Portable Gray Map

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The PGM(Portable Gray Map) file format is the lowest common denominator grayscale file format. It simply contains the grayscale pixel value matrix in raw form. This makes for a format that is extremely easy to learn and manipulate.

There are many variations on the PGM, one of which is the transparency mask. A transparency mask is represented by a PGM image, except that in place of pixel intensities, there are opaqueness values.

Structure

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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.

Each PGM image consists of the following:

1. A "magic number" for identifying the file type. A pgm image'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 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.
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.
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 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.
13. Characters from a "#" to the next end-of-line, before the maxval line, are comments and are ignored.

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 for some commentary on how plain and raw formats relate to one another and how to use them.

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.

Source: http://netpbm.sourceforge.net/doc/pgm.html