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.