|Linux & Unix Commands - Search Man Pages
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 pbmre-
duce. 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)
All times are GMT -4. The time now is 08:11 PM.