Improved implementation of point location in general two-dimensional subdivisions

• Authors: Michael Hemmer;Michal Kleinbort;Dan Halperin
• Venue: ESA'12 Proceedings of the 20th Annual European conference on Algorithms
• Year: 2012

We present a major revamp of the point-location data structure for general two-dimensional subdivisions via randomized incremental construction, implemented in Cgal, the Computational Geometry Algorithms Library. We can now guarantee that the constructed directed acyclic graph $\mathcal G$ is of linear size and provides logarithmic query time. Via the construction of the Voronoi diagram for a given point set S of size n, this also enables nearest-neighbor queries in guaranteed O(logn) time. Another major innovation is the support of general unbounded subdivisions as well as subdivisions of two-dimensional parametric surfaces such as spheres, tori, cylinders. The implementation is exact, complete, and general, i.e., it can also handle non-linear subdivisions. Like the previous version, the data structure supports modifications of the subdivision, such as insertions and deletions of edges, after the initial preprocessing. A major challenge is to retain the expected O(n logn) preprocessing time while providing the above (deterministic) space and query-time guarantees. We describe efficient preprocessing algorithms, which explicitly verify the length $\mathcal L$ of the longest query path. However, instead of using $\mathcal L$, our implementation is based on the depth $\mathcal D$ of $\mathcal G$. Although we prove that the worst case ratio of $\mathcal D$ and $\mathcal L$ is Θ(n/logn), we conjecture, based on our experimental results, that this solution achieves expected O(n logn) preprocessing time.