# Arrangements And Duality

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1 Arrangements And Duality 3.1 Introduction 3 Point configurations are tbe most basic structure we study in computational geometry. But what about configurations of more complicated shapes? For example, a configuration of lines, or curves, or surfaces? Such a configuration is generally called an arrangement and is the object of study here. Figure 3.1: Examples of arrangements An arrangement of d 1 dimensional objects in d dimensions generates objects of all dimensions that are at most d. Vertices are 0-dimensional, edges are 1 dimensional, and so on. The description of an arrangement consists of all the different dimensional structures present in the arrangement, as well as how they connect to each other. For example, consider the arrangement formed by four intersecting lines, show in Figure 3.2. The face f is bounded by edges e 1, e 2, e 3, e 4, and by vertices v 1, v 2, v 3, v 4. A complete description of this face requires all of this information, as well as their adjacency structure. Examples of arrangements One example of an arrangement comes from robotics. Suppose you re trying to plan a path for a moving robot from a start to a finish in a region with obstacles. We can define a configuration space where each point is a placement of the robot (we assume for now that the robot can only translate). Now we can mark all points in the space where the robot can be placed without collision. This will be a collection of regions with boundaries that describe critical locations of the robot (where it touches an obstacle). Finding a path from start to finish then reduces to the problem of determining whether the cell containing the start posiition and the cell containing the finish are connected. 19

2 20 CHAPTER 3. ARRANGEMENTS AND DUALITY v 2 v 1 e 1 e 2 f v 3 e 4 e 3 v 4 Figure 3.2: Labelled cell of an arrangement In general, the robot may have more degrees of freedom than just its position. While this changes the configuration space and increases its dimension, it does not change the principle. Even in this higher-dimensional space, the arrangement can be used to solve the planning problem. A second example of an arrangement arises from minimization problems. Suppose I am given a collection of functions f i : R 2! R on the plane. The goal is to find a point x 2 reals 2 such that max i f i (x) is minimized. For instance, f i (x) could be the distance from x to a fixed point p i, in which case max i f i (x) is the radius of the ball centered at x that contains all the p i. A third example comes from collections of inequalities. Suppose we have a set of linear equations. As we have seen, we can think of these linear equations as hyperplanes. Now consider a single region in space carved out by these linear equations. All points in this region represent the same set of inequalities associated with the linear equations. In fact, a polyhedron is one cell of this arrangement; if we flip an inequality, we get a different adjacent cell in the arrangement, and so on. In all of these cases, studying the arrangement of shapes allows us to break down a continuous domain into a discrete set of pieces, such that in each piece the problem either has a fixed solution or is easy to solve. 3.2 Duality Perhaps the most important reason to study arrangements of lines is that they re the same as point configurations via a duality mapping. What is duality? It s a way to map between points and hyperplane (also called projective duality). Consider a point in the plane p =(x, y). It has two parameters that describe it. If we consider a line in the plane (say y = mx + c), it also has two coordinates that

