Voxel cone tracing
Real-time global illumination that voxelises the scene and marches a few cones through the mip-mapped voxel volume.
Cyril Crassin and colleagues introduced the technique in 2011 as a way to get indirect light in real time. The scene is voxelised into a sparse octree where each voxel stores outgoing radiance; indirect light is approximated by shooting a handful of cones from each surface through the volume. A widening cone samples coarser mip levels, so a broader cone returns a pre-filtered average over a larger area — a cheap way to integrate incoming light. NVIDIA built VXGI on the idea and early Unreal Engine 4 shipped a prototype (SVOGI). It was memory-hungry and has largely been superseded by probe-based GI, screen-space GI and hardware ray tracing.