NVidia: Please give us an overview of how the algorithm works from generating the octree, to cone tracing, to the gathering pass.
Tim Sweeney: The technique is known as SVOGI Sparse Voxel Octree Global Illumination, and was developed by Andrew Scheidecker at Epic. UE4 maintains a real-time octree data structure encoding a multi-resolution record of all of the visible direct light emitters in the scene, which are represented as directionally-colored voxels. That octree is maintained by voxelizing any parts of the scene that change, and using a traditional set of Direct Lighting techniques, such as shadow buffers, to capture first-bounce lighting.
Performing a cone-trace through this octree data structure (given a starting point, direction, and angle) yields an approximation of the light incident along that path.
The trick is to make cone-tracing fast enough, via GPU acceleration, that we can do it once or more per-pixel in real-time. Performing six wide cone-traces per pixel (one for each cardinal direction) yields an approximation of second-bounce indirect lighting. Performing a narrower cone-trace in the direction of specular reflection enables metallic reflections, in which the entire scene is reflected off each glossy surface.
