Calculate diffuse reflectance
R = diffuse(Nx,Ny,Nz,S)
R = diffuse(Nx,Ny,Nz,S) returns the reflectance
of a surface with normal vector components
S specifies the direction to the light source.
You can specify these directions as three vectors
[Theta Phi (in spherical coordinates).
R = cos(PSI) where
the angle between the surface normal and light source.