When rendering a volume represented as a 3D grid, it takes a long time to
check the cells that do not intersect the surface. Wilhelms et al. proposed
using an octree as a hierarchical index, keeping the maximum and minimum cell
value for each voxel, in order to avoid processing empty cells. Their method
used a complete octree which required a large amount of space.
We propose a new indexing method that uses an incomplete octree and which
requires less memory to be stored.
