Shadow photons

In my blog post about Occlusion Mapping I presented a method to reduce the variance in the probabilistic visiblity estimate. Since this method looked promising, my promotor encouraged me to do a bit more research towards photon mapping. Therefore I read the book Realistic Image Synthesis using Photon Mapping by Henrik Wann Jensen, which is cited in a lot of papers on photon mapping.

To my surprise, this books already proposes a similar method, using a photon map with "Shadow Photons" to reduce the number of shadow rays in a scéne. The description of their method is presented in the paper Efficiently rendering shadows using the photon map also by Henrik Wann Jensen.

Similar to my method they trace photons from the light source, which are only stored after a first intersection. These photons are the "Shadow Photons". In the paper they also trace the regular photons called "Illumination Photons" (see figure 1)

Figure 1: Shadow photon creation.
Courtesy: Henrik Wann Jensen, "Efficiently Rendering Shadow Using the Photon Map", 1995

These shadow and illumination photons are used to identify regions that are completely in shadow and completely illuminated. When a ray is traced through the scéne, a photon lookup is performed. Based on the number of shadow and illumination photons, three distinct cases can be identified:

1. there are only shadow photons: no shadow rays have to be traced since the point is completely in shadow
2. there are only illumination photons: no shadow rays have to be traced since the point is completely illuminated
3. there is a mix of shadow and illumination photons: shadow rays have to be traced.

Jensen also proposes to estimate the visibility v_i as n_i / (n_i+n_s) where n_i is the number of illumination photons and _s is the number of shadow photons. However, this requires a great number of photons.

The results from the paper indicate an average reduction in the number of shadow rays of 90%.