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pbmreduce(1) Unix Programmer's Manual 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.
02 August 1989 1