Always borrow money from a pessimist.  He doesn't
expect to be paid back.

Concept 8.2 : Multiple Lights 
=============================

Idea:
====
Here we have two lights.  A regular light (Light 1) 
and a spot light.  In the default condition both lights
are enabled.
Right clicking the mouse on the sphere will give a pop
up menu.  Use this and the following sections to become
more familiarized with spot light settings and effects.

Experiment 
==========
1.  Disable Light1.  You see only the spotlight effect
2.  Enable Light1.
3.  Set the Diffuse component of the spot light to red.
    Why do we still see a white patch to the right of the
    spot?. Can you get rid of it?
4.  Set the Diffuse component to white light.  Now set
    the ambient component to red.  What happens to the
    sphere and why?
5.  Reset the ambient component to the default condition
    and disable Light1 to better see the next few examples.
6.  Set the specular component to all red.  Does this 
    answer question 3.?
7.  Similarly try out the different positions for the 
    spotlight.  When changing the spot light position, one
    has to correctly specify its direction also.  Usually
    the negative of the position vector will yield a
    suitable direction.
8.  With Light1 still disabled, try out the various 
    attenuation parameters.
    In the default condition the constant attenuation was
    set at 1.5, the linear attenuation at 0.5 and the 
    quadratic attenuation at 0.2
    Which one of these attenuation factors do you think has
    the greatest effect?
9.  The spot can be made bigger or smaller by specifying a 
    bigger or smaller cutoff angle.  Here the default cutoff
    angle is 10 degrees.  Try the other values provided.
    What happens when the cutoff angle is 180?

Answers
=======
3.  That is the specular component.  Try setting the 
    specular component to be the same as the diffuse
    component to remove the red patch.
4.  Ambient lighting contributes to the overall 
    environmental light and hence bathes the entire sphere 
    in a reddish hue.
8.  Quadratic attenuation followed by  linear attenuation
    and finally constant attenuation factor.
9.  At a cutoff angle of 180 degrees the cone angle (twice 
    the cutoff angle) is now 360 degrees. Hence it is no 
    longer a spotlight, but a regular light, and the whole 
    sphere is illuminated instead of just a part of it.