Call on God but row away from the rocks
- Indian proverb

Concept 1.5 : Three Dimensional Blending 
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Idea:
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The order in which objects are rendered has a
big effect on blending.

Experiment
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In this example everything is stationary. The 
farthest object is the grey cube, next the cyan
cube.  The sphere is closest to the viewpoint. We
draw from back to front i.e.  first we draw the 
grey cube, followed by a cyan cube and then a
reddish sphere.  

Experiment by setting different alpha values.  Also
toggle the order of rendering and see the effect.
When the objects are drawn in reverse order i.e. sphere
first etc. blending no longer occurs and all the
objects are rendered opaque!

Hence order is extremely important.  
What do we do when objects are in relative motion
i.e. order is changing (Concept 1.6)