pbmreduce(1)                                         pbmreduce(1)

       pbmreduce - read a portable bitmap and reduce it N times

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

       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 pgm-
       topbm; 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  thresh-
       olding.   This  gives  better  results  when reducing line

       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

       All flags can be abbreviated to their shortest unique pre-

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

       Copyright (C) 1988 by Jef Poskanzer.

                          02 August 1989                        1