3 3.2. DUALITY 21 describe it. This suggests that we can construct a mapping between points and lines. Standard Dual The standard dual mapping from points to lines takes the point p =(a, b) to the line p = ` : y = ax b. Equivalently, the line ` : y = ax + b is mapped to the dual point ` =(a, b). This mapping has many properties: Self-inverse (p ) = p Incidence If p lies on `, then ` lies on p Ordering If p lies above (below) `, then p lies below (above) `. But not all lines have a well defined dual. Consider any line parallel to the y-axis. It cannot be written in the form y = mx + c. It is helpful to think of what happens in the limit as the line slope increases. Consider the line y = mx + c and let m! (this corresponds to swinging the line around a pivot at (0, c). In the dual, this corresponds to the point ` =(m, c) which moves along the horizontal line y = c as m increases. In other words, the dual of the line x = 0 is the point at infinity. We will return to this topic later when we talk about the projective plane. Polar Dual There are other dual mappings that often come up in practice. The polar dual takes a point (a, b) and maps it to the line p : ax + by + 1 = 0. Geometrically, this dual can be visualized as the line we get when we draw a ray from p to the origin, extend it further by an amount 1/kpk, and then draw a line perpendicular to it. You can verify that this mapping also satisfies the incidence property. It also satisfies an order mapping, in that ` lies between p and the origin, then p lies between ` and the origin. Dual Constructions To gain intuition about dual mappings, it helps to examine some simple cases. We know that a point maps to a line and vice versa. Suppose we have a configuration with two points. The corresponding dual configuration consists of two lines. What does the intersection of these two (dual) lines correspond to in the primal plane? Since the dual point intersects both lines, its dual line must intersect both priaml points. Therefore, an intersection of two lines in the dual corresponds to the (unique) line passing through two points in the primal. Figure 3.3 also illustrates the ordering principle. The red point is above the line in the primal, and therefore its dual line is below the dual point.

4 22 CHAPTER 3. ARRANGEMENTS AND DUALITY Figure 3.3: Dual Mapping example Another interesting example is a line segment. We know that the dual of a point is a line. But what the dual of a collection of points (i.e a line segment). Imagine walking from one endpoint of the line segment to the other. Since at any stage, we are on the line connecting the two points, we know from the above observation that the dual line corresponding to our position must pass through the (dual) point corresponding to the primal line supporting the line segment. Figure 3.4: Double Wedge This means that in the dual, we can imagine a line sweeping around a pivot point. As a result, the dual of a line segment is a double wedge. Convex Hulls and Envelopes Let us take an even more complex example. What is the dual of a convex hull? It s easier to use the properties of the dual mapping, rather than direct reasoning. Let s consider the

5 3.2. DUALITY 23 lower hull of the set of points. Consider each of the support lines of the lower hull (i.e lines defined by pairs of adjacent points on the lower hull). Each support line by definition lies below all the points in the set. By the order property of the standard dual, this means that the dual of such a line must lie above all the lines in the dual. Moreover, by the incidence property, these (dual) points will be incident on the dual lines corresponding to the points of the lower hull. Now let us move to the dual and consider the arrangement of lines formed from the duals of the primal points. Imagine a ray being shot from the top of the diagram vertically down. It will intersect the arrangement at some line `. Let s call this point of intersection p. Now we know that p is some primal line passing through `, and by the order property p is below all the points of the set. Thus, if we mark off all such points in the dual, we will trace out the dual of the lower hull. Figure 3.5: Convex hull dual envelopes This set of points is called the upper envelope of the arrangements. Formally, it is the set of all points from which we can draw a vertical ray that does not intersect any line of the arrangement. Similarly, we can define the lower envelope as the set of points from which rays drawn downward do not cross any line of the arrangement. By a similar argument, we can show that the lower envelope of the arrangement is the dual of the upper hull of the primal set of points.

6 24 CHAPTER 3. ARRANGEMENTS AND DUALITY 3.3 Constructing Arrangements An arrangement is described by the collection of all faces of all dimensions, and how they connect to each other. What is the total complexity of the description of an arrangement? Let us consider the case of an arrangement of lines. Firstly, assume that no two lines are parallel (because this can only decrease the complexity). Then there are ( n 2 ) vertices (every pair of lines can intersect). Since each line eventually extends down to, the n lines form n + 1 faces that are unbounded below. Similarly, there are n + 1 faces unbounded above. Each bounded face has a unique vertex at its base, and no vertex is the base for two faces. Therefore, the number of bounded faces is precisely the number of vertices. Finally, since the arrangement is planar, we can apply Euler s formula to bound the number of edges. In summary, an arrangement of n lines has ( n 2 ) vertices, O(n2 ) faces and O(n 2 ) edges. What about the complexity of a single cell? Any cell in the arrangement is the intersection of at most n halfplanes and so has complexity at most n. Another important structure in an arrangement is the zone of a line `. This is the set of faces, edges and vertices intersected by the line. Figure 3.6: The zone of a line Theorem 3.1 (Zone Theorem). In an arrangement of n lines, the zone of a line has complexity O(n). The proof is by a careful induction. Note that this is not entirely obvious. A line clearly intersects at most n edges, but maybe it cuts many faces. The zone theorem says that this can t happen.

