pbmreduce(1)pbmreduce(1)NAMEpbmreduce - read a portable bitmap and reduce it N times
SYNOPSISpbmreduce [-floyd|-fs|-threshold ] [-value val] N [pbm-
file]
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 pgm-
topbm; 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 thresh-
olding. 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 pre-
fix.
SEE ALSOpnmenlarge(1), pnmscale(1), pgmtopbm(1), pbm(5)AUTHOR
Copyright (C) 1988 by Jef Poskanzer.
02 August 1989 1
