pnmnorm

   Updated: 7 October 1993

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, pgmhist, pnmgamma, ppmbrighten, ppmdim, pnm
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