There are two tricks which together can get up to 5 additional bytes per pixel in PAM file.
First trick:
You can try store additional byte of information in alpha channel and then choose to ignore that information in decoder. Enabling alpha channel in PAM is done by adding _APLHA
to TUPLTYPE
argument, so instead TUPLTYPE RGB
you have TUPLTYPE RGB_ALPHA
.
Second trick:
You can set MAXVAL
in PAM (or equivalent field in PPM and others) to 65535
instead of 255
, which means that every pixel will be described by three 16-bit values instead of three 8-bit ones. Now, for these 16-bit values the 8 least significant bits can be used to store information as they do not affect visual properties of image when shown on typical computer screen.
First + second trick:
This gives you additional 3 x 8 = 24 bits for RGB planes and 16 bits in alpha channel. Which means: 5 bytes.