What is Portable Pixmap file format?
PXM is a collective name for portable pixmap / greymap / bitmap file formats (PPM, PGM, PBM) which are basic file formats for exchanging graphics files.
Depending on the identification of the file format three similar file formats are distinguished, each with two versions:
pbm - portable bitmap file format (P1/P4) - 1 bit per pixel
pgm - portable graymap file format (P2/P5) - 8 bits per pixel
ppm - portable pixmap file format (P3/P6) - 24 bits per pixel, 8 for red, 8 for green, 8 for blue
In each case, the lower-numbered version refers to a human-readable, ASCII-based format similar to the one in the example above and the higher-numbered version refers to a binary format which is not human-readable but saves some space in the file, as well as being easier to parse due to the lack of white space.
The original definition of the PGM and the PPM binary formats (the P5 and P6 formats) did not support bit-depths greater than 8 bits. Then there came some attempts to extend the format. But the problem is that the various implementations could not agree on which bit depth to use. Therefore, Netpbm which is the de facto standard implementation of the PXM formats uses big-endian.
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