Computing the Reflected Vector R in Lighting Computations

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For lighting, at a point P on the surface, we have unit vectors L from P
towards the light, and R from P in the direction of reflected light. Also a
normal N, also a unit vector.

A     D       B
_____________
\     |     /
 \    |N   /
  \   |   /
 L \  |  /R
    \ | /
     \|/
      P

By simple trigonometry we can show that: R = 2N(N.L) - L
See section 16.1.4.