What if two points along lie on a vertical line that slices through ? What happens when one of the edges of is vertical? These are special cases that have been ignored so far. Throughout much of combinatorial motion planning it is common to ignore such special cases and assume is in general position. This usually means that if all of the data points are perturbed by a small amount in some random direction, the probability that the special case remains is zero. Since a vertical edge is no longer vertical after being slightly perturbed, it is not in general position. The general position assumption is usually made because it greatly simplifies the presentation of an algorithm (and, in some cases, its asymptotic running time is even lower). In practice, however, this assumption can be very frustrating. Most of the implementation time is often devoted to correctly handling such special cases. Performing random perturbations may avoid this problem, but it tends to unnecessarily complicate the solutions. For the vertical decomposition, the problems are not too difficult to handle without resorting to perturbations; however, in general, it is important to be aware of this difficulty, which is not as easy to fix in most other settings.
Steven M LaValle 2012-04-20