July 26-28, 2017
Carleton University
Ottawa, Ontario

# CCCG 2017

29th Canadian Conference on Computational Geometry

## Program

Click on the title of a talk to see the paper abstract, and on the PDF icon to download the paper itself. Recordings of most talks are available on YouTube.

The winner of the Best Student Presentation Award is Patrick Schnider, with his talk on Sharing a pizza: bisecting masses with two cuts. If you missed it, you can watch the talk on YouTube.

### Tuesday July 25

 18:00 Play food & wine Reception 20:30 Reception ends

### Wednesday July 26

8:50TB 360Opening remarks
9:00TB 360 Forbidden Configurations in Discrete Geometry David Eppstein (Invited talk)

We review and classify problems in discrete geometry that depend only on the order-type or configuration of a set of points, and that can be characterized by a family of forbidden configurations. These include the happy ending problem, no-three-in-line problem, and orchard-planting problem from classical discrete geometry, as well as Harborth’s conjecture on integer edge lengths and the construction of universal point sets in graph drawing. We investigate which of these properties have characterizations involving a finite number of forbidden subconfigurations, and the implications of these characterizations for the computational complexity of these problems.

10:00Break
10:30

#### Session 1A

TB 360
• Power domination on triangular grids Prosenjit Bose, Claire Pennarun and Sander Verdonschot.

The concept of power domination emerged from the problem of monitoring electrical systems. Given a graph $G$ and a set $S \subseteq V(G)$, a vertex $v$ is said to be monitored if either $v$ is a neighbor of $S$, or there is a vertex $u \in N(v)$ such that $v$ is the only non-monitored neighbor of $u$. The power domination number of a graph is then minimum size of a set $S$ such that this process monitors every vertex of the graph. We here show that the power domination number of a triangular grid $T_k$ with hexagonal-shape border of length $k-1$ is exactly $\big\lceil \dfrac{k}{3} \big\rceil$.

• Monochromatic Matching in Bicolored Point Set A. Karim Abu-Affash, Paz Carmi and Sujoy Bhore.

Motivated by networks interplay, we consider the monochromatic matching problem in bicolored point set, where the goal is to compute two bottleneck plane matchings of the red and the blue points separately, such that number of crossings between their edges is minimized. That is, given a bicolored set $P$ of $n$ red and $m$ blue points in the plane, where $n$ and $m$ are even, the goal is to compute a plane matching $M_R$ of the red points and a plane matching $M_B$ of the blue points that minimize the number of crossing between $M_R$ and $M_B$ as well as the longest edge in $M_R \cup M_B$.

• Bottleneck Bichromatic Full Steiner Trees A. Karim Abu-Affash, Sujoy Bhore, Paz Carmi and Dibyayan Chakraborty.

Given two sets of points in the plane, Q of n (terminal) points and S of m (Steiner) points, where each of Q and S contains bichromatic points (red and blue points), a full bichromatic Steiner tree is a Steiner tree in which all points of Q are leaves and each edge of the tree is bichromatic (i.e., connects a red and a blue point). In the bottleneck bichromatic full Steiner tree (BBFST) problem, the goal is to compute a bichromatic full Steiner tree T, such that the length of the longest edge in T is minimized. In k-BBFST problem, the goal is to find a bichromatic full Steiner tree T with at most $k\le m$ Steiner points from S, such that the length of the longest edge in T is minimized. In this paper, we present an $O((n+m)\log^2 m)$ time algorithm that solves the BBFST problem. Moreover, we show that k-BBFST problem is NP-hard and we give a polynomial-time 9-approximation algorithm for the problem.

#### Session 1B

TB 340
• Exploring Increasing-Chord Paths and Trees Yeganeh Bahoo, Stephane Durocher, Sahar Mehrpour and Debajyoti Mondal.

A straight-line drawing Γ of a graph G = (V,E) is a drawing of G in the Euclidean plane, where every vertex in G is mapped to a distinct point, and every edge in G is mapped to a straight line segment between their endpoints. A path P in Γ is called increasing-chord if for every four points (not necessarily vertices) a, b, c, d on P in this order, the Euclidean distance between b, c is at most the Euclidean distance between a, d. A spanning tree T rooted at some vertex r in Γ is called increasing chord if T contains an increasing-chord path from r to every vertex in T. We prove that given a vertex r in a straight-line drawing Γ, it is NP-complete to decide whether Γ contains an increasing-chord spanning tree rooted at r, which answers a question posed by Mastakas and Symvonis. We also shed light on the problem of finding an increasing-chord path between a pair of vertices in Γ, but the computational complexity question remains open.

