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)