Normal of a Vertex in a Mesh
•`n1
•`n2
•`n3
•`nk
•`nv
`nv = (`n1 +`n2 + … +`nk) / k = å`ni / k
=
average of adjacent triangle normals
or better:
`nv = å (Ai `ni) / (k å (Ai) )
= area-weighted average of adjacent triangle normals