This example demonstrates the effect of distance attenuation with point lights
The three rows of spheres are each lit by lights of the same color and 
intensity.  However, moving from left to right across the screen, the distance
from the light to the sphere increases linearly.  The top row uses constant
attenuation only.  The second row uses pure linear attenuation, and the bottom
row uses pure quadratic.  The intensity of each of these attenuation 
components may be adjusted using the keystrokes below.  Note that even in the
pure constant case, there are some differences in shading from left to right,
due to the interaction of the light vector with the sphere normals.  This is
most pronounced on the left edge, where the light is quite close to the sphere
when compared to the sphere's radius. 

The key commands are:

q, a - increase, decrease constant attenuation in the range [0, inf]
w, s - increase, decrease linear attenuation in the range [0, inf]
e, d - increase, decrease quadratic attenuation in the range [0, inf]
Space - reset attenuation values to 1.0
