Netpbm Format is a collection of graphic formats designed to be easily read an understood by any platform. To accomplish this, they are stored in plain text and do not use compression. Naturally, these graphics are much larger in size than their machine-optimized counterparts like JPEG and PNG.
Netpbm supports three main image types, portable pixmap format (PPM) for color graphics, portable graymap format (PGM), for gray scale, and portable bitmap format (PBM) for black and white. Each is essentially the same format, just with a different header. PPM has multiple variations to support more than 8-bit color depth and alternate channels.
- en.wikipedia.org/wiki/Netpbm_format - Wikipedia.