The angle between the surface normal and the incoming light ray is called
the angle of incidence and we can express a intensity of the light in terms
of this angle.
The Ilight term represents the intensity of the
incoming light at the particular wavelength (the wavelength determines
the light's color). The kd term represents the diffuse
reflectivity of the surface at that wavelength.
In practice we use vector analysis to compute cosine term indirectly.
If both the normal vector and the incoming light vector are normalized (unit length)
then diffuse shading can be computed as follows:
