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This paper presents a progressive refinement algorithm for volume rendering which uses a pyramidal volume representation. Besides storing average values, the pyramid stores estimated error, so an octtree can be fit to the pyramid given a user-supplied precision. This octtree is then drawn using a set of splats, or footprints, each scaled to match the size of the projection of a cell. The splats themselves are approximated with RGBA Gouraud-shaded polygons, so that they can be drawn efficiently on modern graphics workstations. The result is a real-time rendering algorithm suitable for interactive applications.
Laur et al. (Mon,) studied this question.