ppmnorm(1)                                             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 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.

       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-

       pgmnorm(1), ppmhist(1), ppm(5)

       Wilson H. Bent. Jr. (whb@usc.edu), heavily  based  on  the
       pgmnorm filter by Jef Poskanzer.

                          7 October 1993                        1