> http://www.imagico.de/fast_iso/patch.html

Tangentally related to the current beta, but... I once though about the 
isosurface tracing algorithm. If I evaluate the function at a particular 
point in space and the result is a large positive value, then given that 
value and the max_gradiant, I know that there can be no surface within a 
sphere of some radius centered at that point.

If POV-Ray would cache this information, we could potentially save a 
hell of a lot of function evaluations. (For example, if the whole length 
of the ray falls within these "empty" spheres, then the ray clearly 
strikes no surface, and there's nothing to evaluate!)

OTOH, how much effort does it take to test a ray against a growing 
multitude of spheres?

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