How to compute surface normals for a general vertex/face geometry:

   (1) For each face, compute a faceNormal as follows:

       -  Set faceNormal to [0,0,0]

       -  For every triple A,B,C of successive vertices around the face:
                faceNormal += crossProduct(C-B, B-A)

   (2) For each vertex:

       -  Sum together faceNormals of all faces containing this vertex

       -  Normalize this sum to unit length

Note:

   The cross product of two vectors U and V is the vector:

	[ Uy * Vz - Uz * Vy , Uz * Vx - Ux * Vz , Ux * Vy - Uy * Vx ]