man > pbmreduce(1)

Pbmreduce User Manual(1)               General Commands Manual              Pbmreduce User Manual(1)

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


SYNOPSIS
       pbmreduce [-floyd|-fs|-threshold] [-value val] [-randomseed=integer] N [pbmfile]

       You can abbreviate any option to its shortest unique prefix.


DESCRIPTION
       This program is part of Netpbm(1).

       pbmreduce  reads  a PBM image as input and reduces it by a factor of N, producing a PBM image
       as output.

       pbmreduce duplicates a lot of the functionality of pamditherbw; you could do  something  like
       pamscale | pamditherbw, but pbmreduce is a lot faster.

       You can use pbmreduce to "re-halftone" an image.  Let's say you have a scanner that only pro‐
       duces 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 res‐
       olution,  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 option.


OPTIONS
       In addition to the options common to all programs based on libnetpbm  (most  notably  -quiet,
       see ]8;;index.html#commonoptions\ Common Options]8;;\ ), pbmreduce recognizes the following command line options:




       -threshold
              By  default,  pbmreduce  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.


       -floyd, -fs
              Specify the Floyd-Steinberg error diffusion method.  This is the default.


       -value

              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.


       -randomseed=integer
              This is the seed for the random number generator that controls the halftoning.

              Use this to ensure you get the same image on separate invocations.

              This option was new in Netpbm 10.75 (June 2016).





SEE ALSO
       pamenlarge(1), pamscale(1), pamditherbw(1), pbm(1)


AUTHOR
       Copyright (C) 1988 by Jef Poskanzer.

DOCUMENT SOURCE
       This manual page was generated by the Netpbm tool 'makeman' from  HTML  source.   The  master
       documentation is at

              http://netpbm.sourceforge.net/doc/pbmreduce.html

netpbm documentation                        13 April 2016                   Pbmreduce User Manual(1)