26-Issue 2
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Item Automatic Light Source Placement for Maximum Visual Information Recovery(The Eurographics Association and Blackwell Publishing Ltd, 2007) Vazquez, P.-P.The automatic selection of good viewing parameters is a very complex problem. In most cases, the notion of good strongly depends on the concrete application. Moreover, when an intuitive definition of good view is available, it is often difficult to establish a measure that brings it to the practice. Commonly, two kinds of viewing parameters must be set: camera parameters (position and orientation) and lighting parameters (number of light sources, its position and eventually the orientation of the spot). The first parameters will determine how much of the geometry can be captured and the latter will influence on how much of it is revealed (i.e., illuminated) to the user. Unfortunately, ensuring that certain parts of a scene are lit does not make sure that the details will be communicated to the user, as the amount of illumination might be too small or too high. In this paper we define a metric to calculate the amount of information relative to an object that is effectively communicated to the user given a fixed camera position. This measure is based on an information-based concept, the Shannon entropy, and will be applied to the problem of automatic selection of light positions in order to adequately illuminate an object. In order to validate the results, we have carried out an experiment on users, this experiment helped us to explore other related measures.Item Editorial(The Eurographics Association and Blackwell Publishing Ltd, 2007) Duke, David; Scopigno, RobertoItem Efficient RANSAC for Point-Cloud Shape Detection(The Eurographics Association and Blackwell Publishing Ltd, 2007) Schnabel, R.; Wahl, R.; Klein, R.In this paper we present an automatic algorithm to detect basic shapes in unorganized point clouds. The algorithm decomposes the point cloud into a concise, hybrid structure of inherent shapes and a set of remaining points. Each detected shape serves as a proxy for a set of corresponding points. Our method is based on random sampling and detects planes, spheres, cylinders, cones and tori. For models with surfaces composed of these basic shapes only, for example, CAD models, we automatically obtain a representation solely consisting of shape proxies. We demonstrate that the algorithm is robust even in the presence of many outliers and a high degree of noise. The proposed method scales well with respect to the size of the input point cloud and the number and size of the shapes within the data. Even point sets with several millions of samples are robustly decomposed within less than a minute. Moreover, the algorithm is conceptually simple and easy to implement. Application areas include measurement of physical parameters, scan registration, surface compression, hybrid rendering, shape classification, meshing, simplification, approximation and reverse engineering.Item Eurographics Symposium on Rendering Nicosia, Cyprus, June 26-28, 2006(The Eurographics Association and Blackwell Publishing Ltd, 2007)Item Eurographics Symposium on Rendering Nicosia, Cyprus, June 26-28, 2006(The Eurographics Association and Blackwell Publishing Ltd, 2007)Item A Lagrangian Approach to Dynamic Interfaces through Kinetic Triangulation of the Ambient Space(The Eurographics Association and Blackwell Publishing Ltd, 2007) Pons, J.-P.; Boissonnat, J.-D.In this paper, we propose a robust and efficient Lagrangian approach for modeling dynamic interfaces between different materials undergoing large deformations and topology changes, in two dimensions. Our work brings an interesting alternative to popular techniques such as the level set method and the particle level set method, for two-dimensional and axisymmetric simulations. The principle of our approach is to maintain a two-dimensional triangulation which embeds the one-dimensional polygonal description of the interfaces. Topology changes can then be detected as inversions of the faces of this triangulation. Each triangular face is labeled with the type of material it contains. The connectivity of the triangulation and the labels of the faces are updated consistently during deformation, within a neat framework developed in computational geometry: kinetic data structures. Thanks to the exact computation paradigm, the reliability of our algorithm, even in difficult situations such as shocks and topology changes, can be certified. We demonstrate the applicability and the efficiency of our approach with a series of numerical experiments in two dimensions. Finally, we discuss the feasibility of an extension to three dimensions.Item Massless Cable for Real-time Simulation(The Eurographics Association and Blackwell Publishing Ltd, 2007) Servin, M.; Lacoursiere, C.A technique for real-time simulation of hoisting cable systems based on a multibody nonideal constraint is presented. The hoisting cable constraint is derived