In distribution ray tracing [Glas95], interreflection effects are additionally taken into account. To compute the irradiances in this case, again a low resolution image will be used for the metering. The diffusors are again placed into the scene according to a coarse regular grid on the projection plane. Diffusors are, just as before, elementary, white, perfectly diffuse surfaces, not displayed in the final image.
Let us assume a distribution ray tracing method using multiple reflected single rays starting from the view point. Now we place a white diffusor metering element at each first intersection surface point (see fig. 7.3). Each of these individual diffusors gathers the irradiance from its half space caused by direct lighting and by interreflections. Interreflections are realized with single rays starting from the actual elementary metering surface according to a cosine distribution. More precisely, by this rendering calculation the correct values (see eq. 7.2) are estimated. From these values the median value is determined and used to compute the scale factor m (see eq. 7.8) which is then applied to the original radiance image.
Figure 7.3: Irradiance metering with distribution ray tracing