MPI-I-94-162

We describe a robust and efficient implementation of the Bentley-Ottmann sweep line algorithm based on the LEDA library of efficient data types and algorithms. The program computes the planar graph $G$ induced by a set $S$ of straight line segments in the plane. The nodes of $G$ are all endpoints and all proper intersection points of segments in $S$. The edges of $G$ are the maximal relatively open subsegments of segments in $S$ that contain no node of $G$. All edges are directed from left to right or upwards. The algorithm runs in time $O((n+s) log n)$ where $n$ is the number of segments and $s$ is the number of vertices of the graph $G$. The implementation uses exact arithmetic for the reliable realization of the geometric primitives and it uses floating point filters to reduce the overhead of exact arithmetic.