Provided by: netpbm_10.0-12.2_i386 bug


       pbmreduce - read a portable bitmap and reduce it N times


       pbmreduce [-floyd|-fs|-threshold ] [-value val] N [pbmfile]


       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.


       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.


       pnmenlarge(1), pnmscale(1), pgmtopbm(1), pbm(5)


       Copyright (C) 1988 by Jef Poskanzer.

                                02 August 1989                    pbmreduce(1)