all things are possible except skiing through a
revolving door.

Concept 8.1 : Spotlights and Multiple Lights
============================================

Spotlights:
===========
A positional light source can act as a spotlight
- by restricting its shape to that of a cone.
To create a spotlight : 
 o Determine the spread of the cone of light
 o Position the spotlights (always positional, w = 1)

                       / A
		      /
                     /
                    /
                   / 
                  /)   GL_SPOT_CUTOFF
   ------------(-/ )
   SPOTLIGHT   ( B---------------C
   ------------(-\
                  \ 
                   \
                    \
                     \ 
                      \
                      
GL_SPOT_CUTOFF specifies a value equal to one half the 
angle of the cone.

By default the GL_SPOT_ CUTOFF parameter is set to 180.0
degrees and hence the spotlight feature is disabled.  

By default the direction is (0.0, 0.0, -1.0) i.e.
the negative Z direction.  
As described before, we can set attenuation factors 
also for spotlights.  We can also set the 
GL_SPOT_EXPONENT parameter to control the intensity 
of the light i.e. to define how concentrated the light 
is.

Multiple Lights in OpenGL
=========================
Depending upon your OpenGL implementation you can have 
as many as or more than eight lights in a scene.  The 
constants used to refer to the eight lights are GL_LIGHT0,
GL_LIGHT1, GL_LIGHT2, GL_LIGHT3 and so on.  In previous 
examples we have used only one light source.  If we want 
an additional light(s) we needto specify the parameters 
for it and enable it.