Captain Penny's Law:
You can fool all the people some of the time, and
some of the people all the time, but you Can't fool MOM

Concept 1.6 : 3D Blending Moving Objects- Depth Buffer
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Idea:
=====
In the previous concept we saw the importance of the 
order in which the objects are rendered.  This 
problems is aggravated in the case of scenes
where either the viewpoint or the object/s
are moving.  

Animation:
==========
In this example we draw an opaque sphere and a
translucent cube.  Pressing the 'a' key triggers
an animation sequence in which the translucent cube
moves through the opaque sphere.  Pressing 'r' 
resets the objects to their original positions
Notice how as the cube gets in front of the sphere
it still retains its translucency.  How did we achieve
this ?

Solution:
=========
The solution is to enable depth buffering 
but to make the depth buffer read-only while
drawing the translucent objects.

First draw the sphere with the depth buffer in the
normal operational mode.   Then make the depth buffer
read only and draw the cube. When the cube is drawn its
depth values are still compared to the values established
by the sphere and blended accordingly.

To control if the depth buffer is writable, use 
glDepthMask(); if GL_FALSE is the arguement, the
buffer becomes read-only, whereas GL_TRUE restores
the normal, writable operation.