don@andante.att.com (Don Mitchell) (11/08/90)
"Robust Ray Tracing with Interval Arithmetic", by Don Mitchell, pp. 68-74 in the Proceedings of Graphics Interface '90, Canadian Infor- mation Processing Society (Toronto), 1990. "Very interesting paper, but I believe that the convergence of the method is quite slow, even slower than bisection." The interval-arithmetic algorithm is a root-isolation algorithm. It gets used in combination with root-refinement (something much faster than bisection and much safer than Newton's method, hopefully). The speed of convergence depends a lot on how you are refining intersections. If you are just performing the interval algorithm until it converges to a root, that would be very slow and not really a proper application of the method. For a benchmark image containing a number of 4th degree algebraic surfaces, the interval method was about three times slower than a specialized polynomial solver (e.g., Sturm sequences). Its hard to compare its run time on transcendental surfaces, since I don't know any other way except Kalra's Lipschitz method (described in SIGGRAPH 89). I think interval arithmetic is more straightforward than the Lipschitz methods, particularly for surfaces with singular gradients (like concave superquadrics). It does not require an off-line human calculation to derive Lipschitz conditions. There are also a lot of improvements that I didn't mention (or implemented later). For example, there are faster interval multiply algorithms than the one I gave. You can do the root isolation algorithm with (F, dF/dt) instead of (F, grad F). There are also ways of using the derivative intervals to narrow the function intervals with the mean-value theorem. Like a lot of techniques, there is a long journey between theory and implementation.