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)