Manual: pnmnorm
Pnmnorm User Manual(1) General Commands Manual Pnmnorm User Manual(1)
NAME
pnmnorm - normalize the contrast in a Netpbm image
SYNOPSIS
pnmnorm
[-bpercent=percent | -bvalue=N | -bsingle]
[-wpercent=percent | -wvalue=N | -wsingle]
[-midvalue=N]
[-middle=N]
[-maxexpand=percent]
[-keephues]
[-luminosity | -colorvalue | -saturation]
[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
This program is part of Netpbm(1).
pnmnorm reads a PNM image (PBM, PGM, or PPM). It normalizes the contrast by forcing the brightest pixels to white,
the darkest pixels to black, and spreading out the ones in between. It produces the same kind of file as output.
This is pretty useless for a PBM image.
The program offers two ways of spreading out the pixels in between the darkest and brightest: linear and quadratic.
In the quadratic case, you specify some in between brightness and specify what brightness that should become in the
output. With those three constraints: the brightness that becomes black, the brightness that becomes white, and the
brightness that becomes that middle value, pnmnorm computes a quadratic equation that maps all the other brightnesses
from input values to output values.
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.
For a PPM image, you have a choice of three ways to define brightness:
⢠luminosity
⢠color value
⢠saturation
In the case of saturation, "brightness" is pretty much a misnomer, but you can use the brightness analogy to
see what it does. In the analogy, bright means saturated and dark means unsaturated.
Note that all of these are different from separately normalizing the individual color components.
An alternative way to spread out the brightnesses in an image is pnmhisteq. pnmhisteq stretches the brightest pixels
to white and the darkest pixels to black, but rather than linearly adjusting the ones in between, it adjusts them so
that there are an equal number of pixels of each brightness throughout the range. This gives you more contrast than
pnmnorm does, but can considerably change the picture in exchange.
OPTIONS
In addition to the options common to all programs based on libnetpbm (most notably -quiet, see Common Options ), pnâ
mnorm recognizes the following command line options:
By default, the darkest 2 percent of all pixels are mapped to black, and the brightest 1 percent are mapped to white.
You can override this behavior and specify either a different percentage, or specific brightness values to map to
black and to white, or just have the single greatest brightness map to white and the least brightness map to black.
-bpercent
-wpercent
-bvalue
-wvalue
To specify a percentage, use the -bpercent and -wpercent options, or you can specify the exact pixel values to
be mapped by using the -bvalue and -wvalue options. You can get appropriate numbers for the options from
ppmhist. 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
brighten 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.
If you specify both -bvalue and -bpercent, pnmnorm uses the one that produces the least change. The same goes
for -wvalue and -wpercent. (In Netpbm 10.26 (January 2005), the -bvalue/-wvalue takes precedence, and before
that, it's a syntax error to specify both).
If you want to maximize the change instead of minimizing it, just cascade two runs of pnmnorm, specifying valâ
ues for the first and percentages for the second.
-bpercent and -wpercent values are floating point decimal. Zero is valid and is the same as -bvalue=0 or
-wvalue=maxval, respectively.
Because there are whole numbers of pixels at each brightness, pnmnorm obviously can't guarantee the exact perâ
centage, so it arranges that at least the percentage of pixels you specify get remapped as promised.
It is possible for your -bpercent or -wpercent to overlap your -wvalue or -bvalue, respectively. For example,
you say -bpercent=20 and -wvalue=100 for an image in which only 10 percent of the pixels are darker than 100.
In that case, pnmnorm adjusts the percentile value as required. In the example, it uses 99 as the black value
(like -bvalue=99).
It is also possible for your -bpercent and -wpercent options to select the same brightness value for the
stretch-to-white and stretch-to-black value because of the fact that pnmnorm can't subdivide a histogram cell.
E.g. if an image is all brightness 100, then no matter what -bpercent and -wpercent values you choose, it's
the same as saying -bvalue=100 -wvalue=100. In that case, pnmnorm changes one of the values by 1 to make it
legal. In the example, pnmnorm would either make the black value 99 or the white value 101.
Before Netpbm 10.43 (June 2008), pnmnorm fails if the -wpercent and/or -bpercent values specify an overlap.
The stretch points are further constrained by the -maxexpand option. Sometimes, too much contrast is a bad
thing. If your intensities are all concentrated in the middle, -bpercent=2 and -wpercent=1 might mean that an
intensity of 60 gets stretched up to 100 and intensity of 20 gets stretched down to zero, for a range expanâ
sion of 150% (from a range of 40 to a range of 100). That much stretching means two adjacent pixels that used
to differ in intensity by 4 units now differ by 10, and that might be unsightly.
