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085-093.Pdf (289.0Kb) Eurographics Symposium on Point-Based Graphics (2006) M. Botsch, B. Chen (Editors) Approximating Geodesics on Point Set Surfaces M. R. Ruggeri, T. Darom2, D. Saupe & N. Kiryati2 † Dept. of Computer and Information Science, University of Konstanz, Konstanz, Germany 2 School of Electrical Engineering, Tel Aviv University, Tel Aviv, Israel Abstract We present a technique for computing piecewise linear approximations of geodesics on point set surfaces by min- imizing an energy function defined for piecewise linear path. The function considers path length, closeness to the surface for the nodes of the piecewise linear path and for the intermediate line segments. Our method is robust with respect to noise and outliers. In order to avoid local minima, a good initial piecewise linear approximation of a geodesic is provided by Dijkstra’s algorithm that is applied to a proximity graph constructed over the point set. As the proximity graph we use a sphere-of-influence weighted graph extended for surfel sets. The convergence of our method has been studied and compared to results of other methods by running experiments on surfaces whose geodesics can be computed analytically. Our method is presented and optimized for surfel-based repre- sentations but it has been implemented also for MLS surfaces. Moreover, it can also be applied to other surface representations, e.g., triangle meshes, radial-basis functions, etc. Categories and Subject Descriptors (according to ACM CCS): I.3.3 [Computer Graphics]: Line and Curve Gener- ation; I.3.5 [Computer Graphics]: Curve, surface, solid, and object representations; I.3.5 [Computer Graphics]: Geometric algorithms, languages, and systems. 1. Introduction or elliptical disks containing color information that describe surface characteristics within the neighborhood of each sam- In many fields, including cultural heritage, reverse engineer- ple point. Neither connectivity nor other topology informa- ing, architecture, and medical applications, 3D surfaces must tion is given. The importance of point-based representations be acquired from reality. The current 3D acquisition tech- and point-based primitives such as surfels, is rapidly increas- nologies produce large sets of 3D point clouds from which a ing. During the past years several paradigms known from proper and convenient representation of the real surface must triangle meshes have been transferred to point-based geom- be extracted for rendering, analysis and processing. In com- etry and many new techniques have been developed and op- puter graphics surfaces are commonly represented by trian- timized for point-based representations [KB04]. gular meshes. However, in recent years a significant trend in computer graphics has been the shift towards point-sampled Point sets geometry represents the typical output of surfaces, especially for rendering. Pfister et al [PZvBG00] the current 3D acquisition techniques, which produce sets presented surfels as a powerful point-based surface repre- of noisy 3D point clouds that subsequently have to be sentation scheme for rendering. Surfels are oriented circular processed for different purposes. The problem of comput- ing geodesics, i.e., shortest paths on surfaces, arises in dif- † ferent applications such as robot motion planning [HPJS01, This research was supported by the Kurt Lion Foundation. At HP04], shape analysis [EK01] and geographical informa- Tel-Aviv University, it was supported also by MUSCLE: Multime- tion systems. Furthermore, geodesics have been widely used dia Understanding through Semantics, Computation and Learning, a in 3D surface analysis and processing, including parame- European Network of Excellence funded by the EC 6th Framework ∗ IST Programme. At Konstanz University, it was supported also by terization [ZKK02, SWG 03], partitioning [KT03], flatten- the DFG Graduiertenkolleg “Explorative Analysis and Visualiza- ing [GKK02], modelling [KCVS98, ZG04], segmentation tion of Large Information Spaces”. We also thank Anna Dowden- [PC04], remeshing [OSG03], texture mapping [ZKK02], Williams for proofreading the paper. and also compression [MD04]. We propose a new technique c The Eurographics Association 2006. M. R. Ruggeri, T. Darom, D. Saupe & N. Kiryati / Approximating Geodesics on Point Set Surfaces for computing approximations of geodesics directly on noisy cision of our method for noisy point samples of smooth sur- point set surfaces with outliers. faces whose geodesics can be computed analytically. We also compare the precision of our approach with the tech- Given a surface Ψ represented by a set of N three dimen- 3 niques presented by Hofer and Pottmann in [HP04]. sional sample points Π = {pi ∈ R ,i = 1..N} we compute an approximation of the geodesic path and the correspond- ing