Team Alice

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Section: Scientific Foundations

Keywords : Mesh processing, parameterization, texture mapping, fairing, splines.

Geometry Processing

Participants : Olivier Génevaux, Bruno Lévy, Aurélien Martinet, Nicolas Ray, Bruno Vallet.

Figure 2. Our geometry processing tools applied to a triangulated CAD model of the Avantime car (courtesy of Renault). The constructed 2D parameter space (right) can be used for texturing, real-time visualization and conversion into parametric surfaces.
avantime

Geometry processing recently appeared (in the middle of the 90's) as a promising avenue to solve the geometric modeling problems encountered when manipulating meshes composed of million elements. Since a mesh may be considered to be a sampling of a surface - in other words a signal - the digital signal processing formalism was a natural theoretic background for this subdomain (see e.g., [31] ). Researchers of this subdomain then studied different aspects of this formalism applied to geometric modeling.

Although many advances have been made in the geometry processing area, important problems still remain open. Even if shape acquisition and filtering is much easier than 30 years ago, a scanned mesh composed of 30 millions of triangles cannot be used directly in real-time visualization or complex numerical simulation. For this reason, automatic methods to convert those large meshes into higher level representations are necessary. However, these automatic methods do not exist yet. For instance, the pioneer Henri Gouraud often mentions in his talks that the data acquisition problem is still open. Malcolm Sabin, another pioneer of the “Computer Aided Geometric Design” and “Subdivision” approaches, mentioned during several conferences of the domain that constructing the optimum control-mesh of a subdivision surface so as to approximate a given surface is still an open problem. More generally, converting a mesh model into a higher level representation, consisting of a set of equations, is a difficult problem for which no satisfying solutions have been proposed. This is one of the long-term goals of international initiatives, such as the AIMShape European network of excellence.

Motivated by gridding application for finite elements modeling for oil and gas exploration, in the frame of the Gocad project, we started studying geometry processing in the late 90's and contributed to this area at the early stages of its development. We developed the LSCM method (Least Squares Conformal Maps) in cooperation with Alias Wavefront [6] . This method has become the de-facto standard in automatic unwrapping, and was adopted by several 3D modeling packages (including Maya and Blender). We experimented various applications of the method, comprising normal mapping, mesh completion and light simulation [2] .

However, classical mesh parameterization requires to partition the considered object into a set of topological disks. For this reason, last year (2006), we designed a new method (Periodic Global Parameterization) that generates a continuous set of coordinates over the object [8] . We also showed the applicability of this method, by proposing the first algorithm that converts a scanned mesh into a Spline surface automatically [7] .

We are still not fully satisfied with these results, since the method remains quite complicated. We think that a deeper understanding of the underlying theory is likely to lead to both efficient and simple methods. For this reason, this year (2007), we studied different ways of discretizing partial differential equations on meshes, including Finite Element Modeling and Discrete Exterior Calculus.


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