• Angle-monotone Paths in Non-obtuse Triangulations

We reprove a result of Dehkordi, Frati, and Gudmundsson: every two vertices in a non-obtuse triangulation of a point set are connected by an angle-monotone path—an xy-monotone path in an appropriately rotated coordinate system. We show that this result cannot be extended to angle-monotone spanning trees, but can be extended to boundary-rooted spanning forests. The latter leads to a conjectural edge-unfolding of sufficiently shallow polyhedral convex caps.

• A General Algorithm for the Maximum Span of Fixed-Angle Chains David Goering and Ronald Gentle.

Fixed angle chains have been used to model protein backbones and other molecular structures. Benbernou and O'Rourke proved several structural theorems for finding the maximum 3D span of fixed-angle chains: the largest distance achievable between the two endpoints. They used their results to develop an algorithm which can find the maximum span of chains with all angles equal to 90 degrees in quadratic time. We use their most general structural theorem to develop a new algorithm that computes the maximum span for any fixed-angle chain and the configuration in which this is achieved. The construction is purely geometric in nature, meaning that it consists of only straight-edge and compass constructions together with some list-keeping. It can be implemented with or without the use of a coordinate system. The algorithm has complexity $O(n^2)$ for any chains with equal fixed angles but arbitrary link lengths (alpha-chains). We do not claim that it runs in polynomial time for all chains but discuss why it will do so for those likely to be used in any modeling application.

13:30

#### Session 2A

TB 360
• Upward order-preserving 8-grid drawings of binary trees

This paper concerns upward order-preserving straight-line drawings of binary trees with the additional constraint that all edges must be routed along edges of the 8-grid (i.e., horizontal, vertical, diagonal) or some subset thereof. We give an algorithm that creates such drawings of width $O(\log^2 n)$, while the previous best result were drawings of width $O(n^{0.48})$. If only some of the grid-lines are allowed to be used, then the algorithm gives (with minor modifications) the same upper bounds for the {/,|,\}-grid and the {|,\,—}-grid. On the other hand, in the {/,\,— }-grid sometimes $\Omega(\sqrt{n/\log n})$ width is required.

• On the Planar Spherical Depth and Lens Depth Rasoul Shahsavarifar and David Bremner.

For a distribution function $F$ on $\mathbb{R}^d$ and a point $q\in \mathbb{R}^d$, the spherical depth $\SphD(q;F)$ (lens depth $\LD(q;F)$) is defined to be the probability that a point $q$ is contained inside a random closed hyperball (hyperlens) obtained from a pair of points from $F$. The spherical depth $\SphD(q;S)$ (lens depth $\LD(q;S)$) is also defined for an arbitrary data set $S\subseteq \mathbb{R}^d$ and $q\in \mathbb{R}^d$. This definition is based on counting all of the closed hyperballs (hyperlenses), obtained from pairs of points in $S$, that contain $q$. The straightforward algorithm for computing the spherical depth (lens depth) in dimension $d$ takes $O(dn^2)$. The main result of this paper is an optimal algorithm for computing the bivariate spherical depth. The algorithm takes $O(n \log n)$ time. By reducing the problem of \textit{Element Uniqueness}, we prove that computing the spherical depth (lens depth) requires $\Omega(n \log n)$ time. Some geometric properties of spherical depth (lens depth) are also investigated in this paper. These properties indicate that simplicial depth ($\SD$) is linearly bounded by spherical depth and lens depth (in particular, $\LD\geq \SphD\geq \frac{2}{3}SD$). To illustrate these relationships, some experimental results are provided. In these experiments on random point sets, the bounds of $\SphD\geq 2\SD$ and $\LD \geq 1.2\SphD$ are achieved.

• Minimum Enclosing Circle Problem with Base Point Binay Bhattacharya and Lily Li.

This article presents a linear time algorithm to solve a variant of the minimum enclosing circle (MEC) problem. The inputs are a point set $S$ of size $n$, and a point $b$ in the plane called the free point. Our goal is to locate a circle center $o∗$ such that the maximum distance of all points in $S$ to $o∗$ divided by the distance from $o∗$ to $b$ is minimized. The original investigation by Qiu et al. found an $O(n \log n)$ algorithm using the furthest point Voronoi diagram of the point set $S$. Our approach is based on Meggido’s $O(n)$ algorithm for solving the standard problem minimum enclosing circle. We extend our technique to solve in linear time similar variants of the MEC problem where the free point is replaced with other geometric objects such as a free line, a free line segment, and a set of free points.

