Mathematical Topics from 3D Graphics - 3D Math Primer for Graphics and Game Development - page 404

are empirical models, and the Blinn model should not be considered an
“approximation” to the “correct” Phong model. In fact, Ngan et al. [48]
have demonstrated that the Blinn model has some objective advantages
and more closely matches experimental data for certain surfaces.
One detail we have omitted is that in either model, cosθ may be less
than zero. In this case, we usually clamp the specular contribution to zero.

10.6.3 The Diffuse Component

The next component in the standard lighting model is the diffuse compo-
nent. Like the specular component, the diffuse component also models light
that traveled directly from the light source to the shading point. However,
whereas specular light accounts for light that reflects preferentially in a
particular direction, diffuse light models light that is reflected randomly in
all directions due to the rough nature of the surface material. Figure 10.24
compares how rays of light reflect on a perfectly reflective surface and on a
rough surface.
To compute specular lighting, we needed to know the location of the
viewer, to see how close the eye is to the direction of the perfect mirror
bounce. For diffuse lighting, in contrast, the location of the viewer is not
relevant, since the reflections are scattered randomly, and no matter where
we position the camera, it is equally likely that a ray will be sent our way.
However, the direction if incidence l , which is dictated by the position of
the light source relative to the surface, is important. We've mentioned
Lambert's law previously, but let's review it here, since the diffuse portion
of Blinn-Phong is the most important place in real-time graphics that it
comes into play. If we imagine counting the photons that hit the surface
of the object and have a chance of reflecting into the eye, a surface that is
perpendicular to the rays of light receives more photons per unit area than
a surface oriented at a more glancing angle, as shown in Figure 10.25.

Figure 10.24. Diffuse lighting models scattered reflections