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ppmnorm(1) AMIGA (7 October 1993) ppmnorm(1)
NAME
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 pixels
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
levels. 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
prefix.
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.
Page 1 (printed 3/1/94)