# Surface Reconstruction

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1 WDS'09 Proceedings of Contributed Papers, Part I, , ISBN MATFYZPRESS Surface Reconstruction P. Surynková Charles University in Prague, Faculty of Mathematics and Physics, Prague, Czech Republic Department of Mathematics Education Abstract. We describe the problem of surface reconstruction from the point clouds which is applied in many scientific and engineering applications. Our aim is to obtain a visualization of data from 3D scanning and document the real objects digitally. Digital documentation brings the possibility to manipulate with surfaces in mathematical and modelling computer software. It can help with reconstruction and documentation of historical buildings and sculptures with 3D scanners or restoring of monuments. We mention several known algorithms for surface reconstruction and focus on the algorithms which are necessary for pre-processing of the input set of points Delaunay triangulation and Voronoi diagram and the thinning algorithm. We explain choice of software for implementation of the algorithms. Introduction The problem of surface reconstruction appeared in medical imaging. Recently digital documentation is very modern and expanding method in many scientific and engineering applications. The input is a finite set of points in the space, we know 3D coordinates. The input set is called point cloud in computer graphics. Real data can be obtained from 3D scanning of the real object and may contain as many as millions points. The output is a reconstructed surface so that the points of the input set lie on or close to surface. If it is possible we want to get the parametrical or implicit description and the final step is to create 3D model of reconstructed surface. How to obtain the output is the subject of our research; see Figures 1, 2. Figure 1. Example of the input set of points obtained by 3D scanning of the Corinthian column. On the left the real object, on the right the input set of points which represents the object. (the source:

2 Figure 2. On the left piecewise linear interpolant of the input set of points and on the right one of the possible outputs 3D model. (the source: The input set of points is mostly redundant, some points are useless, don t contain any new or important information. For that reason it is necessary to reduce the input set of points and remove the useless points. It is the first step. The surface reconstruction follows. The majority of methods is based on the dividing of the 3D space (so-called spatial subdivision). It means that the circumscribed box of the input set of points is divided into disjoint cells e.g. tetrahedrization, we obtain a tetrahedral mesh. Then we find those parts of mesh which are connected with the surface and calculate the surface from the selected cells. The algorithms In this part we introduce several algorithms. We start with the algorithms for partitioning of a planar or a spatial domain which are necessary for the pre-processing Delaunay triangulation and Voronoi diagram. These algorithms can be generalized into n-dimensional space. Then we mention the algorithms for reducing of the input set of points the thinning algorithms and finally some algorithms for surface reconstruction alpha-shapes, crust algorithm, which are based on spatial subdivision. 1 The algorithms for partitioning We always explain the planar case. Everything can be generalized into three-dimensional space (or higher dimensions). Definition 1 (Triangulation) in [Iske, A., 2004]. A triangulation of a discrete set X of points in the plane is a collection of triangles in the plane with these conditions: a) the vertices of triangles are points from the input set X of points, b) any pair of two distinct triangles intersect at most at one vertex or along one edge, c) the convex hull of X coincides with the area covered by the union of triangles. From this some another properties follow. The triangulation may not be unique. The triangulation can also be regarded as a planar graph whose nodes are the vertices of triangles and whose edges provide the connections in the graph. There are many different triangulations of a given planar set X of points. The most frequently used triangulation is Delaunay triangulation. The Delaunay triangulation is a triangulation of X, such that the circumcircle for each of its triangles doesn t contain any point from X in its interior. The Delaunay triangulation isn t unique for four points on the same circle, maximizes the minimal angle and it doesn t exist for a set of points on the same line. There exist several algorithms for the 205

3 construction of the Delaunay triangulation e.g. Post optimize, Iteratively build or Divide and conquer. We can generalize this decomposition into three-dimensional space. We obtain tetrahedra instead of triangles and circumsphere instead of circumcircle. Figure 3 shows an example of the Delaunay triangulation of a set of points in the plane with circumcircle of one triangle and three-dimensional case Delaunay tetrahedrization. Figure 3. On the left an example of the Delaunay triangulation of a set of points in the plane with circumcircle of one triangle. On the right the Delaunay tetrahedrization the input set of points are the vertices of the cube and its centre. Figure 4. On the left an example of the Voronoi region of a set of points in the plane. Point p is the nearest point for each point of Voronoi region of point p. On the right the duality of the Delaunay triangulation and Voronoi diagram. Centre of circumcircle of the triangle is the vertex of Voronoi region. 206

4 The dual graph of the Delaunay triangulation is Voronoi diagram. Voronoi diagram of a set X of points in the plane is decomposition of the plane into Voronoi regions. Definition 2 (Voronoi region) in [Edelsbrunner, H., 2001]. Voronoi region of p X is the set of points in the plane which are at least as close to p as to any other point in X. Voronoi diagram is a planar graph and Voronoi regions cover the whole plane. Voronoi regions don t share interior points, and if a point belongs to two Voronoi regions then it lies on the bisector. Voronoi regions are convex polygons, possibly unbounded, and together with their shared edges and vertices form the Voronoi diagram. The algorithms for the construction of the Voronoi diagram e.g. Divide and conquer or construction as the dual of Delaunay triangulation. Again we can generalize it into three-dimensional space. Then the Voronoi regions are convex polyhedra. Figure 4 shows the Voronoi diagram of a set of points in the plane and the duality of the Delaunay triangulation and the Voronoi diagram. 2 The algorithms for reducing In this part we focus on the algorithms for reducing of the input set of points. We explain the thinning which is concerned with the approximation of a bivariate function from the point clouds by piecewise linear functions over a suitable triangulation. a ) b) c) Figure 5. a) The input a finite planar set X of points, b) the Delaunay triangulation of X, c) the final step addition of z-coordinates to each point and calculating the piecewise linear interpolant. 207

