.TH pbmreduce 1 "02 August 1989"
.IX pbmreduce
.SH NAME
pbmreduce - read a portable bitmap and reduce it N times
.SH SYNOPSIS
.B pbmreduce
.RB [ -floyd | -fs | -threshold
.RB ]
.RB [ -value
.IR val ]
.I N
.RI [ pbmfile ]
.SH DESCRIPTION
Reads a portable bitmap as input.
Reduces it by a factor of
.IR N ,
and produces a portable bitmap as output.
.IX shrinking
.PP
.I pbmreduce
duplicates a lot of the functionality of
.I pgmtopbm;
.IX pgmtopbm
you could do something like
.B pnmscale | pgmtopbm,
.IX pnmscale
but
.I pbmreduce
is a lot faster.
.PP
.I pbmreduce
can be used to "re-halftone" an image.
.IX halftoning
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
.IR pbmreduce .
You can even correct the brightness of an image, by using the
.B -value
flag.
.SH OPTIONS
.PP
By default, the halftoning after the reduction is done via
boustrophedonic Floyd-Steinberg error diffusion; however, the
.IX Floyd-Steinberg
.IX "error diffusion"
.B -threshold
flag can be used to specify simple thresholding.  This gives better
.IX thresholding
results when reducing line drawings.
.PP
The
.B -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.
.PP
All flags can be abbreviated to their shortest unique prefix.
.SH "SEE ALSO"
pnmenlarge(1), pnmscale(1), pgmtopbm(1), pbm(5)
.SH AUTHOR
Copyright (C) 1988 by Jef Poskanzer.
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