
5. A height, again in ASCII decimal.
6. Whitespace.
7. The maximum gray value (Maxval), again in ASCII decimal.
Must be less
than 65536, and more than zero.
8. Newline or other single whitespace character.
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 Max‐
val 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.
A row of an image is horizontal. A column is vertical. The
pixels in the
image are square and contiguous.
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 there‐
fore black. A
value of Maxval represents CIE D65 white and the most in‐
tense value in
the image and any other image to which the image might be
compared.
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. pnmgamma takes such a PGM variant as input and
produces a
true PGM as output.
12. In the transparency mask variation on PGM, the value
represents
opaqueness. It is proportional to the fraction of intensi‐
ty 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.
13. Characters from a "#" to the next end‐of‐line, before the
maxval line,
are comments and are ignored.
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 consid‐
ered the normal
one, is known as the "raw" PGM format. See pbm for some com‐
mentary on how
plain and raw formats relate to one another.
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 af‐
ter 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 possi‐
ble, accepting
anything that looks remotely like a PGM.
COMPATIBILITY
Before April 2000, a raw format PGM file could not have a max‐
val 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
pnm, pbm, ppm, pam, libnetpbm, programs that process PGM,
AUTHOR
Copyright (C) 1989, 1991 by Jef Poskanzer.
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Table Of Contents
* NAME
* DESCRIPTION
* COMPATIBILITY
* SEE ALSO
* AUTHOR
