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  reso-
       lution,  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 boustrophe-
       donic 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)
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