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NAME

       pnmnorm - normalize the contrast in a Netbpm image

SYNOPSIS

       pnmnorm [-bpercent N | -bvalue N] [-wpercent N | -wvalue N] [-keephues]
       [-brightmax]

       [ppmfile]

       All options can be abbreviated to their shortest  unique  prefix.   You
       may use two hyphens instead of one to designate an option.  You may use
       either white space or an equals sign between an  option  name  and  its
       value.

DESCRIPTION

       Reads  a  PNM  image  (PBM,  PGM,  or PPM).  Normalizes the contrast by
       forcing the lightest pixels to white, the darkest pixels to black,  and
       linearly  rescaling  the ones in between; and produces the same kind of
       file as output.  This is pretty useless for a PBM image.

       The program first  determines  a  mapping  of  old  brightness  to  new
       brightness.   For  each  possible  brightness  of  a pixel, the program
       determines a corresponding brightness for the output image.

       Then for each pixel in the image, the program computes  a  color  which
       has  the desired output brightness and puts that in the output.  With a
       color image, it is not always possible to  compute  such  a  color  and
       retain  any semblance of the original hue, so the brightest and dimmest
       pixels may only approximate the desired brightness.

       Note  that  for  a  PPM  image,  this  is  different  from   separately
       normalizing the individual color components.

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.

       The  -keephues  option says to keep each pixel the same hue as it is in
       the input; just adjust its intensity.  By default,  pnmnorm  normalizes
       contrast  in  each  component independently (except that the meaning of
       the  -wpercent  and  -bpercent  options  are  based  on   the   overall
       intensities of the colors, not each component taken separately).  So if
       you have a color which is intensely red but dimly green, pnmnorm  would
       make  the  red  more  intense and the green less intense, so you end up
       with a different hue than you started with.

       If you specify -keephues, pnmnorm would likely leave this pixel  alone,
       since its overall intensity is medium.

       -keephues  can  cause  clipping, because a certain color may be below a
       target intensity while one  of  its  components  is  saturated.   Where
       that’s  the  case, pnmnorm uses the maximum representable intensity for
       the saturated component  and  the  pixel  ends  up  with  less  overall
       intensity, and a different hue, than it is supposed to have.

       This option is meaningless on grayscale images.

       Before March 2002, there was no -keephues option.

       The -brightmax option says to use the intensity of the most intense RGB
       component of a pixel as the pixel’s brightness.   By  default,  pnmnorm
       uses the luminosity of the color as its brightness.

       This option is meaningless on grayscale images.

       Before March 2002, there was no -brightmax option.

SEE ALSO

       ppmhist(1), pgmhist(1), pnmgamma(1), ppmbrighten(1), ppmdim(1), pnm(5)

                                7 October 1993                      pnmnorm(1)