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)