7 3.4. LEVELS OF AN ARRANGEMENT 25 The zone theorem leads to a simple algorithm for computing an arrangement: add each line in one at a time, updating the arrangement each time. The zone theorem guarantees that only a linear number of cells are changed in each iteration, and these can also be found in linear time (see Exercise 6). 3.4 Levels of an arrangement We ve already encountered the lower and upper envelope of an arrangement. If we imagine a ray shooting upwards from in the vertical dimension, then the lower envelope is the set of all points of the arrangement that are hit first, and the upper envelope is the set of all points of the arrangement hit last. We can also define intermediate levels of the arrangement. The k-level of an arrangement is the set of all points on the shapes of the arrangement such that a ray shooting upwards from hits k 1 curves of the arrangement before hitting the point. By this definition, the lower envelope of a set of shapes is the 1-level. Levels of an arrangement are very important. Many problems can be formulated as an optimization over a set of levels of an arrangement. Near Neighbors For example, if we wanted to compute the nearest neighbor of a point, this can be viewed as a query on the lower envelope of an arrangement of 45 cones with apex at each point. MEBs If we want to find the minimum enclosing ball (MEB of a set of points, we can draw the cones as above. For any point x, the height of the upper envelope of this arrangement is the size of an MEB centered there. Therefore, the center of the optimal enclosing ball is found by finding the point corresponding to the minimum of the upper envelope. Best Fit Circle Imagine you re given a set of points {p 1,...,p n }, and you wish to find a circle of radius r centered at c such that Â i kp i ck r is minimized. This is one version of the best-fit circle problem. As before, we draw cones centered at each of the input points. Now consider a candidate center (x, y) and a candidate radius r. The distance of any point from this circle is kp i ck r. This is equivalent to the vertical distance of the point (x, y, r) from the cone corresponding to p i. Therefore, if we fix c and draw the corresponding vertical line through (c, 0), the optimal radius can be found by marking the intersections of the cones along the line, and picking a point that minimizes the sum of distances to these intersections. The problem of finding r such that Â i r i r is minimized (where here r i is the height at which p i s cone is intersected) is easy to solve. The solution is the median of the r i. In other words, the optimal center and radius can be found by finding the lowest point on the medial level of the arrangement of cones[2].

8 26 CHAPTER 3. ARRANGEMENTS AND DUALITY We know that the lower and upper envelope of an arrangement can be bounded by the complexity of the convex hull. But what about intermediate levels? It s not hard to see that the complexity of the k-level of an arrangement of lines in the plane is O(nk), but it remains one of the biggest open problems in computational geometry to give a tight bound. The best known upper bound is O(nk 1/3 )[1]. 3.5 Quantifier Elimination and Tarski s Theorem Coming soon After Notes Topics for later Duality via lifting map and tangents The topology of the projective plane Exercises 6. Describe a linear time algorithm for inserting a line into an arrangement. You may assume the Zone theorem. 7. Let a bitonic function be one that increases monotonically upto a point, and then decreases monotonically. Show that the k-level of an arrangement of lines in the plane can be expressed as the upper envelope of k bitonic functions, and then use this fact to show that the complexity of the k-level of an arrangement of lines is O(nk). 3.7 Bibliography [1] T. Dey. Improved bounds for planar k-sets and related problems. Discrete & Computational Geometry, 19(3): , [2] S. Har-Peled. Approximation algorithm for the l1-fitting circle problem. In EuroCG, pages Technische Universiteit Eindhoven, 2005.

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