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 [Nx,Ny,Nz].
S specifies the direction to the light source.
You can specify these directions as three vectors[x,y,z] or
two vectors [Theta Phi (in spherical coordinates).
Lambert's Law: R = cos(PSI) where PSI is
the angle between the surface normal and light source.