pnmdepth - change the maxval in a portable anymap
Reads a portable anymap as input. Scales all the pixel
values, and writes out the image with the new maxval.
Scaling the colors down to a smaller maxval will result in
some loss of information.
Be careful of off-by-one errors when choosing the new
maxval. For instance, if you want the color values to be
five bits wide, use a maxval of 31, not 32.
One important use of pnmdepth is to convert a new
format 2-byte-per-sample PNM file to the older
1-byte-per-sample format. Before April 2000, essentially all
raw (binary) format PNM files had a maxval less than 256 and
one byte per sample, and many programs may rely on that. If
you specify a newmaxval less than 256, the resulting
file should be readable by any program that worked with PNM
files before April 2000.
pnm(5), ppmquant(1), ppmdither(1)
Copyright (C) 1989, 1991 by Jef Poskanzer.