#### Session 2B

TB 340
• Snipperclips: Cutting Tools into Desired Polygons using Themselves Erik Demaine, Matias Korman, André van Renssen and Marcel Roeloffzen.

We study Snipperclips, a computer puzzle game whose objective is to create a target shape with two tools. The tools start as constant-complexity shapes, and each tool can snip (i.e., subtract its current shape from) the other tool. We study the computational problem of, given a target shape represented by a polygonal domain of $n$ vertices, is it possible to create it as one of the tools' shape via a sequence of snip operations? If so, how many snip operations are required? We show that a polynomial number of snips suffice for two different variants of the problem.

• Rep-cubes: Unfolding and Dissection of Cubes Dawei Xu, Takashi Horiyama and Ryuhei Uehara.

Last year, a new notion of rep-cube was proposed. A rep-cube is a polyomino that is a net of a cube, and it can be divided into some polyominoes such that each of them can be folded to a cube. This notion was inspired by the notions of polyomino and rep-tile, which were introduced by Solomon W. Golomb. It is proved that there are infinitely many distinct rep-cubes. In this paper, we investigate this new notion and obtain three new results. First, we prove that there does not exist a regular rep-cube of order 3, which solves an open question proposed in the paper. Next, we enumerate all regular rep-cubes of order 2 and 4. For example, there are 33 rep-cubes of order 2; that is, there are 33 dodecominoes that can fold to a cube of size √2 ×√2 ×√2 and each of them can be divided into two nets of unit cube. Similarly, there are 7185 rep-cubes of order 4. Lastly, we focus on pythagorean triples that consist of three positive integers (a, b, c) with a² + b² = c². For each of these triples, we can consider a rep-cube problem that asks whether a net of a cube of size c×c×c can be divided into two nets of two cubes of size a × a × a and b × b × b. We give a partial answer to this natural open question by dividing into more than two pieces. For any given pythagorean triple (a, b, c), we construct five polyominoes that form a net of a cube of size c×c×c and two nets of two cubes of size a×a×a and b×b×b.

• On the Shortest Separating Cycle

According to a result of Arkin et al. (2016), given $n$ point pairs in the plane, there exists a simple polygonal cycle that separates the two points in each pair to different sides; moreover, a $O(\sqrt{n})$-factor approximation with respect to the minimum length can be computed in polynomial time. Here we extend the problem to geometric hypergraphs, and obtain the following characterization of feasibility. Given a geometric hypergraph on points in the plane with hyperedges of size at least $2$, there exists a simple polygonal cycle that separates each hyperedge if and only if the hypergraph is $2$-colorable.

We extend the $O(\sqrt{n})$-factor approximation in the length measure as follows: Given a geometric graph $G=(V,E)$, a separating cycle (if it exists) can be computed in $O(m+ n\log{n})$ time, where $|V|=n$, $|E|=m$. Moreover, a $O(\sqrt{n})$-approximation of the shortest separating cycle can be found in polynomial time. Given a geometric graph $G=(V,E)$ in $\mathbb{R}^3$, a separating polyhedron (if it exists) can be found in $O(m+ n\log{n})$ time, where $|V|=n$, $|E|=m$. Moreover, a $O(n^{2/3})$-approximation of a separating polyhedron of minimum perimeter can be found in polynomial time.

14:30Break
15:00TB 360 Open problem session
16:30Break
18:00Richcraft HallBanquet

### Thursday July 27

9:00TB 360 Burning the Medial Axis Erin Wolf Chambers (Invited talk)

Initially proposed by Blum in 1967, the medial axis of a shape consists of the union of all centers of maximally inscribed balls. The medial axis is one of the most commonly used tools for understanding shape, as it is homotopy equivalent to the original object, has codimension one, and is centrally located. In addition, it is used as a component in building skeletons that are of smaller dimension than the original object, but which capture the shape in a more compact but still useful representation. However, the medial axis is unstable to perturbations; even small changes in the boundary of the shape result in large changes in the medial axis.

Methods for pruning the medial axis are usually guided by some measure of significance, with considerable work done for both 2 and 3 dimensional shapes. However, the majority of significance measures over the medial axis are locally defined and hence unable to capture more global features, or are difficult to compute and sensitive to perturbations on the boundary. In general, there are no skeletons which provably capture the correct topology, are central to the object, are always result in a curve skeleton for a 3-dimensional input.

