Diffuse light is the directional component of a light source and was the
subject of our previous example lighting shader. In the Phong lighting
model, the diffuse material and lighting values are multiplied together, as
is done with the ambient components. However, this value is then scaled
by the dot product of the surface normal and light vector, which is the
direction vector from the point being shaded to the light. Again, in shader
speak, this might look something like this: