read a portable bitmap and reduce it N times
see also :
pnmenlarge - pnmscale - pgmtopbm
[-floyd|-fs|-threshold ] [-value
val] N [pbmfile]
add an example, a script, a trick and tips
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portable bitmap as input. Reduces it by a factor of
N, and produces a portable bitmap as output.
duplicates a lot of the functionality of pgmtopbm;
you could do something like pnmscale | pgmtopbm, but
pbmreduce is a lot faster.
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
-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.
pnmscale , pgmtopbm , pbm
1988 by Jef Poskanzer.