In this talk, I will present recent work done in 2d and 3d to compute new significance measures on the medial axis. In 2d, the extended distance function (EDF), also called the burn time, was recently developed by Liu et al., as well as related measures such as erosion thickness and weighted EDF. The EDF function was later generalized to the burn time function for 3 dimensional shapes, yielding both a mathematical framework for quantifying shape as well as an algorithm for approximating this function for a union of balls, which are commonly used for surface reconstruction and approximation. In 3d, this also allows us to develop a definition of topologically accurate 1-dimensional skeletons. These measures give practical methods for differentiating boundary noise from primary features, and can be used for shape alignment and recognition. In addition, there is both practical and theoretical evidence that these measures are robust under certain types of noise in the boundary, unlike the medial axis itself.

10:00Break
10:30

#### Session 3A

TB 360
• The most-likely skyline problem for stochastic points Akash Agrawal, Yuan Li, Jie Xue and Ravi Janardan.

For a set $O$ of $n$ points in $\mathbb{R}^d$, a skyline is a subset of points where no point is dominated by any other point of $O$. Suppose that each point $o_i$ in $O$ has an associated probability of existence $p_i$ in $(0,1]$. The problem of computing the skyline with the maximum probability of occurrence is considered. It is shown that in $\mathbb{R}^d$, $d \geq 3$, the problem is NP-hard and that the desired skyline cannot even be well-approximated in polynomial-time unless P=NP. In $\mathbb{R}^2$, an optimal $O(n \log n)$-time and $O(n)$-space algorithm is given.

• Visibility Testing and Counting for Uncertain Segments Mohammad Ali Abam, Sharareh Alipour, Mohammad Ghodsi and Mohammad Mahdian.

We study two well-known planar visibility problems, namely visibility testing and visibility counting, in a model where there is uncertainty about the input data. The standard versions of these problems are defined as follows: we are given a set $S$ of $n$ segments in $\mathbb{R}^2$, and we would like to preprocess $S$ so that we can quickly answer queries of the form: is the given query segment $s \in S$ visible from the given query point $q \in \mathbb{R}^2$ (for visibility testing) and how many segments in $S$ are visible from the given query point $q \in \mathbb{R}^2$ (for visibility counting).

In our model of uncertainty, each segment may or may not exist, and if it does, it is located in one of finitely many possible locations, given by a discrete probability distribution. In this setting, the probabilistic visibility testing problem (PVTP, for short) is to compute the probability that a given segment $s \in S$ is visible from a given query point $q$ and the probabilistic visibility counting problem (PVCP, for short) is to compute the expected number of segments in $S$ that are visible from a query point $q$. We first show that PVTP is $\#P$-complete. In the special case where uncertainty is only about whether segments exist and not about their location, we show that the problem is solvable in $O(n\log n)$ time. Using this, together with a few old tricks, we can show that one can preprocess $S$ in $O(n^5\log n)$ time into a data structure of size $O(n^4)$ so that PVTP queries can be answered in $O(\log n)$ time. Our algorithm for PVTP combined with linearity of expectation gives an $O(n^2\log n)$ time algorithm for PVCP. We also give a faster 2-approximation algorithm for this problem.

• Nearest-Neighbor Search Under Uncertainty Boris Aronov, John Iacono and Khadijeh Sheikhan.

We study the problem of Nearest-Neighbor Searching under locational uncertainty. Here, an uncertain query or site consists of a set of points in the plane, and their distance is defined as distance between the two farthest points within them. In $L_\infty$ metric, we present an algorithm with $O(n\log^2n+s)$ preprocessing time, $O(n\log n)$ space, and $O(\log^2n+k)$ query time, where $s$ is the total number of site points, $n$ is the number of sites, and $k$ is the size of the query. We also propose a $\sqrt{2}$-approximation algorithm for the $L_2$ version of the problem.

#### Session 3B

TB 340
• A Seed Placement Strategy for Conforming Voronoi Meshing

We show how to place a set of seed points such that a given piecewise linear complex is the union of some faces in the resulting Voronoi diagram. The seeds are placed on sufficiently small spheres centered at input vertices and are arranged into little circles around each half-edge where every seed is mirrored across the associated triangle. The Voronoi faces common to the seeds of such arrangements yield a mesh conforming to the input complex. If the input contains sharp angles, then additional seeds are needed, analogous to nonobtuse refinement. Finally, we propose local optimizations to reduce the number of seeds and output facets.

