Children are natural mimics who act like their 
parents despite every effort to teach them good
manners.

Concept 2.3 : Antialiasing Polygons
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Idea:
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Antialiasinng the edges of filled polygons is similar 
to antialiasing points, and lines.  When different 
polygons have over-lapping edges, we need to blend the 
color values appropriately.  

To antialias polygons in RGBA mode:

  o   Use the alpha value to represent coverage 
      values of polygon edges
 
  o   Enable polygon antialiasing by passing
      GL_POLYGON_SMOOTH to glEnable().  This causes
      pixels on the edges of the polygons to be
      assigned fractional alpha values based on
      their coverage as though they were lines
      being antialiased.

Remember :
==========
When performing antialiasing, OpenGL calculates a 
coverage value for each fragment based on the fraction 
pixel square on the screen that it would cover.  In RGBA
mode, OpenGL multiplies the fragments coverage value by 
its alpha value.  

  o   Supply a value for GL_POLYGON_SMOOTH_HINT
      if desired.

  o   Next we need to blend overlapping images 
      properly.  To do this, first turn off the
      depth buffer by calling glDepthMask()
      with GL_FALSE as the arguement.  This way
      we have control over how overlapping
      pixels are drawn.  

  o   Set the blending factors to GL_SRC_ALPHA_
      SATURATE for the source and GL_ONE for the
      destination.  With this blending, the final
      color is the sum of the destination color
      and the scaled source color.

  o   finally sort all the polygons in the scene
      so that they are ordered from front to back
      before drawing them.

Example
=======
The example to your left shows how to antialias
filled polygon.  Pressing the 't' key toggles the 
antialiasing on and off.

Note:
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For this technique to work correctly make sure to include 
GLUT_ALPHA as an option for the glutInitDisplayMode()