Computing nice sweeps for polyhedra and polygons

The plane sweep technique is one of the best known paradigms for the design of geometric algorithms [2]. Here an imaginary line sweeps over the plane while computing the property of interest at the moment the sweep line passes the required information needed to compute that property. There are also three-dimensional problems that are solved by space sweep, where a plane sweeps the space. This paper does not deal with the sweeping paradigm itself; it deals with testing polygons and polyhedra to determine if they have a certain property. The properties that we consider are related to sweeping. We will test for a simple polygon or polyhedron if it can be swept by a line or plane such that every cross-section has a property like being convex or simply-connected. For example, to determine for a simple polygon (with interior) in the plane whether there is a sweep direction such that every cross-section is simplyconnected (a point, line segment, or empty) is the well-known question of determining whether a simple polygon is monotone in some direction. We solve two extensions of this problem in 3-space, and solve another extension in the plane. The first question we address applies to a polyhedron in 3space. We want to determine if there is a vector , such that if a sweeping plane with normal passes over , every cross-section of is convex. Toussaint [7] calls this property weakly monotonic in the convex sense. Obviously, for convex polyhedra, any vector gives only convex cross-sections during the sweep. For many nonconvex polyhedra no such vector exists. We give an