• On Compatible Triangulations with a Minimum Number of Steiner Points

Two polygons are compatible if they have the same clockwise cyclic ordering of vertices. The definition extends to polygonal regions (polygons with holes) and to triangulations - for every face, the clockwise cyclic order of vertices on the boundary must be the same. It is known that every pair of compatible $n$-vertex polygonal regions can be extended to compatible triangulations by adding $O(n^2)$ Steiner points. Furthermore, $\Omega(n^2)$ Steiner points are sometimes necessary, even for a pair of polygons. Compatible triangulations provide piecewise linear homeomorphisms and are also a crucial first step in morphing planar graph drawings, aka "2D shape animation". An intriguing open question, first posed by Aronov, Seidel, and Souvaine in 1993, is to decide if two compatible polygons have compatible triangulations with at most $k$ Steiner points. In this paper we prove the problem to be NP-hard for polygons with holes. The question remains open for simple polygons.

• Planarity Preserving Augmentation of Topological and Geometric Plane Graphs to Meet Parity Constraints Israel Aldana Galván, José Luis Álvarez Rebollar, Juan Carlos Catana-Salazar, Erick Solís Villarreal, Jorge Urrutia and Carlos Bruno Velarde Valázquez.

We introduce the augmentation problem to meet parity constraints in topological and geometric plane graphs. We show a family of topological plane graphs such that any augmentation leaves at least $2n/5$ vertices without meeting their parity constraints, and a family of geometric plane trees such that any augmentation leaves at least $n/10$ vertices without meeting their parity constraints. We prove that the problem of adding a minimum number of edges to plane geometric graphs is NP-Complete. When the input graph is a topological tree finding a minimum set of edges that needed to be added to meet a parity constrain is solvable in $O(n)$ time and $O(1)$ space. We also establish a lower bound of $11n/15$ on the number of necessary edges to augment a topological graph when the graph is augmentable, and a lower bound of $6n/11$ on the number of necessary edges to augment a geometric tree when the tree is also augmentable.

12:00TB 340Industry talk with Shopify

Come join us for lunch and learn from one of Shopify’s data engineers about what it is like to work at Shopify and how computational geometry can be applied in industry.

Shopify is the leading cloud-based, multi-channel commerce platform designed for small and medium-sized businesses. Merchants can use the software to design, set up, and manage their stores across multiple sales channels, including web, mobile, social media, marketplaces, brick-and-mortar locations, and pop-up shops. The Shopify platform was engineered for reliability and scale, making enterprise-level technology available to businesses of all sizes. Shopify currently powers over 400,000 businesses in approximately 175 countries and is trusted by brands such as Tesla, Red Bull, Nestle, GE, Kylie Cosmetics, and many more.

13:30

#### Session 4A

TB 360
• Interference Minimization in k-Connected Wireless Networks Stephane Durocher and Sahar Mehrpour.

Given a set of positions for wireless nodes, the $k$-connected interference minimization problem seeks to assign a transmission radius to each node such that the resulting network is $k$-connected and the maximum interference is minimized. We show there exist sets of $n$ points on the line for which any $k$-connected network has maximum interference $\Omega(\sqrt{kn})$. We present polynomial-time algorithms that assign transmission radii to any given set of $n$ nodes to produce a $k$-connected network with maximum interference $O(\sqrt{kn})$ in one dimension and $O(\min \{ k\sqrt{n}, k\log\lambda \})$ in two dimensions, where $\lambda$ denotes the ratio of the longest to shortest distances between any pair of nodes.

In this paper‎, ‎we study the problem of optimal orientation of directional antennas in the symmetric model of communication‎. ‎We propose an optimal algorithm to find the minimum radius and the orientation of antennas‎, ‎when antennas are placed on a set $P$ of points on a line‎, ‎and each antenna has angle less than $\pi$‎. We show that the connected graph induced by this optimal orientation is a 7-hop spanner‎ with respect to the unit disk graph of $P$‎. ‎Moreover‎, ‎we present a deterministic local routing algorithm that is guaranteed to find a path between any pair of antennas in the communication graph whose number of edges is at most 7 times the number of edges between that pair in the unit disk graph‎.