5 Let SURYNKOVÁ: SURFACE RECONSTRUCTION 2 X be a finite planar set of points and let only function values f X f : 2 denote an unknown function,, at the points in X, are given ( f is optional function which defines the elevation field). The aim of the thinning algorithm is the construction of subsets Y X, such that each piecewise linear interpolant L( f, T Y )(satisfying the following equation L( f, TY )( y) = f( y), for all y Y ), over a suitable triangulation T Y of Y, is close to the given function values f X ; see Figure 5. We mostly work with the Delaunay triangulation T Y of Y. There exist several removal criteria which depend on the underlying application. We mention the most frequently used removal criterion. More detailed information can be found in [Iske, A., 2004]. Definition 3 (Removal Criterion) in [Iske, A., 2004]. For Y X removable from Y, if and only if it satisfies, a point * y Y is said to be η * ( Y \ y, X) min η( Y \ y, X) y Y =, where η denotes the approximation error η ( Y, X) = L( f, T ) f. Y X X Note that η ( Y, X) depends on both the input values f X and on the selected triangulation method. The convex hull of Y X must coincide with the convex hull of X. For that reason we don t remove the extremal points from X. 3 The algorithms for surface reconstruction There is a wide range of applications for which surface construction from point cloud is important. We focus on the algorithm based on the spatial subdivision. The goal of these algorithms is to find the cells related to the surface described by the input set of points. We can select the cells in two ways surface-oriented and volume-oriented. Alpha-shapes. This algorithm by Edelsbrunner and Mücke is an example of surface-oriented cell selection. The first step of the algorithm is the decomposition of the input set of points using the Delaunay tetrahedrization. The second step is to remove tetrahedra, triangles and edges of the Delaunay tetrahedrization using α -balls as eraser sphere with radius α. Each tetrahedron, triangle or edge of the tetrahedrization whose minimum surrounding sphere doesn t fit into the eraser sphere is eliminated. We obtain so-called α -shape which consists of points, edges, faces and tetrahedra. The third step of the algorithm is the extraction of triangles from α -shape using following rule let us have two possible spheres with radius α through all three points of a triangle of the α -shape. If at least one of these doesn t contain any other point of the input set, the triangle belongs to the surface. This algorithm may have some disadvantages. Parameter α must be chosen experimentally. And the algorithm is used for the uniform input set of points. More detailed information can be found in [Mencl, R., 1997]. Crust algorithm. An example of volume-oriented cell selection is the crust algorithm by Amenta and Bern. In contrast to the previous one, this algorithm is based on the decomposition by Voronoi diagram. So the first step of the algorithm is to compute the Voronoi diagram of the given set of points. Then for each point s of the input set S do: a) if s doesn t lie on the convex hull, let p + be the farthest Voronoi vertex of Voronoi region of s from s. Let n + be the vector sp +, 208

6 b) if s lies on the convex hull, let n + be the average of the normals of the adjacent triangles, c) let p be the Voronoi vertex of Voronoi region of s with negative projection + on n that is farthest from s. Then the algorithm computes the Delaunay tetrahedrization of S P, where P is the set of all poles p + and p. Then it keeps only those triangles for which all three vertices are points from the input set of points. The output of the algorithm is three-dimensional crust consisting of triangles that resembles the surface. Some triangles in three-dimensional crust aren t correct. We have to check the output again. More detailed information can be found in [Mencl, R., 1997]. Choice of software We choose MATLAB for implementation of the algorithms. MATLAB is modern language and interactive environment for technical computing. It includes many 2D and 3D graphics functions for data visualization and it is commonly used for these purposes. Figures 3, 4, 5 are created using visualization skills of MATLAB system. Conclusion and future work Our contribution introduced the outline of several methods and algorithms for surface reconstruction. In our future work we will focus on the implementation of known and eventually new algorithms for surface reconstruction. We will start with computer-generated data and we will continue with real data from 3D scanners. Our next goal is to obtain data visualization and to create 3D models. References Amenta, N., et al., A Simple Algorithm for Homeomorphic Surface Reconstruction, International Journal of Computational Geometry and Applications, Amenta, N., et al., A New Voronoi-Based Surface Reconstruction Algorithm, International Conference on Computer Graphics and Interactive Techniques, Edelsbrunner, H., Geometry and topology for mesh generation, Cambridge University Press, Cambridge, United Kingdom, Iske, A., Multiresolution Method in Scattered Data Modelling, Technische Universität München, Germany, Mencl, R., et al., Interpolation and Approximation of Surfaces from Three-Dimensional Scattered Data Points, State of the Art Report, EURORAPHICS 98, Preparata, F. P., et al., Computational geometry, Springer-Verlag, New York, USA,

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