xenial (1) pbmreduce.1.gz

Provided by: netpbm_10.0-15.3_amd64 bug

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 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.

OPTIONS

       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.

SEE ALSO

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

AUTHOR

       Copyright (C) 1988 by Jef Poskanzer.

                                                 02 August 1989                                     pbmreduce(1)