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[Apologies for multiple postings]

Robust Geometric Computations for Neutron Tracking

GRAND Seminar <https://cs.gmu.edu/~robotics/pmwiki.php/Main/GrandSeminar>
Thursday,
Oct 19, 11am, Room 4201

Jack Snoeyink <http://www.cs.unc.edu/~snoeyink/>
Professor
Department of Computer Science
UNC Chapel Hill
(on leave at the National Science Foundation, CISE CCF Algorithmic
Foundations)

*Abstract:*

As computer scientists, it is important for us to recognize when our
favorite abstractions break and to consider alternatives. In this talk we
will look at a particular geometric example exposing that computers
actually don't do real number arithmetic, but typically do use fast IEEE
754 floating-point hardware to do arithmetic. The problem is suggested by
David Greisheimer of Bettis labs: track neutrons in a CAD model of a
nuclear reactor. As I'll describe, his reactor is defined by a hierarchical
Constructive Solid Geometry representation, which forms unions and
intersections of geometric primitives bounded by quadratic surfaces. A
neutron is a point, so the task is to follow a point as it crosses each
surface in order. This is easier said than done; analysis of the degrees of
the geometric operations used reveals why. A point representing a neutron
can actually get stuck in a reactor -- something that makes a nuclear
physicist very nervous, since it breaks basic physical laws of
conservation. Degree-driven analysis suggests changing the problem to
tracking line segments where we apply geometric rounding to the segment
endpoints. By doing so, we can continue to use fast floating-point, but can
give guarantees on both geometric and topological properties (e.g., every
neutron going into a bounded region must come out). This is work with David
Millman and Michael Deakin.