pgmnorm(1) - man - phpman

pnmnorm(1)                             General Commands Manual                            pnmnorm(1)



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 in‐
       stead of one to designate an option.  You may use either white space or an  equals  sign  be‐
       tween 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  pro‐
       duces 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 possi‐
       ble 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 per‐
       cent 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 (ex‐
       cept that the meaning of the -wpercent and -bpercent options are based on the overall  inten‐
       sities  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 in‐
       tense, 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 in‐
       tensity 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 repre‐
       sentable intensity for the saturated component and the pixel ends up with less overall inten‐
       sity, 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)