• Range Assignment of Base-Stations Maximizing Coverage Area without Interference

This note is a study on the problem of maximizing the sum of area of non-overlapping disks centered at a set of given points in $I\!\!R^2$. If the points of $P$ are placed on a straight-line, then the problem is solvable in polynomial time. Eppstein [CCCG, pages 260–265, 2016] proposed an $O(n^\frac{3}{2})$ time algorithm, for maximizing the sum of radii of non-overlapping balls or disks when the points are arbitrarily placed on a plane. We show that the solution to this problem gives a $2$-approximation solution for the area maximization problem by non-overlapping disks or balls. We also present simulation results. Finally, we propose a PTAS for our problem.

#### Session 4B

TB 340
• Nonoverlapping Grid-aligned Rectangle Placement for High Value Areas Stephen Rowe, Christopher Valicka, Scott Mitchell and Simon Zou.

We consider heuristic and optimal solutions to a discrete geometric bin packing problem that arises in a resource allocation problem. An imaging sensor is assigned to collect data over a large area, but some subregions are more valuable than others. To capture these high-value regions with higher fidelity, we can assign some number of non-overlapping rectangular subsets, called “subfootprints.” The sensor image is partitioned into squares called “chips”, and each chip is further partitioned into pixels. Pixels may have different values. Subfootprints are restricted to rectangular collections of chips, but we are free to choose different rectangle heights, widths, and areas. We seek the optimal arrangement over the family of possible rectangle shapes and sizes.

We provide a mixed-integer linear program optimization formulation, as well as a greedy heuristic, to solve this problem. For the meta-problem, we have some freedom to align the chip boundaries to different pixels. However, it is too expensive to solve the optimization formulation for each alignment. However, we show that the greedy heuristic can inform which pixel alignments are worth solving the optimization over. We use a variant of $k$-means clustering to group greedy solutions by their transport shape-similarity. For each cluster, we run the optimization problem over the greedy layout with the highest value. In practice this efficiently explores the geometric configuration space, and produces solutions close to the global optimum. We show a contrived example using surveillance of the Mississippi River. Our software is available as open-source in the Github repository “GeoPlace.”

• Packing Boundary-Anchored Rectangles Therese Biedl, Ahmad Biniaz, Anil Maheshwari and Saeed Mehrabi.

In this paper, we study the boundary-anchored rectangle packing problem in which we are given a set $P$ of points on the boundary of an axis-aligned square $Q$. The goal is to find a set of disjoint axis-aligned rectangles in $Q$ such that each rectangle is anchored at some point in $P$, each point in $P$ is used to anchor at most one rectangle, and the total area of the rectangles is maximized. We show how to solve this problem in linear-time in the number of points of $P$, provided that the points of $P$ are given in sorted order along the boundary of $Q$. The solvability of the general version of this problem, in which the points of $P$ can also lie in the interior of $Q$, in polynomial-time, is still open.

• Dominating Set of Rectangles Intersecting an Inclined Line

We study the minimum dominating set problem using axis-parallel rectangles and unit squares on the plane. These geometric objects are constrained to be intersected by a line which makes an angle with the $x$-axis. For axis-parallel rectangles, we prove that this problem is NP-complete. When the objects are axis-parallel unit square, we give a polynomial time algorithm. For unit squares which touches the inclined line at a single point from either side of the inclined line, we give an $O(n\log n)$ time algorithm.

14:30Break
15:00

#### Session 5A

TB 360
• On Guarding Orthogonal Polygons with Bounded Treewidth

There exist many variants of guarding an orthogonal polygon in an orthogonal fashion: sometimes a guard can see an entire rectangle, or along a staircase, or along a orthogonal path with at most $k$ bends. In this paper, we study all these guarding models in the special case of orthogonal polygons that have bounded treewidth in some sense. Exploiting algorithms for graphs of bounded treewidth, we show that the problem of finding the minimum number of guards in these models becomes linear-time solvable in polygons of bounded treewidth.

• Beacon Coverage in Orthogonal Polyhedra José Luis Álvarez-Rebollar, Israel Aldana-Galván, Juan Carlos Catana-Salazar, Jesús Nestaly Marín-Nevárez, Erick Solís-Villarreal, Jorge Urrutia and Carlos Bruno Velarde.

