\
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.TH "Pbmreduce User Manual" 1 "13 April 2016" "netpbm documentation"

.SH NAME
pbmreduce - read a PBM image and reduce it N times

.UN synopsis
.SH SYNOPSIS

\fBpbmreduce\fP
[\fB-floyd\fP|\fB-fs\fP|\fB-threshold\fP]
[\fB-value\fP \fIval\fP]
[\fB-randomseed=\fP\fIinteger\fP]
\fIN\fP
[\fIpbmfile\fP]
.PP
You can abbreviate any option to its shortest unique prefix.

.UN description
.SH DESCRIPTION
.PP
This program is part of
.BR "Netpbm" (1)\c
\&.
.PP
\fBpbmreduce\fP reads a PBM image as input and reduces it by a
factor of \fIN\fP, producing a PBM image as output.
.PP
\fBpbmreduce\fP duplicates a lot of the functionality of
\fBpamditherbw\fP; you could do something like \f(CWpamscale |
pamditherbw\fP, but \fBpbmreduce\fP is a lot faster.
.PP
You can use \fBpbmreduce\fP 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 \fBpbmreduce\fP.  You can even
correct the brightness of an image, by using the \fB-value\fP option.

.UN options
.SH OPTIONS
.PP
In addition to the options common to all programs based on libnetpbm
(most notably \fB-quiet\fP, see 
.UR index.html#commonoptions
 Common Options
.UE
\&), \fBpbmreduce\fP recognizes the following
command line options:



.TP
\fB-threshold\fP
By default, \fBpbmreduce\fP does the halftoning after the reduction via
boustrophedonic Floyd-Steinberg error diffusion; however, you can use this
option to specify simple thresholding.  This gives better results when
reducing line drawings.

.TP
\fB-floyd\fP, \fB-fs\fP
Specify the Floyd-Steinberg error diffusion method.  This is the
default.

.TP
\fB-value\fP
.sp
This option 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.

.TP
\fB-randomseed=\fP\fIinteger\fP
This is the seed for the random number generator that controls the
halftoning.
.sp
Use this to ensure you get the same image on separate invocations.
.sp
This option was new in Netpbm 10.75 (June 2016).




.UN seealso
.SH SEE ALSO
.BR "pamenlarge" (1)\c
\&,
.BR "pamscale" (1)\c
\&,
.BR "pamditherbw" (1)\c
\&,
.BR "pbm" (1)\c
\&

.UN author
.SH AUTHOR

Copyright (C) 1988 by Jef Poskanzer.
.SH DOCUMENT SOURCE
This manual page was generated by the Netpbm tool 'makeman' from HTML
source.  The master documentation is at
.IP
.B http://netpbm.sourceforge.net/doc/pbmreduce.html
.PP