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RedHat 9 (Linux i386) - man page for ppmnorm (redhat section 1)

ppmnorm(1)						      General Commands Manual							ppmnorm(1)

ppmnorm - normalize the contrast in a portable pixmap
ppmnorm [-bpercent N | -bvalue N] [-wpercent N | -wvalue N] [ppmfile]
Reads a portable pixmap as input. Normalizes the contrast by forcing the lightest pixels to white, the darkest pixels to black, and lin- early 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.
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.
pgmnorm(1), ppmhist(1), ppm(5)
Wilson H. Bent. Jr. (, heavily based on the pgmnorm filter by Jef Poskanzer. 7 October 1993 ppmnorm(1)