Reads a portable bitmap as input. Reduces it by a factor of N, and produces a portable bitmap as output.
pbmreduce duplicates a lot of the functionality of pgmtopbm; you could do something like pnmscale | pgmtopbm, but pbmreduce is a lot faster.
By default, the halftoning after the reduction is done via boustrophedonic Floyd-Steinberg error diffusion; however, the -threshold flag can be used to specify simple thresholding. This gives better results when reducing line drawings.
The -value flag alters the thresholding value for all quantizations. It should be a real number between 0 and 1. Above 0.5 means darker images; below 0.5 means lighter.