Algorithms for distance problems in planar complexes of global nonpositive curvature

Shortest path problem in rectangular complexes of global nonpositive curvature

CAT(0) metric spaces constitute a far-reaching common generalization of Euclidean and hyperbolic spaces and simple polygons: any two points x and y of a CAT(0) metric space are connected by a unique shortest path γ(x,y). In this paper, we present an efficient algorithm for answering two-point distance queries in CAT(0) rectangular complexes and two of theirs subclasses, ramified rectilinear polygons (CAT(0) rectangular complexes in which the links of all vertices are bipartite graphs) and squaregraphs (CAT(0) rectangular complexes arising from plane quadrangulations in which all inner vertices…

Certified Computation of planar Morse-Smale Complexes

The Morse-Smale complex is an important tool for global topological analysis in various problems of computational geometry and topology. Algorithms for Morse-Smale complexes have been presented in case of piecewise linear manifolds. However, previous research in this field is incomplete in the case of smooth functions. In the current paper we address the following question: Given an arbitrarily complex Morse-Smale system on a planar domain, is it possible to compute its certified (topologically correct) Morse-Smale complex? Towards this, we develop an algorithm using interval arithmetic to com…

Delta Complexes in Digital Images. Approximating Image Object Shapes

In a computational topology of digital images, simplexes are replaced by Delta sets in approximating image object shapes. For simplicity, simplexes and Delta sets are restricted to the Euclidean plane. A planar simplex is either a vertex, a line segment or a filled triangle. In this study of image shapes, a planar Delta set is a sequence of ordered simplicial complexes. The basic approach is to approximate an image shape by decomposing an image region containing the shape into combinations of Delta sets called Delta complexes. This approach to image shapes is motivated by the ease with which s…

Flat Folding an Unassigned Single-Vertex Complex (Combinatorially Embedded Planar Graph with Specified Edge Lengths) without Flat Angles

A foundational result in origami mathematics is Kawasaki and Justin's simple, efficient characterization of flat foldability for unassigned single-vertex crease patterns (where each crease can fold mountain or valley) on flat material. This result was later generalized to cones of material, where the angles glued at the single vertex may not sum to $360^\circ$. Here we generalize these results to when the material forms a complex (instead of a manifold), and thus the angles are glued at the single vertex in the structure of an arbitrary planar graph (instead of a cycle). Like the earlier chara…

图嵌入二维单纯复形的FPT算法

An FPT algorithm for the embeddability of graphs into two-dimensional simplicial complexes

针对图嵌入二维单纯复形的NP难问题，提出基于无关顶点法和动态规划的FPT算法（O(2^{poly(c)}·n²)时间），可计算嵌入表示，归约多个已知问题，结合现有结果可优化至线性时间。

Sensor-topology based simplicial complex reconstruction

We propose a new method for the reconstruction of simplicial complexes (combining points, edges and triangles) from 3D point clouds from Mobile Laser Scanning (MLS). Our main goal is to produce a reconstruction of a scene that is adapted to the local geometry of objects. Our method uses the inherent topology of the MLS sensor to define a spatial adjacency relationship between points. We then investigate each possible connexion between adjacent points and filter them by searching collinear structures in the scene, or structures perpendicular to the laser beams. Next, we create triangles for eac…