Are you thinking of a voxel grid when your are talking about a grid? A grid acceleration structure is different from that: each cell is a variable length list o

Are you thinking of a voxel grid when your are talking about a grid? A grid acceleration structure is different from that: each cell is a variable length list of the contents of the cell. So in case of a sparse voxel grid you would have a coarse grid with the cells pointing to voxel grids iff there is at least one non-empty voxel inside the volume. So you're only allocating memory for blocks that have some voxel information to store. The coarse grid itself is just a set of list headers which don't have a high memory requirement (a few MB maybe).