# info > pbmreduce

[pbmreduce(1)](https://www.chedong.com/phpMan.php/man/pbmreduce/1/markdown)                General Commands Manual               [pbmreduce(1)](https://www.chedong.com/phpMan.php/man/pbmreduce/1/markdown)

## NAME
       pbmreduce - read a portable bitmap and reduce it N times

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

## DESCRIPTION
       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.

## OPTIONS
       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 im-
       ages; below 0.5 means lighter.

       All flags can be abbreviated to their shortest unique prefix.

## SEE ALSO
       [pnmenlarge(1)](https://www.chedong.com/phpMan.php/man/pnmenlarge/1/markdown), [pnmscale(1)](https://www.chedong.com/phpMan.php/man/pnmscale/1/markdown), [pgmtopbm(1)](https://www.chedong.com/phpMan.php/man/pgmtopbm/1/markdown), [pbm(5)](https://www.chedong.com/phpMan.php/man/pbm/5/markdown)

## AUTHOR
       Copyright (C) 1988 by Jef Poskanzer.

                                02 August 1989                    [pbmreduce(1)](https://www.chedong.com/phpMan.php/man/pbmreduce/1/markdown)