A file with a PGM (Portable Gray Map) extension is a grayscale image file saved in that format and encoded with one or two bytes (8 or 16 bits) per pixel.
It contains header information and a grid of numbers representing different shades of gray, from black (0) to white (up to 65,536).
PGM files are normally stored in ASCII text format, but they also have a binary representation.
The information included in a PGM file encodes the image header, the dimensions, and the number of shadows.
The PGM format is part of the NetPBM project, which is an open source package of graphics programs.
PGM images can be displayed on most platforms either with the use of the NetPBM package or any multi-format image viewer.
For more general information about files, file extensions and registry you can read one of the following articles:
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