ppmnorm (1)
NAME
pnmnorm - normalize the contrast in a Netbpm imageSYNOPSIS
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.