We consider a variant of the Art Galllery Problem in orthogonal polygons and orthogonal polyhedra using beacons as guards. A beacon is a device that attracts objects toward itself within a given domain. A beacon $b$ covers a point if when a beacon attracts it, it reaches $b$. In this paper, we prove that there exist orthogonal polyhedra that cannot be covered even if we place a beacon at each of its vertices. We also give another example for covering the exterior of an orthogonal polyhedra. We also study the beacon coverage problem in orthogonal polyhedra, by extending the notion of vertex beacons to edge beacons. We prove that $e/12$ edge beacons are always sufficecient while $e/21$ edge beacons are sometimes necessary to cover any orthogonal polyhedron. We also prove that $e/6$ edge beacons are always sufficient to cover both the interior and the exterior of any orthogonal polyhedra.

• Discrete Surveillance Tours in Polygonal Domains Elmar Langetepe, Bengt J. Nilsson and Eli Packer.

The shortest watchman route of a polygon is the shortest closed tour that sees all points of an environment. A reasonable extension of this concept is to require that the tour minimizes the hiding time of the points in the environment, i.e., the maximum time during which any points is not seen by the agent following the tour at unit speed. We call such tours surveillance routes. It is known that the shortest watchman route is a 2-approximation for the best surveillance route in any simple polygon and any metric and that computing the optimum weighted discrete version of the surveillance route in a simple polygon is NP-hard, even for two weight values.

We show a linear time $3/2$-approximation algorithm for the optimum surveillance tour problem in rectilinear polygons using the $L_1$ metric. In addition, we present a $w_{\max}^{\frac{O(1)}{\sqrt{\log w_{\max}}}}$-approximation algorithm for the optimum weighted discrete surveillance route in a simple polygon with weight values in the range $[1,w_{\max}]$. We observe that the above complexity function grows asymptotically slower than any function $O(\sqrt[k]{w_{\max}})$, for any constant $k$.

• Guarding Monotone Polygons with Half-Guards Matt Gibson, Erik Krohn and Matthew Rayford.

We consider a variant of the art gallery problem where all guards are limited to seeing to the right inside a monotone polygon. We provide a polynomial-time 4-approximation algorithm for a version of the problem where we wish to point guard the vertices of the polygon. We then extend this algorithm and provide a $O(1)$-approximation to point guard the boundary of the polygon and ultimately the entire polygon.

#### Session 5B

TB 340
• Sharing a pizza: bisecting masses with two cuts Luis Barba and Patrick Schnider.

Assume you have a pizza consisting of four ingredients (e.g. bread, tomatoes, cheese and olives) that you want to share with your friend. You want to do this fairly, meaning that you and your friend should get the same amount of each ingredient. How many times do you need to cut the pizza so that this is possible? We will show that two straight cuts always suffice. More formally, we will show the following extension of the well-known Ham-Sandwich theorem: Given four mass distributions in the plane, they can be simultaneously bisected with two lines. That is, there exist two oriented lines with the following property: let $R^+_1$ be the region of the plane that lies to the positive side of both lines and let $R^+_2$ be the region of the plane that lies to the negative side of both lines. Then $R^+=R^+_1\cup R^+_2$ contains exactly half of each mass distribution. Additionally, we prove that five mass distributions in $\mathbb{R}^3$ can be simultaneously bisected by two planes.

• Balanced $k$-Center Clustering When $k$ Is A Constant

The problem of constrained $k$-center clustering has attracted significant attention in the past decades. In this paper, we study balanced $k$-center cluster where the size of each cluster is constrained by the given lower and upper bounds. The problem is motivated by the applications in processing and analyzing large-scale data in high dimension. We provide a simple nearly linear time $4$-approximation algorithm when the number of clusters $k$ is assumed to be a constant. Comparing with existing method, our algorithm improves the approximation ratio and significantly reduces the time complexity. Moreover, our result can be easily extended to any metric space.

• 2D Closest Pair Problem: A Closer Look Ovidiu Daescu and Ka Yaw Teo.

A closer look is taken at the well-known divide-and-conquer algorithm for finding the closest pair of a set of points in the plane under the Euclidean distance. An argument is made that it is sufficient, and sometimes necessary, to check only the next three points following the current point associated with the $y$-sorted array in the combine phase of the algorithm.

• Supporting Ruled Polygons Nicholas Cavanna, Marc Khoury and Donald Sheehy.

We explore several problems related to ruled polygons. Given a ruling of a polygon $P$, we consider the Reeb graph of $P$ induced by the ruling. We define the Reeb complexity of $P$, which roughly equates to the minimum number of points necessary to support $P$. We give asymptotically tight bounds on the Reeb complexity that are also tight up to a small additive constant. When restricted to the set of parallel rulings, we show that the Reeb complexity can be computed in polynomial time.

