We introduce an app for generating spiroplots, based on a new discrete-time, linear, dynamic system that repeatedly rotates a pair of points, and plots points where they land. The app supports easy definition of the initial situation and has various visualization settings. It can be accessed at this webpage.

In this video, we present theoretical and practical methods for achieving arbitrary reconfiguration of a set of objects, based on the use of external forces, such as a magnetic field or gravity: Upon actuation, each object is pushed in the same direction. This concept can be used for a wide range of applications in which particles do not have their own energy supply or in which they are subject to the same global control commands. A crucial challenge for achieving any desired target configuration is breaking global symmetry in a controlled fashion. Previous work (some of which was presented during SoCG 2015) made use of specifically placed barriers; however, introducing precisely located obstacles into the workspace is impractical for many scenarios. In this paper, we present a different, less intrusive method: making use of the interplay between static friction with a boundary and the external force to achieve arbitrary reconfiguration. Our key contributions are theoretical characterizations of the critical coefficient of friction that is sufficient for rearranging two particles in triangles, convex polygons, and regular polygons; a method for reconfiguring multiple particles in rectangular workspaces, and deriving practical algorithms for these rearrangements. Hardware experiments show the efficacy of these procedures, demonstrating the usefulness of this novel approach.

In this video, we consider recognition and reconfiguration of lattice-based cellular structures by very simple robots with only basic functionality. The underlying motivation is the construction and modification of space facilities of enormous dimensions, where the combination of new materials with extremely simple robots promises structures of previously unthinkable size and flexibility. We present algorithmic methods that are able to detect and reconfigure arbitrary polyominoes, based on finite-state robots, while also preserving connectivity of a structure during reconfiguration. Specific results include methods for determining a bounding box, scaling a given arrangement, and adapting more general algorithms for transforming polyominoes.

In this video we describe why producing a Computational Geometry video is a good idea, what it takes to make one, and how to actually do it. This includes a guide for the overall process, a number of examples, and a variety of tips and tricks.

In this video, we motivate and visualize a fundamental result for covering a rectangle by a set of non-uniform circles. We describe the structure of the proof, and show animations of some of the main components.

We present two software packages for computing straight skeletons: Monos, our implementation of an algorithm by Biedl et al. (2015), computes the straight skeleton of a monotone input polygon, and Surfer2 implements a generalization of an algorithm by Aichholzer and Aurenhammer (1998) to handle multiplicatively-weighted planar straight-line graphs as input. The graphical user interfaces that ship with our codes support step-by-step computations, where each event can be investigated and studied by the user. This makes them a canonical candidate for educational purposes and detailed event analyses. Both codes are freely available on GitHub.

We present a piece of software for computing animations of linkages with rotational symmetry in the plane. We construct these linkages from an algorithm that utilises a special type of edge colouring to embed graphs with rotational symmetry.

Face-connected configurations of cubes are a common model for modular robots in three dimensions. In this abstract and the accompanying video we study reconfigurations of such modular robots using so-called sliding moves. Using sliding moves, it is always possible to reconfigure one face-connected configuration of n cubes into any other, while keeping the robot connected at all stages of the reconfiguration. For certain configurations Ω(n²) sliding moves are necessary. In contrast, the best current upper bound is O(n³). It has been conjectured that there is always a cube on the outside of any face-connected configuration of cubes which can be moved without breaking connectivity. The existence of such a cube would immediately imply a straight-forward O(n²) reconfiguration algorithm. However, we present a configuration of cubes such that no cube on the outside can move without breaking connectivity. In other words, we show that this particular avenue towards an O(n² reconfiguration algorithm for face-connected cubes is blocked.

We present a new game, "Dots & Polygons", played on a planar point set. We prove that its NP-hard and discuss strategies for the case when the point set is in convex position.

We present a method for generating interesting levels based on several NP-hardness reductions for a puzzle game based on the Art Gallery problem.

The order in which plane-filling curves visit points in the plane can be exploited to design efficient algorithms. Typically, the curves are useful because they preserve locality: points that are close to each other along the curve tend to be close to each other in the plane, and vice versa. However, sketches of plane-filling curves do not show this well: they are hard to read on different levels of detail and it is hard to see how far apart points are along the curve. This paper presents a software tool to produce compelling visualisations that may give more insight in the structure of the curves.

Discrete stratified Morse theory, first introduced by Knudson and Wang, works toward a discrete analogue of Goresky and MacPherson's stratified Morse theory. It is inspired by the works of Forman on discrete Morse theory by generalizing stratified Morse theory to finite simplicial complexes. The class of discrete stratified Morse functions is much larger than that of discrete Morse functions. Any arbitrary real-valued function defined on a finite simplicial complex can be made into a discrete stratified Morse function with the proper stratification of the underlying complex. An algorithm is given by Knudson and Wang that constructs a discrete stratified Morse function on any finite simplicial complex equipped with an arbitrary real-valued function. Our media contribution is an open-sourced visualization tool that implements such an algorithm for 2-complexes embedded in the plane, and provides an interactive demo for users to explore the algorithmic process and to perform homotopy-preserving simplification of the resulting stratified complex.