pbmreduce (1)
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.
shrinking
pbmreduce
duplicates a lot of the functionality of
pgmtopbm;
pgmtopbm
you could do something like
pnmscale | pgmtopbm,
pnmscale
but
pbmreduce
is a lot faster.
pbmreduce
can be used to "re-halftone" an image.
halftoning
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
Floyd-Steinberg
"error diffusion"
-threshold
flag can be used to specify simple thresholding. This gives better
thresholding
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
AUTHOR
Copyright (C) 1988 by Jef Poskanzer.
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