Similar Technique
A convenient and practical implementation of front/back facing detection is to use the unit normal of the plane (which is commonly precomputed for lighting effects anyway), then simply applying the dot product of the light position to the plane's unit normal and adding the D component of the plane equation (a scalar value):
Where plane_D is easily calculated as a point on the plane dot product with the unit normal of the plane:
Note: The homogeneous coordinates, L_w and d, are not always needed for this computation.
After doing this calculation, you may notice indicator is actually the signed distance from the plane to the light position. This distance indicator will be negative if it is behind the face, and positive if it is in front of the face.
This is also the technique used in the 2002 SIGGRAPH paper, "Practical and Robust Stenciled Shadow Volumes for Hardware-Accelerated Rendering"
Read more about this topic: Silhouette Edge
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