NEW BOUNDS FOR LOWER ENVELOPES IN 3 DIMENSIONS, WITH APPLICATIONS TO VISIBILITY IN TERRAINS

We consider the problem of bounding the complexity of the lower envelope of n surface patches in 3-space, all algebraic of constant maximum degree, and bounded by algebraic arcs of constant maximum degree, with the additional property that the interiors of any triple of these surfaces intersect in at most two points. We show that the number of vertices on the lower envelope n such surface patches is O(n(2).2(c root logn)), for some constant c depending on the shape and degree of the surface patches. We apply this result to obtain an upper bound on the combinatorial complexity of the ''lower envelope'' of the space of all rays in 3-space that lie above a given polyhedral terrain K with n edges. This envelope consists of all rays that touch the terrain (but otherwise lie above it). We show that the combinatorial complexity of this ray-envelope is O(n(3).2(c root logn)) for some constant c; in particular, there are at most that many rays that pass above the terrain and touch it in four edges. This bound, combined with the analysis of de Berg et al. [4], gives an upper bound (which is almost tight in the worst case) on the number of topologically different orthographic views of such a terrain.