ppmnorm
normalize the contrast in a Netbpm image
see also :
ppmhist - pgmhist - pnmgamma - ppmbrighten - ppmdim
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.
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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