NAME
pbmreduce - read a portable bitmap and reduce it N times
SYNOPSIS
pbmreduce [-floyd|-fs|-threshold ] [-value val] N [pbmfile]
DESCRIPTION
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.
pbmreduce can 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.
OPTIONS
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.
SEE ALSO
pnmenlarge(1), pnmscale(1), pgmtopbm(1), pbm(5)
AUTHOR
Copyright (C) 1988 by Jef Poskanzer.
02 August 1989 pbmreduce(1)