Sparse voxel octree (SVO)
An octree that stores only the occupied parts of a voxelised scene, so empty space costs no memory and high resolutions become tractable.
A dense voxel volume grows cubically with resolution and quickly becomes unmanageable. A sparse voxel octree subdivides only the branches that contain geometry and leaves empty regions unallocated; a ray then traverses the tree top-down and skips past the empty cubes on the way. Memory follows the surface rather than the volume.
The structure broke through around 2009–2011: Cyril Crassin's GigaVoxels and voxel cone tracing used it for global illumination, and Jon Olick's 2008 SIGGRAPH demo showed geometry rendered as an SVO via ray marching instead of polygons. The promise of "unlimited detail" in real-time games never quite materialised, but the structure lives on in GI solutions and ray tracing.