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pgm
!!!pgm
NAME
DESCRIPTION
COMPATIBILITY
SEE ALSO
AUTHOR
----
!!NAME


pgm - portable graymap file format
!!DESCRIPTION


The portable graymap format is a lowest common denominator
grayscale file format.


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


-


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
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
pnmgamma__
takes such a PGM variant as input and produces a true PGM as
output.


-


Characters from a


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:
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 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), 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.
----
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Last edited on Monday, June 3, 2002 6:56:00 pm by "perry"
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