ABSTRACT
This chapter presents a novel approach to compute ambient occlusion for direct volume rendering. It shows how to derive the probability of a random ray to be blocked by a shell. The chapter suggests that ambient occlusion in comparison with obscurance can provide cues that are closer to realistic lighting because voxels behind the point of interest are not taken into account. It deals with more complex combinations of the shells, especially, as the assumption of independence of the occlusion probabilities is usually not true in most datasets. Ambient occlusion is a compelling approach to improve depth and shape perception, to give the illusion of global illumination, and to efficiently approximate low-frequency outdoor lighting. The occlusion values can be calculated directly from the opacity of the original volume. Instead of applying costly ray casting to determine the accessibility of a voxel, this technique employs a probabilistic heuristic in concert with three-dimensional image filtering.
