PGM (Portable graymap format)

The PGM file format is a bitmap image format used to store grayscale images in either a text or binary format. The text format, which is known as P2, enables easy manual editing of the image data due to it being stored in a human-readable format.

Greyscale data is stored within the file using values between 0 and 255; this gives 256 levels of gray, with 0 being black and 255 being white, and everything in between being a different shade of gray.