Calculate specular reflectance
R = specular(Nx,Ny,Nz,S,V)
R = specular(Nx,Ny,Nz,S,V) returns the
reflectance of a surface with normal vector components [Nx,Ny,Nz].
S and V specify the direction
to the light source and to the viewer, respectively. You can specify
these directions as three vectors[x,y,z] or two
vectors [Theta Phi (in spherical coordinates).
The specular highlight is strongest when the normal vector is
in the direction of (S+V)/2 where S is
the source direction, and V is the view direction.
The surface spread exponent can be specified by including a
sixth argument as in specular(Nx,Ny,Nz,S,V,spread).