pgmnorm - normalize the contrast in a portable graymap
SYNOPSIS
pgmnorm [-bpercent N | -bvalue N] [-wpercent N | -wvalue
N] [pgmfile]
DESCRIPTION
Reads a portable graymap as input. Normalizes the con
trast by forcing the lightest pixels to white, the darkest
pixels to black, and linearly rescaling the ones in
between; and produces a portable graymap as output.
OPTIONS
By default, the darkest 2 percent of all pixels are mapped
to black, and the lightest 1 percent are mapped to white.
You can override these percentages by using the -bpercent
and -wpercent flags, or you can specify the exact pixel
values to be mapped by using the -bvalue and -wvalue
flags. Appropriate numbers for the flags can be gotten
from the pgmhist tool. If you just want to enhance the
contrast, then choose values at elbows in the histogram;
e.g. if value 29 represents 3% of the image but value 30
represents 20%, choose 30 for bvalue. If you want to
lighten the image, then set bvalue to 0 and just fiddle
with wvalue; similarly, to darken the image, set wvalue to
maxval and play with bvalue.
All flags can be abbreviated to their shortest unique pre
fix.
SEE ALSO
pgmhist(1), ppmnorm(1), pgm(5)
AUTHOR
Partially based on the fbnorm filter in Michael Mauldin's
"Fuzzy Pixmap" package.
Copyright (C) 1989 by Jef Poskanzer.
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