16-bit Extensions
The original definition of the PGM and the PPM binary formats (the P5 and P6 formats) did not allow bit depths greater than 8 bits. One can of course use the ASCII format, but this format both slows down reading and makes the files much larger. Accordingly, many programmers extended the format to allow higher bit depths. Using higher bit depths encounters the problem of having to decide on the endianness of the file. The various implementations could not agree on which byte order to use, and some connected the 16-bit endianness to the pixel packing order. In Netpbm, the de facto standard implementation of the PNM formats, the most significant byte is first.
Read more about this topic: Netpbm Format
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