Mark de Berg

The Priority R-tree: a practically efficient and worst-case optimal R-tree

We present the Priority R-tree, or PR-tree, which is the first R-tree variant that always answers a window query using O((N/B)1 1/d + T/B) I/Os, where N is the number of d-dimensional (hyper-) rectangles stored in the R-tree, B is the disk block size, and T is the output size. This is provably asymptotically optimal and significantly better than other R-tree variants, where a query may visit all N/B leaves in the tree even when T = 0. We also present an extensive experimental study of the practical performance of the PR-tree using both real-life and synthetic data. This study shows that the PR-tree performs similar to the best known R-tree variants on real-life and relatively nicely distributed data, but outperforms them significantly on more extreme data.

Algorithms – ESA 2010

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Realistic input models for geometric algorithms

The traditional worst-case analysis often fails to predict the actual behavior of the running time of geometric algorithms in practical situations. One reason is that worst-case scenarios are often very contrived and do not occur in practice. To avoid this, models are needed that describe the properties that realistic inputs have, so that the analysis can take these properties into account.We try to bring some structure to this emerging research direction. In particular, we present the following results: • We show the relations between various models that have been proposed in the literature. • For several of these models, we give algorithms to compute the model parameter(s) for a given (planar) scene; these algorithms can be used to verify whether a model is appropriate for typical scenes in some application area. • As a case study, we give some experimental results on the appropriateness of some of the models for one particular type of scene often encountered in geographic information systems, namely certain triangulated irregular networks.

TSP with neighborhoods of varying size

In TSP with neighborhoods we are given a set of objects in the plane, called neighborhoods, and we seek the shortest tour that visits all neighborhoods. Until now constant-factor approximation algorithms have been known only for cases where the objects are of approximately the same size. We present the first polynomial-time constant-factor approximation algorithm for disjoint convex fat objects of arbitrary size. We also show that the problem is APX-hard and cannot be approximated within a factor of 391/390 in polynomial time, unless P = NP.

On levels of detail in terrains

In many applications it is important that one can view a scene at di erent levels of detail. A prime example is ight simulation: a high level of detail is needed when ying low, whereas a low level of detail su ces when ying high. More precisely, one would like to visualize the part of the scene that is close at a high level of detail, and the part that is far away at a low level of detail. We propose a hierarchy of detail levels for a polyhedral terrain (or, triangulated irregular network) that allows this: given a view point, it is possible to select the appropriate level of detail for each part of the terrain in such a way that the parts still t together continuously. The main advantage of our structure is that it uses the Delaunay triangulation at each level, so that triangles with very small angles are avoided. This is the rst method that uses the Delaunay triangulation and still allows to combine di erent levels into a single representation.

On lazy randomized incremental construction

We introduce a new type of randomized incremental algorithms. Contrary to standard randomized incremental algorithms, theselazy randomized incremental algorithms are suited for computing structures that have a “nonlocal” definition. In order to analyze these algorithms we generalize some results on random sampling to such situations. We apply our techniques to obtain efficient algorithms for the computation of single cells in arrangements of segments in the plane, single cells in arrangements of triangles in space, and zones in arrangements of hyperplanes.

Topologically Correct Subdivision Simplification Using the Bandwidth Criterion

The line simplification problem is an old and well studied problem in cartography. Although there are several algorithms to compute a simplification there seem to be no algorithms that perform line simplification in the context of other geographical objects. This paper presents a nearly quadratic time algorithm for the following line simplification problem: Given a polygonal line, a set of extra points, and a real e> 0, compute a simplification that guarantees (i) a maximum error e (ii) that the extra points remain on the same side of the simplified chain as of the original chain; and (iii) that the simplified chain has no self-intersections. The algorithm is applied as the main subroutine for subdivision simplification and guarantees that the resulting subdivision is topologically correct.

On rectilinear link distance

In this paper we study two link distance problems for rectilinear paths inside a simple rectilinear polygon P.First, we present a data structure using O(n log n) storage such that a shortest path between two query points can be computed efficiently. If both query points are vertices of P, the query time is O(1 + l), where l is the number of links. If the query points are arbitrary points inside P, then the query time becomes O(log n + l). The resulting path is not only optimal in the rectilinear link metric, but it is optimal in the L1-metric as well. Secondly, it is shown that the rectilinear link diameter of P can be computed in time O(n log n). We also give an approximation algorithm that runs in linear time. This algorithm produces a solution that differs by at most three links from the exact diameter.The solutions are based on a rectilinear version of Chazelles polygon cutting theorem. This new theorem states that any simple rectilinear polygon can be cut into two rectilinear subpolygons of size at most 34 times the original size, and that such a cut segment can be found in linear time.

Improved Bounds on the Union Complexity of Fat Objects

Abstract We introduce a new class of fat, not necessarily convex or polygonal, objects in the plane, namely locally γ-fat objects. We prove that the union complexity of any set of n such objects is O(λs+2(n)log 2n). This improves the best known bound, and extends it to a more general class of objects.

OPTIMAL BINARY SPACE PARTITIONS FOR SEGMENTS IN THE PLANE

An optimal BSP for a set S of disjoint line segments in the plane is a BSP for S that produces the minimum number of cuts. We study optimal BSPs for three classes of BSPs, which differ in the splitting lines that can be used when partitioning a set of fragments in the recursive partitioning process: free BSPs can use any splitting line, restricted BSPs can only use splitting lines through pairs of fragment endpoints, and auto-partitions can only use splitting lines containing a fragment. We obtain the following two results: • It is NP-hard to decide whether a given set of segments admits an auto-partition that does not make any cuts. • An optimal restricted BSP makes at most 2 times as many cuts as an optimal free BSP for the same set of segments.