Returns a point which represents the direction of the reflected ray.
refer to page 328 in the RenderMan Companion.

The entering ray direction.  This must be a vector. 
The default is the normalized incidence vector, I.

This must be a normal.  The default is the surface normal,
N, which has been normalized and run through faceforward.
This is the normal of the surface point to be shaded, P.
If the surface point P has been changed, calculatenormal(P)
should be used to recalculate N before shading.

Returns a vector which represents the direction of the reflected ray.
refer to page 328 in the RenderMan Companion.