A professor is one who talks in someone else's sleep

Concept 3.5 : Light Position
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Idea:
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GLfloat light_position[] = {x, y, z, w};
glLightfv(GL_FLOAT0, GL_POSITION, light_position);
define the position of the light in the scene.

If w = 0 the light source is considered an infinite
one (i.e. directional eg. the sun) and the x, y, and z
values specify the direction.  If w = 1 the light source 
is positional and the x, y, and z values specify the
location of the light source.

Experiment:
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Set the 'w' value to 0.  Then set it to 1.  You will
notice that when 'w'= 0 the lighting is more dispersed
whereas it is more directed when 'w'= 1