Volume 25 (2006)

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    An Improved Physically-Based Soft Shadow Volume Algorithm
    (The Eurographics Association and Blackwell Publishing, Inc, 2006) Lehtinen, Jaakko; Laine, Samuli; Aila, Timo
    We identify and analyze several performance problems in a state-of-the-art physically-based soft shadow volume algorithm, and present an improved method that alleviates these problems by replacing an overly conservative spatial acceleration structure by a more efficient one. The new technique consistently outperforms both the previous method and a ray tracing-based reference solution in several realistic situations while retaining the correctness of the solution and other desirable characteristics of the previous method. These include the unintrusiveness of the original algorithm, meaning that our method can be used as a black-box shadow solver in any offline renderer without requiring multiple passes over the image or other special accommodation. We achieve speedup factors from 1.6 to 12.3 when compared to the previous method.Categories and Subject Descriptors (according to ACM CCS): I.3.7 [Three-Dimensional Graphics and Realism]: Shadowing
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    Preface
    (The Eurographics Association and Blackwell Publishing, Inc, 2006)
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    Computational Aesthetics 2005 Eurographics Workshop on Computational Aesthetics in Graphics, Visualization and Imaging Girona, Spain, 18-20 May 2005
    (The Eurographics Association and Blackwell Publishing Ltd., 2006) Neumann, Laszlo; Sbert, Mateu; Gooch, Bruce; Purgathofer, Werner
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    A Hierarchical Topology-Based Model for Handling Complex Indoor Scenes
    (The Eurographics Association and Blackwell Publishing Ltd, 2006) Fradin, D.; Meneveaux, D.; Lienhardt, P.
    This paper presents a topology-based representation dedicated to complex indoor scenes. It accounts for memory management and performances during modelling, visualization and lighting simulation. We propose to enlarge a topological model (called generalized maps) with multipartition and hierarchy. Multipartition allows the user to group objects together according to semantics. Hierarchy provides a coarse-to-fine description of the environment. The topological model we propose has been used for devising a modeller prototype and generating efficient data structure in the context of visualization, global illumination and 1 GHz wave propagation simulation. We presently handle buildings composed of up to one billion triangles.
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    Beyond Tone Mapping: Enhanced Depiction of Tone Mapped HDR Images
    (The Eurographics Association and Blackwell Publishing, Inc, 2006) Smith, Kaleigh; Krawczyk, Grzegorz; Myszkowski, Karol; Seidel, Hans-Peter
    High Dynamic Range (HDR) images capture the full range of luminance present in real world scenes, and unlike Low Dynamic Range (LDR) images, can simultaneously contain detailed information in the deepest of shadows and the brightest of light sources. For display or aesthetic purposes, it is often necessary to perform tone mapping, which creates LDR depictions of HDR images at the cost of contrast information loss. The purpose of this work is two-fold: to analyze a displayed LDR image against its original HDR counterpart in terms of perceived contrast distortion, and to enhance the LDR depiction with perceptually driven colour adjustments to restore the original HDR contrast information. For analysis, we present a novel algorithm for the characterization of tone mapping distortion in terms of observed loss of global contrast, and loss of contour and texture details. We classify existing tone mapping operators accordingly. We measure both distortions with perceptual metrics that enable the automatic and meaningful enhancement of LDR depictions. For image enhancement, we identify artistic and photographic colour techniques from which we derive adjustments that create contrast with colour. The enhanced LDR image is an improved depiction of the original HDR image with restored contrast information.Categories and Subject Descriptors (according to ACM CCS): I.3.3 [Computer Graphics]: Picture/Image Generation I.4.0 [Image Processing and Computer Vision]: GeneralImage processing software
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    Compact Representation of Spectral BRDFs Using Fourier Transform and Spherical Harmonic Expansion
    (The Eurographics Association and Blackwell Publishing Ltd, 2006) Xu, Huiying; Sun, Yinlong
    This paper proposes a compact method to represent isotropic spectral BRDFs. In the first step, we perform a Fourier transform in the wavelength dimension. The resulting Fourier coefficients of the same order depend on three angles: the polar angle of the incident light, and the polar and azimuth angles of the outgoing light. In the second step, given an incident light angle, when the Fourier coefficients of the same order have an insensitive dependency on the outgoing direction, we represent these Fourier coefficients using a linear combination of spherical harmonics. Otherwise, we first decompose these Fourier coefficients into a smooth background that corresponds to diffuse component and a sharp lobe that corresponds to specular component. The smooth background is represented using a linear combination of spherical harmonics, and the sharp lobe using a Gaussian function. The representation errors are evaluated using spectral BRDFs obtained from measurement or generated from the Phong model. While maintaining sufficient accuracy, the proposed representation method has achieved data compression over a hundred of times. Examples of spectral rendering using the proposed method are also shown.
