pgmnorm - normalize the contrast in a portable graymap
pgmnorm [-bpercent N | -bvalue N] [-wpercent N | -wvalue
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.
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-
pgmhist(1), ppmnorm(1), pgm(5)
Partially based on the fbnorm filter in Michael Mauldin's
"Fuzzy Pixmap" package.
Copyright (C) 1989 by Jef Poskanzer.
28 February 1989 1