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 out-
       put.

       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 dif-
       ferent 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.

                                7 October 1993                      ppmnorm(1)