Man page 1 : pbmreduce

pbmreduce - read a portable bitmap and reduce it N times

pbmreduce [ -floyd | -fs | -threshold ] [ -value val ] N [ pbmfile ]

Reads a portable bitmap as input. Reduces it by a factor of N , and produces a portable bitmap as output.

pbmreduceduplicates a lot of the functionality of pgmtopbm;you could do something like

pnmscale | pgmtopbm, but pbmreduceis a lot faster.

pbmreducecan be used to "re-halftone" an image. Let's say you have a scanner that only produces black&white, not grayscale, and it does a terrible job of halftoning (most b&w scanners fit this description). One way to fix the halftoning is to scan at the highest possible resolution, say 300 dpi, and then reduce by a factor of three or so using pbmreduce . You can even correct the brightness of an image, by using the

-value flag.

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.

All flags can be abbreviated to their shortest unique prefix.

pnmenlarge(1), pnmscale(1), pgmtopbm(1), pbm(5)

Copyright (C) 1988 by Jef Poskanzer.