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.
A height, again in ASCII decimal.
The maximum gray value (Maxval), again in ASCII decimal. Must be less
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 "linear," i.e. as specified above except without the gamma
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, with the same
gamma transfer function 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
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
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: