ppmnorm - normalize the contrast in a portable pixmap
SYNOPSIS
ppmnorm [-bpercent N | -bvalue N] [-wpercent N | -wvalue
N] [ppmfile]
DESCRIPTION
Reads a portable pixmap as input. Normalizes the contrast
by forcing the lightest pixels to white, the darkest pix
els to black, and linearly rescaling the ones in between;
and produces a portable pixmap as output.
Works by computing the realtive grey-level of each pixel
a'la ppmtopgm, and uses those values to scale the RGB lev
els. Note that this is different from using pgmnorm on
the individual red, green, and blue greymaps (as produced
by ppmtorgb3) and recombining them.
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 ppmhist 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
pgmnorm(1), ppmhist(1), ppm(5)
AUTHOR
Wilson H. Bent. Jr. (whb@usc.edu), heavily based on the
pgmnorm filter by Jef Poskanzer.
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