The 2^k Neighborhoods for Grid Path Planning

Abstract

Grid path planning is an important problem in AI. Its understanding has been key for the development of autonomous navigation systems. An interesting and rather surprising fact about the vast literature on this problem is that only a few neighborhoods have been used when evaluating these algorithms. Indeed, only the 4- and 8-neighborhoods are usually considered, and rarely the 16-neighborhood. This paper describes three contributions that enable the construction of effective grid path planners for extended $2^k$-neighborhoods; that is, neighborhoods that admit $2^k$ neighbors per state, where $k$ is a parameter. First, we provide a simple recursive definition of the $2^k$-neighborhood in terms of the $2^{k-1}$-neighborhood. Second, we derive distance functions, for any $kgeq 2$, which allow us to propose admissible heuristics that are perfect for obstacle-free grids, which generalize the well-known Manhattan and Octile distances. Third, we define the notion of canonical path for the $2^k$-neighborhood; this allows us to incorporate our neighborhoods into two versions of A*, namely Canonical A* and Jump Point Search (JPS), whose performance, we show, scales well when increasing $k$. Our empirical evaluation shows that, when increasing $k$, the cost of the solution found improves substantially. Used with the $2^k$-neighborhood, Canonical A* and JPS, in many configurations, are also superior to the any-angle path planner Theta$^*$ both in terms of solution quality and runtime. Our planner is competitive with one implementation of the any-angle path planner, ANYA in some configurations. Our main practical conclusion is that standard, well-understood grid path planning technology may provide an effective approach to any-angle grid path planning.