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pbmreduce.man
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1995-03-17
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pbmreduce(1) AMIGA (02 August 1989) pbmreduce(1)
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.
Page 1 (printed 10/19/91)