Grid resolution issues

The method explained so far will nicely find the solution to many problems when provided with the correct resolution. If the number of points per axis is too high, then the search may be too slow. This motivates selecting fewer points per axis, but then solutions might be missed. This trade-off is fundamental to sampling-based motion planning. In a more general setting, if other forms of sampling and neighborhoods are used, then enough samples have to be generated to yield a sufficiently small dispersion.

There are two general ways to avoid having to select this resolution (or more generally, dispersion):

  1. Iteratively refine the resolution until a solution is found. In this case, sampling and searching become interleaved. One important variable is how frequently to alternate between the two processes. This will be presented shortly.
  2. An alternative is to abandon the adaptation of discrete search algorithms and develop algorithms directly for the continuous problem. This forms the basis of the methods in Sections 5.4.3, 5.4.4, and 5.5.

The most straightforward approach is to iteratively improve the grid resolution. Suppose that initially a standard grid with $ 2^n$ points total and $ 2$ points per axis is searched using one of the discrete search algorithms, such as best-first or $ A^*$. If the search fails, what should be done? One possibility is to double the resolution, which yields a grid with $ 4^n$ points. Many of the edges can be reused from the first grid; however, the savings diminish rapidly in higher dimensions. Once the resolution is doubled, the search can be applied again. If it fails again, then the resolution can be doubled again to yield $ 8^n$ points. In general, there would be a full grid for $ 2^{ni}$ points, for each $ i$. The problem is that if $ n$ is large, then the rate of growth is too large. For example, if $ n = 10$, then there would initially be $ 1024$ points; however, when this fails, the search is not performed again until there are over one million points! If this also fails, then it might take a very long time to reach the next level of resolution, which has $ 2^{30}$ points.

A method similar to iterative deepening from Section 2.2.2 would be preferable. Simply discard the efforts of the previous resolution and make grids that have $ i^n$ points per axis for each iteration $ i$. This yields grids of sizes $ 2^n$, $ 3^n$, $ 4^n$, and so on, which is much better. The amount of effort involved in searching a larger grid is insignificant compared to the time wasted on lower resolution grids. Therefore, it seems harmless to discard previous work.

A better solution is not to require that a complete grid exists before it can be searched. For example, the resolution can be increased for one axis at a time before attempting to search again. Even better yet may be to tightly interleave searching and sampling. For example, imagine that the samples appear as an infinite, dense sequence $ {\alpha }$. The graph can be searched after every $ 100$ points are added, assuming that neighborhoods can be defined or constructed even though the grid is only partially completed. If the search is performed too frequently, then searching would dominate the running time. An easy way make this efficient is to apply the union-find algorithm [243,823] to iteratively keep track of connected components in $ {\cal G}$ instead of performing explicit searching. If $ {q_{I}}$ and $ {q_{G}}$ become part of the same connected component, then a solution path has been found. Every time a new point in the sequence $ {\alpha }$ is added, the ``search'' is performed in nearly5.11constant time by the union-find algorithm. This is the tightest interleaving of the sampling and searching, and results in a nice sampling-based algorithm that requires no resolution parameter. It is perhaps best to select a sequence $ {\alpha }$ that contains some lattice structure to facilitate the determination of neighborhoods in each iteration.

What if we simply declare the resolution to be outrageously high at the outset? Imagine there are $ 100^n$ points in the grid. This places all of the burden on the search algorithm. If the search algorithm itself is good at avoiding local minima and has built-in multi-resolution qualities, then it may perform well without the iterative refinement of the sampling. The method of Section 5.4.3 is based on this idea by performing best-first search on a high-resolution grid, combined with random walks to avoid local minima. The algorithms of Section 5.5 go one step further and search in a multi-resolution way without requiring resolutions and neighborhoods to be explicitly determined. This can be considered as the limiting case as the number of points per axis approaches infinity.

Although this section has focused on grids, it is also possible to use other forms of sampling from Section 5.2. This requires defining the neighborhoods in a suitable way that generalizes the $ k$-neighborhoods of this section. In every case, an infinite, dense sample sequence must be defined to obtain resolution completeness by reducing the dispersion to zero in the limit. Methods for obtaining neighborhoods for irregular sample sets have been developed in the context of sampling-based roadmaps; see Section 5.6. The notion of improving resolution becomes generalized to adding samples that improve dispersion (or even discrepancy).

Steven M LaValle 2012-04-20