Provided by: netpbm_10.0-15_amd64 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)