18:00Cornerstone Bar and GrillDinner

### Friday July 28

9:00TB 360 Tilers, Tilemakers, Transformers! Stefan Langerman (Invited talk)

A tiling is a covering of the plane with copies of a geometric shape (tiles) without gaps or overlaps. A tiler is a shape that tiles the plane. An unfolding is obtained by cutting along the surface of a polyhedron through all its vertices, and opening all the dihedral angles between adjacent faces to obtain a single flat nonoverlapping geometric shape. A dissection is a decomposition of a shape into pieces that, can be rearranged to form another shape. In this hands-on talk, I will explore connections between these fascinating concepts, in an attempt to shed some light on several still unsolved algorithmic problems, among them: How easy (or hard) is it to determine if a given geometric shape can tile the plane?

10:00Break
10:30

#### Session 6A

TB 360
• A problem on track runners

Consider the circle $C$ of length 1 and a circular arc $A$ of length $\ell < 1$. It is shown that there exists $k=k(\ell) \in \mathbb{N}$, and a schedule for $k$ runners along the circle with $k$ distinct but constant positive speeds so that at any time $t \geq 0$, at least one of the $k$ runners is not in $A$.

• Common Development of Prisms, Anti-Prisms, Tetrahedra, and Wedges Amartya Shankha Biswas and Erik Demaine.

We investigate the problem of finding common developments of multiple polyhedra. We construct an uncountably infinite family of unfoldings which can be folded in to twelve distinct convex solids. This seems to be the first studied uncountable family of common developments. An additional useful property of our development is its ability to tile the plane.

• Computing 3SAT on a Fold-and-Cut Machine

This paper introduces a computational model called a fold-and-cut machine which allows as operations simple folds and unfolds, straight-line cuts, and inspection for a “through-hole” (hole through all the layers of paper). We show that a (deterministic) fold-and-cut machine can decide a 3SAT instance with $n$ variables and $m$ clauses using $O(nm+m^2)$ operations (with just one cut and inspection), showing that the machine is at least as powerful as a nondeterministic Turing machine.

#### Session 6B

TB 340
• Data Structures for Frechet Queries in Trajectory Data

Let T be a trajectory in the plane, represented as a polyline with $n$ edges. We show how to preprocess T into a data structure such that for any horizontal query segment Q in the plane and a subtrajectory between two vertices of T, one can quickly determine the Frechet distance between Q and that subtrajectory. We provide data structures for these queries that need $O(n^2\log^2 n)$ preprocessing time, $O(n^2\log^2 n)$ space, and $O(\log^2 n)$ query time. If we are interested only in the Frechet distance between the complete trajectory T and a horizontal query segment Q, we can answer these queries in $O(\log^2 n)$ time using only $O(n^2)$ space.

• Discretized Approaches to Schematization Maarten Löffler and Wouter Meulemans.

For both the Fréchet distance and the symmetric difference, we show that finding the simple polygon $S$ restricted to a grid that best resembles a simple polygon $P$ is NP-complete, even if: (1) we require that $S$ and $P$ have equal area; (2) we require turns to occur in a specified sequence for the Fréchet distance; (3) we permit $S$ to have holes for the symmetric difference.

• On the minimum edge size for 2-colorability and the realizability of hypergraphs by axis-parallel rectangles

Given a hypergraph $\hypergraph=(\vertA,\edgesA)$ what is the minimum integer $\rcn{\hypergraph}$ such that the sub-hypergraph with edges of size at least $\rcn{\hypergraph}$ are $2$-colorable? We consider the computational problem of finding the smallest such integer for a given hypergraph, and show that it is NP-hard to approximate it to within $\log m$ where $m=\cardin{\edgesA}$. For most geometric hypergraphs, i.e., those defined on a set of $n$ points by intersecting it with some shapes, it is well known that there is a coloring with $2$ colors ‘red’ and ‘blue’ such that any hyperedge containing $c \log n$ points, for some constant $c$, is bi-chromatic, i.e., contains points of both colors. We observe that indeed, for several such hypergraph families, this is the best possible – i.e., there are some $n$ points where there will always be a hyperedge with $\Omega(\log n)$ points that is mono-chromatic. These results follow from results on the indecomposability of coverings. We also show that a well known hypergraph, used in the literature on indecomposable coverings cannot be realized by axis-parallel rectangles in the plane. This problem was mentioned in a paper of Pach. et. al. on indecomposable coverings.

11:50End of conference