A language that doesn't affect the way you think
about programming is not worth knowing

Concept 2 : A Hidden-Surface Removal 
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Idea:
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When we render a scene composed of three dimensional
objects, some of them might obscure all or parts
of others.  Changing the viewing point can also
change the obscuring relationship.

The elimination of parts of solid objects that
are obscured by others is called hidden-surface
removal.  

The Depth Buffer (or Z Buffer):
===============================
The depth buffer works by associating a depth
or distance, from the view plane (remember the
near clipping plane of glFrustum() and glOrtho())
with each pixel on the window. 

Before each pixel is drawn a comparision is done 
with the existing depth value.  If the value is greater 
than the currently stored value, the new pixel is not 
rendered.  If the converse is true, the new pixel is drawn.

Experiment:
===========
The sphere rendering commands are followed by the cube
rendering commands in our example. The sphere is rendered
3 units in front of the cube.  Hence with depth buffering
enabled the sphere obscures part of the cube.  With depth
buffering disabled, the cube even though it is located 
behind the sphere in our scene, gets rendered completely
and obscures the sphere in front of it.

glutInitDisplayMode(GLUT_DEPTH | ....);
initializes the depth buffer

glClear(GL_COLOR_BUFFER_BIT | GL_DEPTH_BUFFER_BIT);
Clears the depth buffer initially in the program

glEnable(GL_DEPTH_TEST);
Enables depth testing of pixels before rendering