from the cable internal energies for stretching and twisting. Each hoisting cable introduces two constraint equations, one for stretching and one for torsion, which include all the rigid bodies attached by the same cable. The computation produces the global tension and torsion in the cable as well as the resulting forces and torques on each attached body. The complexity of the computation grows linearly with the number of bodies attached to a given cable and is weakly coupled to the rest of the simulation. The nonideal constraint formulation allows stable simulations of cables over wide ranges of linear and torsional stiffness, including the rigid limit. This contrasts with lumped element formulations including the cable internal degrees of freedom in which computational complexity grows at least linearly with the number of cable elements - usually proportional to cable length - and where numerical stability is sensitive to the mass ratio between the load and the lumped elements.Item A Method for Constructing Interpolatory Subdivision Schemes and Blending Subdivisions(The Eurographics Association and Blackwell Publishing Ltd, 2007) Li, G.; Ma, W.This paper presents a universal method for constructing interpolatory subdivision schemes from known approximatory subdivisions. The method establishes geometric rules of the associated interpolatory subdivision through addition of further weighted averaging operations to the approximatory subdivision. The paper thus provides a novel approach for designing new interpolatory subdivision schemes. In addition, a family of subdivision surfaces varying from the given approximatory scheme to its associated interpolatory scheme, namely the blending subdivisions, can also be established. Based on the proposed method, variants of several known interpolatory subdivision schemes are constructed. A new interpolatory subdivision scheme is also developed using the same technique. Brief analysis of a family of blending subdivisions associated with the Loop subdivision scheme demonstrates that this particular family of subdivisions are globally C1 continuous while maintaining bounded curvature for regular meshes. As a further extension of the blending subdivisions, a volume-preserving subdivision strategy is also proposed in the paper.Item On Exact Error Bounds for View-Dependent Simplification(The Eurographics Association and Blackwell Publishing Ltd, 2007) Eisemann, E.; Decoret, X.In this article we present an analytical closed-form expression to ensure exact error bounds for view-dependent simplification which is of importance for several algorithms. The present work contains proofs and solutions for the general 2D case and particular 3D cases.Most preceding works rely on coarse heuristics, that might fail and/or restrict movements or object representations. We introduce the notion of validity regions as the complete set of possible simplifications respecting a given error bound between the object and its simplification. The approach handles arbitrary polygonal viewcells which allow for free movement in the interior. We show how to compute these regions for mesh points and faces. Since the validity region of a face accounts for all its points, properties like silhouette preservation and textures are gracefully handled. This is not the case if the error is controlled only at the face s vertices or edges.Item Smooth Piecewise Polynomial Blending Operations for Implicit Shapes(The Eurographics Association and Blackwell Publishing Ltd, 2007) Li, Q.In this paper, we present a new set of blending operations for implicitly defined geometric shapes. The proposed shape operators are piecewise polynomial and blending range controllable, and can be constructed to any required degree of smoothness. The key idea behind these techniques is the introduction of the concept of the smooth absolute functions, which in turn lead to the definition of smooth maximum functions. These novel generalized absolute functions can be constructed recursively or through a recursively defined functions, and can thus be computed cheaply. In addition, the underlying mathematical descriptions of these shape operations are very simple and elegant.Item Texture Synthesis using Exact Neighborhood Matching(The Eurographics Association and Blackwell Publishing Ltd, 2007) Sabha, M.; Peers, P.; Dutre, P.In this paper we present an elegant pixel-based texture synthesis technique that is able to generate visually pleasing results from source textures of both stochastic and structured nature. Inspired by the observation that the most common artifacts that occur when synthesizing textures are high-frequency discontinuities, our technique tries to avoid these artifacts by forcing at least one of the direct neighboring pixels in each causal neighborhood to match within a predetermined threshold. This does not only avoid deterioration of the visual quality, but also results in faster synthesis timings. We demonstrate our technique on a variety of stochastic and structured textures.