Different approaches to reduce shaft hierarchy in dynamic radiosity

Hierarchical radiosity provides a good model for the lighting of static scenes. But when an object moves, this model requires an update of all the lighting. In fact, thanks to space-time coherence, we just have to compute some of the geometry parameters (form factors). The problem is to determine which part of the hierarchy of links must change or not. A shaft is a practical mean to represent the interactions of light between two objects. A hierarchy of shaft describes all the interactions in a 3D scene. So the intersection of the moving object and the hierarchy of shafts directly gives the part of the hierarchy that are concerned by the displacement. The time needed to compute the new required form factors is somehow proportional to the total number of shafts of the hierarchy. Therefore, in order to decrease this computation time, we present a method based on the Oriented Bounding Box of the objects and compare it to a more standard Axis Aligned Bounding Box approach.