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pnmgamma
!!!pnmgamma
NAME
SYNOPSIS
DESCRIPTION
WHAT IS GAMMA?
SEE ALSO
AUTHOR
----
!!NAME


pnmgamma - perform gamma correction on a PNM image
!!SYNOPSIS


__pnmgamma__ [[__-ungamma__] [[__-cieramp__]
[[''value'' [[''pnmfile'']]__
pnmgamma__ [[__-ungamma__] [[__-cieramp__] ''redgamma
greengamma bluegamma'' [[''pnmfile'']
!!DESCRIPTION


Reads a psuedo-PNM image as input. Performs gamma
correction, and produces a PNM image as output.


Alternatively, this program can undo gamma
correction.


The PPM specification states that the sample values in the
image are nonlinear, i.e. not directly proportional to light
intensity, i.e. gamma corrected. But there exist images that
are PPM in every respect except that their sample values are
directly proportional to light intensity. People may loosely
refer to these as PPM, but they are not. __pnmgamma__
converts these pseudo-PPM images to true PPM by performing
gamma correction. To get true PPM, you must specify the
__-cieramp__ option and no gamma values. That causes
__pnmgamma__ to apply the CIE Rec. 709 gamma transfer
function, as specified by the PPM format
specification.


On the other hand, you can use the __-ungamma__ option to
convert from true PPM to linear RGB pseudo-PPM. (Again, if
the input is true PPM, specify the __-cieramp__ option
and no gamma values).


The situation for PGM images is analogous. And
__pnmgamma__ treats PBM images as PGM
images.


You can also apply a different transfer function (which
means you don't end up with a true PPM image) by selecting
the gamma values as arguments or omitting the
__-cieramp__ option. The gamma value is the power to
which the input value is raised in the transfer function. A
value of 1 means the output is the same as the input. A
value less than one makes the output samples numerically
less than the input samples; A value greater than one makes
the samples numerically greater.


Without the __-cieramp__ option, the transfer function is
a simple power function. With __-cieramp__, it is a power
function modified with a linear ramp near black, as
described in CIE Rec. 709.


When you feed a linear PPM image to a display program that
expects a true PPM, the display appears darker than it
should, so __pnmgamma__ has the effect of lightening the
image. When you feed a true PPM to a display program that
expects linear sample values, and therefore does a gamma
correction of its own on them, the display appears lighter
than it should, so __pnmgamma__ with a gamma value less
than one (the multiplicative inverse of whatever gamma value
the display program uses) has the effect of darkening the
image.
!!WHAT IS GAMMA?


A good explanation of gamma is in Charles Poynton's GammaFAQ
at


In brief: The simplest way to code an image is by using
sample values that are directly proportional to the
intensity of the color components. But that wastes the
sample space because the human eye can't discern differences
between low-intensity colors as well as it can between
high-intensity colors. So instead, we pass the light
intensity values through a transfer function that makes it
so that changing a sample value by 1 causes the same level
of perceived color change anywhere in the sample range. We
store those resulting values in the image file. That
transfer function is called the gamma transfer function and
the transformation is called gamma correcting.


Virtually all image formats, either specified or de facto,
use gamma-corrected values for their sample
values.


What's really nice about gamma is that by coincidence, the
inverse function that you have to do to convert the
gamma-corrected values back to real light intensities is
done automatically by CRTs. You just apply a voltage to the
CRT's electron gun that is proportional to the
gamma-corrected sample value, and the intensity of light
that comes of the screen is close to the intensity value you
had before you applied the gamma transfer
function!


And when you consider that computer video devices usually
want you to store in video memory a value proportional to
the signal voltage you want to go to the monitor, which the
monitor turns into a proportional drive voltage on the
electron gun, it is really convenient to work with
gamma-corrected sample values.
!!SEE ALSO


pnm(5)
!!AUTHOR


Copyright (C) 1991 by Bill Davidson and Jef
Poskanzer.
----
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Last edited on Tuesday, June 4, 2002 12:22:43 am by "perry"
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