pbmreduce Man page

pbmreduce General Commands Manual pbmreduce

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 reso‐
lution, 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 boustrophe‐
donic 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, pnmscale, pgmtopbm, pbm(5)

AUTHOR

Copyright (C) 1988 by Jef Poskanzer.

02 August 1989 pbmreduce

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