The new draft now includes the following section:-

The ray tracing performed by this model can also operate in the reverse direction, taking the perceived surface brightness signal from every point in the scene, and propagating it backwards along the reverse light path to produce a percept of the illumination profile due to the scene. In the scene depicted in Figure 19 (B) for example, the large number of illuminated surfaces which are pointing upwards to the right produces by reverse ray-tracing a percept of a bright illuminant in that direction, while the shady surfaces in the same scene project a percept of dark illuminant in the opposite direction. This description is only approximate however, because a brightly lit surface does not imply illumination exclusively from the normal direction. The illumination could actually be coming from a range of angles near the normal, so the probability distribution of the possible illuminants suggested by a bright surface defines a spherical cosine function centered on the normal, and it is this cosine illuminant probability distribution that would actually be propagated by a single surface, while the total illumination profile is calculated as the sum of all such probability distributions from every surface in the scene.

Figure 20.

The illumination profile suggested by (A) the bright horizontal surfaces, (B) the dark vertical surfaces, (C) the bright vertical surfaces (on the far side of the block) of the scene, are summed to produce (D) the combined illumination profile suggested by the whole scene.

For example Figure 20 (A) represents the spherical cosine illuminant distribution suggested by the bright horizontal surfaces in the scene; Figure 20 (B) represents the illuminant distribution due to the dark vertical surfaces, which suggests darkness in the general direction of the two surface normals; Figure 20 (C) represents the contribution of the bright vertical surfaces (invisible on the far side of the block that produce bright peaks on the far side of the illuminant sphere), and Figure 20 (D) represents the total illuminant percept calculated by summing all of the individual component illuminant profiles. The final perceived illumination profile therefore is only approximate, although it would clearly distinguish between a uniform v.s. a strongly polarized illumination profile.