geodesic distance between two arbitrary points of Π on 2. Related work the surface Ψ. Different algorithms are available for computing geodes- Geodesics are defined on a surface. In our case the sur- ics and measuring distances on surfaces. They are typi- face is given only indirectly by the set of 3D points Π. Since cally developed for high-dimensional manifolds and differ point based surfaces are typically acquired with 3D scanning with respect to the particular surface representation. One ap- techniques, we assume that our point set Π includes noise proach to approximate geodesics is to apply a Dijkstra-like and outliers. In this scenario extracting a precise approxi- algorithm on a graph generated by a refined representation mation of the real surface may be difficult and computation- of surfaces (e.g. polyhedron surfaces [KS01, KS04]). Kiry- ally expensive. However, several methods can be used to ex- ati and Székely in [KS93] presented an algorithm for geo- tract geometric surfaces from points clouds. The most used desic distance estimation on voxel-based surfaces. Kimmel approaches extract implicit surface representations that lo- and Kiryati in [KK96] proposed an algorithm for finding cally approximates the real surface via moving least-squares shortest paths in two stages. In the first stage the algorithm (MLS) surfaces [ABCO∗03, AK04, DS05, BH05], or radial proposed in [KS93] is used to obtain an approximation of basis functions (RBF) [CBC∗01,OBS04,RTSD03]. Working the globally shortest path. In the second stage the approx- directly with local approximations through implicit surfaces imation is refined to a locally optimal path. Kimmel and allows a rigorous formulation of the problem of computing Sethian in [KS98] proposed an efficient algorithm for com- geodesics on the real surface. Nevertheless, this approach puting geodesic distances and paths on triangulated mani- can be too costly for several applications, such as render- folds based on a fast marching method. This approach has ing and other processing techniques, which need to compute been widely used due to its speed and precision. Further- such information on the fly. Surfels have been invented for more, different efficient variations of this method were im- rendering in order to provide 3D points with information plemented [NK02,Kir04]. Surazhsky et al. in [SSK∗05] pro- about colors, orientations (normals), and their influence in posed an alternative technique to compute exact and approx- covering the underlying surface. Such information can be imate geodesics on triangle meshes. Their technique is based extracted by analyzing the surface samples in the neighbor- on the idea to unfold a set of adjacent triangles into a com- hood of each point pi ∈ Π [PZvBG00,KB04,WK04,AA04]. mon 2D plane and extract geodesics by reconstructing the Thus, the surfel set S = {si,i = 1..N} extends the original distance field associated to each mesh edge. point set Π and can be considered as a discrete approxima- Geodesics for point clouds have been considered before tion of the real surface Ψ. by other authors. Memoli and Sapiro in [MS03] constructed We address the problem of computing piecewise linear ap- an offset band of the point set surface consisting of the union proximations of geodesics on point set surfaces by minimiz- of Euclidean balls centered at the given points. Geodesics ing an energy function defined for piecewise linear paths. were computed by applying the fast marching algorithm The function considers path length, closeness to the surface on a 3D Cartesian grid built inside the constructed band. for the nodes of the piecewise linear path and for the in- The accuracy depends on the resolution of the 3D grid, and termediate line segments. In order to avoid local minima, a the execution time depends on the number of grid points. good initial approximation is provided by Dijkstra’s algo- Their method is robust with respect to noise provided that rithm that is applied to a proximity graph constructed over it is bounded by the radii of the balls defining the offset the point set. Our method robustly handles noisy surfaces band. Klein and Zachmann [KZ04] approximated geodesics with outliers, i. e., typical output of 3D scanning frame- as shortest paths on a geometric proximity graph over point works. We implemented our method for two point based sur- clouds. They experimented with Delaunay and spheres-of- face representations: surfels and MLS surfaces. Since sur- influence graphs (SIG) as proximity graphs and proposed an fels as a general point based primitive, are computation- extended version of SIG. Hofer and Pottmann [HP04] pro- ally easy to manage, suitable for interactive/multi-resolution posed to compute geodesic curves as energy minimizing dis- systems [RL00], and
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