pgm(5)pgm(5)NAMEpgm - 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 integers, 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
opaqueness 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, 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.
- 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 inten
sity 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 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 frac
tion of intensity of a pixel that would show in place of
an underlying pixel, with the same gamma transfer func
tion mentioned above applied. 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).<
- 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:
- 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 dec
imal 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 ALSOfitstopgm(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.
12 November 1991 pgm(5)
