Multibounce : Reducing Energy Loss in Two-Level Radiance Caching for Global Illumination

Sammanfattning

Global illumination is a topic of active research in real-time graphics that has many potential areas of improvement. AMD’s GI-1.0 which was updated to GI-1.1, is a GI technique with probes in screen-space for caching radiance at the primary hit points, as well as caches in world-space, formed by the hash grid cells, for the secondary ray hits. Because the secondary hit caches only cache direct light as well as a lacking indirect estimate, it exhibits considerable energy loss compared to a path-tracer. Energy loss is seen in the reflections as well, particularly the specular reflections. When the reflector isn’t in view and the limited resolution of the world cache doesn’t cover the hit point, the reflection becomes black. In this thesis, an improved technique is presented with an additional bounce. The ray-path is extended from the secondary hits and another hash grid cell is created at the third hit point. The secondary hit cells are referred to as the first bounce cells and the cells at the third hit point are the second bounce cells. The direct radiance of the second bounce cells can be then be used as the indirect radiance for the first bounce cells, effectively adding a bounce. Because of this, the technique is referred to as multibounce in this paper. Over time, multibounce enlightens the scene with notably higher energy levels in the form of increased brightness, and the similarity to the path-tracer is increased in general, compared to GI-1.1 without multibounce, referred to as singlebounce. The reflections are especially affected when the reflector isn’t view, where the use of multibounce improves the quality of the reflections notably compared to singlebounce, due to the increased resolution of the hash grid cells. Even though multibounce improves the reflections and scene in general, some glossy reflections still seem to lack energy, possibly because of the direct light solution used. The frame time increases with multibounce with as much as 1ms in the Sponza scene, but there are simple optimizations that can be made to lessen the impact.