Adolescence, n.:
The stage between puberty and adultery.

Concept 7.1 : Position And Attenuation
======================================

Idea:
=====
As mentioned before we can choose whether to have 
a light source that's directional or positional.
The effect of an infinite location is that the rays of 
light can be considered parallel by the time they reach
an object.  An example would be our sun.

For the positional source its exact position within the 
scene determines the effect it has on a scene, and 
specifically, the direction from which the light rays come.  
An example would be a desk lamp.  

Light Position Vector
====================

Typically we supply four values to specify the 
position of a light source, as shown above.  These
values (x, y, z, w) supply the position for the 
GL_POSITION parameter.  If the last value, w is zero
the light source is a directional one, i.e. infinitely
located, and the (x,y,z) values in that case describe
its direction.  

If the w value is nonzero, the light is positional, and
the (x,y,z) values specify the location of the light.

Attenuation Factor
==================

For real-world lights, the intensity of light decreases 
as distance from the light increases.   Since a directional
light is infinitely far away, it doesn't make sense to 
attenuate its intensity over distance, so attenuation is
disabled for a directional light.  However we might want to
attenuate the light from a positional light source.  OpenGL
attenuates a light source by multiplying the contribution
of that source by an attenuation factor.

The attenuation factor is defined as :

                 1
     -------------------------
        Kc   +  Kl*d  +  Kq*d*d
where
d = distance between the light's position and the vertex
Kc = GL_CONSTANT_ATTENUATION
Kl = GL_LINEAR_ATTENUATION
Kq = GL_QUADRATIC_ATTENUATION

By default Kc is 1.0, and both Kl and Kq are zero but we
can give these parameters different values by calls to
glLightfv() as the following example shows:

glLightf(GL_LIGHT0, GL_CONSTANT_ATTENUATION, 2.0);
glLightf(GL_LIGHT0, GL_LINEAR_ATTENUATION, 1.0);
glLightf(GL_LIGHT0, GL_QUADRATIC_ATTENUATION, 0.5);

Note : The ambient, diffuse, and specular contributions
       are all attenuated.  Only the emission and global
       ambient values are not attenuated.