-bsingle
To specify that the single least brightness in the image should stretch to black in the output, specify -bsinâ
gle. To specify that the single greatest brightness in the image should stretch to white in the output, specâ
ify -wsingle. -bsingle and -wsingle were new in Netpbm 10.69 (December 2014).
-maxexpand
So that you can put a limit on the amount of expansion without having to examine the image first, there is the
-maxexpand option. It specifies the maximum expansion you will tolerate, as an additional percentage. In the
example above, you could say -maxexpand=50 to say you want the range to expand by at most 50%, regardless of
your other options. pnmnorm figures out what intensity to stretch to full intensity and what intensity to
stretch to zero intensity as described above, and then raises the former and lowers the latter as needed to
limit the expansion to the amount you specified.
When pnmnorm limits the expansion because of -maxexpand, it tells you about it with a message like this:
limiting expansion of 150% to 50%
In any case, pnmnorm tells you exactly what expansion it's doing, like this:
remapping 25..75 to 0..100
Before Netpbm 10.26 (December 2004), it was not valid to specify both -bvalue and -bpercent or -wvalue and
-wpercent.
-maxexpand was new in Netpbm 10.32 (February 2006).
-keephues
This option says to keep each pixel the same hue as it is in the input; just adjust its brightness. You norâ
mally want this; the only reason it is not the default behavior is backward compatibility with a design misâ
take.
By default, pnmnorm normalizes contrast in each component independently (except that the meaning of the -wperâ
cent and -bpercent options are based on the overall brightnesses of the colors, not each component taken sepaâ
rately). So if you have a color which is intensely red but dimly green, pnmnorm would make the red more inâ
tense and the green less intense, so you end up with a different hue than you started with.
-midvalue=N
-middle=N
When you specify -midvalue=N, pnmnorm uses a quadratic function to map old brightnesses to new ones, making
sure that an old brightness of N becomes 50% bright in the output. You can override that 50% default with
-middle. The value of -middle is a floating point number in the range 0 through 1 with 0 being full darkness
and 1 full brightness. If your -midvalue and -middle indicate an ambiguous or impossible quadratic function
(e.g. -midvalue is the same as -bvalue, so an infinite number of quadratic functions fit), pnmnorm just igâ
nores your -midvalue and maps linearly.
-midvalue and -middle were new in Netpbm 10.57 (December 2011).
If you specify -keephues, pnmnorm would likely leave this pixel alone, since its overall brightness is medium.
-keephues can cause clipping, because a certain color may be below a target intensity while one of its compoâ
nents 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.
When you don't specify -keephues, the -luminosity, -colorvalue, and -saturation options affect the transfer
function (which is the same for all three RGB components), but are meaningless when it comes to applying the
transfer function (since it is applied to each individual RGB component).
Before Netpbm 9.25 (March 2002), there was no -keephues option.
-luminosity
-colorvalue
-saturation
-luminosity, -colorvalue, and -saturation determine what property of the pixels pnmnorm normalizes. I.e.,
what kind of brightness. You cannot specify more than one of these.
The -luminosity option says to use the luminosity (i.e. the "Y" in the YUV or YCbCr color space) as the
pixel's brightness. The luminosity is a measure of how bright a human eye would find the color, taking into
account the fact that the human eye is more sensitive to some RGB components than others.
This option is default.
This option is meaningless on grayscale images.
Before Netpbm 10.28 (August 2005), there was no -luminosity option, but its meaning was still the default.
Before Netpbm 10.28 (August 2005), there was no -colorvalue option.
The -colorvalue option says to use the color value (i.e. the "V" in the HSV color space) as the pixel's
brightness. The color value is the gamma-adjusted intensity of the most intense RGB component.
This option is meaningless on grayscale images.
Before Netpbm 10.28 (August 2005), there was no -colorvalue option.
The -saturation option says to use the saturation (i.e. the "S" in the HSV color space) as the pixel's brightâ
ness. The saturation is the ratio of the intensity of the most intense RGB component to the difference beâ
tween the intensities of the most and least intense RGB component (all intensities gamma-adjusted).
In this case, "brightness" is more of a metaphor than anything. "bright" means saturated and "dark" means unâ
saturated.
This option is meaningless on grayscale images.
Before Netpbm 10.28 (August 2005), there was no -colorvalue option.
SEE ALSO
pnmhisteq(1), pamlevels(1), ppmhist(1), pgmhist(1), pambrighten(1), ppmdim(1), pnmgamma(1), pnm(1)
DOCUMENT SOURCE
This manual page was generated by the Netpbm tool 'makeman' from HTML source. The master documentation is at
http://netpbm.sourceforge.net/doc/pnmnorm.html
netpbm documentation 19 December 2014 Pnmnorm User Manual(1)