3D graphics eBook - Course Materials Repository

Ambient occlusion 6
ambient occlusion diffuse only combined ambient and diffuse
The occlusion at a point on a surface with normal can be computed by integrating the visibility function
over the hemisphere with respect to projected solid angle:
where is the visibility function at , defined to be zero if is occluded in the direction and one otherwise,
and is the infinitesimal solid angle step of the integration variable . A variety of techniques are used to
approximate this integral in practice: perhaps the most straightforward way is to use the Monte Carlo method by
casting rays from the point and testing for intersection with other scene geometry (i.e., ray casting). Another
approach (more suited to hardware acceleration) is to render the view from by rasterizing black geometry against
a white background and taking the (cosine-weighted) average of rasterized fragments. This approach is an example
of a "gathering" or "inside-out" approach, whereas other algorithms (such as depth-map ambient occlusion) employ
"scattering" or "outside-in" techniques.
In addition to the ambient occlusion value, a "bent normal" vector is often generated, which points in the average
direction of unoccluded samples. The bent normal can be used to look up incident radiance from an environment
map to approximate image-based lighting. However, there are some situations in which the direction of the bent
normal is a misrepresentation of the dominant direction of illumination, e.g.,