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    Third Eurographics Symposium on Geometry Processing (in cooperation with ACM SIGGRAPH)
    (The Eurographics Association and Blackwell Publishing Ltd, 2006) Desbrun, Mathieu
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    Topological Structures in Two-Parameter-Dependent 2D Vector Fields
    (The Eurographics Association and Blackwell Publishing, Inc, 2006) Weinkauf, T.; Theisel, H.; Hege, H.-C.; Seidel, H.-P.
    In this paper we extract and visualize the topological skeleton of two-parameter-dependent vector fields. This kind of vector data depends on two parameter dimensions, for instance physical time and a scale parameter. We show that two important classes of local bifurcations - fold and Hopf bifurcations - build line structures for which we present an approach to extract them. Furthermore we show that new kinds of structurally stable local bifurcations exist for this data, namely fold-fold and Hopf-fold bifurcations. We present a complete classification of them. We apply our topological extraction method to analyze a number of two-parameter-dependent vector fields with different physical interpretations of the two additional dimensions.Categories and Subject Descriptors (according to ACM CCS): I.3.3 [Computer Graphics]: Line and Curve Generation I.3.3 [Computer Graphics]: Picture/Image Generation
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    2006 Eurographics Symposium on Parallel Graphics and Visualization
    (The Eurographics Association and Blackwell Publishing Ltd, 2006) Paulo Santos, Luis
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    Errata
    (The Eurographics Association and Blackwell Publishing Ltd, 2006)
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    A Comparative Evaluation of Metrics for Fast Mesh Simplification
    (The Eurographics Association and Blackwell Publishing Ltd, 2006) Matias van Kaick, Oliver; Pedrini, Helio
    Triangle mesh simplification is of great interest in a variety of knowledge domains, since it allows manipulation and visualization of large models, and it is the starting point for the design of many multiresolution representations. A crucial point in the structure of a simplification method is the definition of an appropriate metric for guiding the decimation process, with the purpose of generating low error approximations at different levels of resolution. This paper proposes two new alternative metrics for mesh simplification, with the aim of producing high-quality results with reduced execution time and memory usage, and being simple to implement. A set of different established metrics is also described and a comparative evaluation of these metrics against the two new metrics is performed. A single implementation is used in the experiments, in order to enable the evaluation of these metrics independently from other simplification aspects. Results obtained from the simplification of a number of models, using the different metrics, are compared.
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    In Memoriam: Professor Dick Grimsdale 1929-2005
    (The Eurographics Association and Blackwell Publishing Ltd., 2006) Willis, Phil
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    AFRIGRAPH: Computer Graphics in Africa
    (The Eurographics Association and Blackwell Publishing Ltd, 2006) Gain, J. E.; Strasser, W.
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    Towards a Unified Dynamical Solver for Computer Graphics
    (The Eurographics Association and Blackwell Publishing, Inc, 2006) Stam, Jos
    In this talk I will present some research I have done over the past few years in developing a unified dynamical solver for computer graphics. Currently many solvers are specialized for a given phenomenon such as fluid flow, cloth, rigid bodies, hair, etc. Having these different solvers interact is sometimes problematic. We propose to model all matter as a linked particle system having the topology of a simplicial complex. The dynamical complex evolves due to external forces like gravity and constraints such as collisions and internal deformation energies. We use a solution scheme that iteratively updates the velocities to achieve all constraints. Key to the stability of our system is to interleave the solve for the different constraints. The talk will cover the main ideas and ingredients of our solver and will be accompanied with live demonstrations.
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    Splicing Upper-Body Actions with Locomotion
    (The Eurographics Association and Blackwell Publishing, Inc, 2006) Heck, Rachel; Kovar, Lucas; Gleicher, Michael
    This paper presents a simple and efficient technique for synthesizing high-fidelity motions by attaching, or splicing, the upper-body action of one motion example to the lower-body locomotion of another. Existing splicing algorithms do little more than copy degrees of freedom (DOFs) from one motion onto another. This naive DOF replacement can produce unrealistic results because it ignores both physical and stylistic correlations between various joints in the body. Our approach uses spatial and temporal relationships found within the example motions to retain the overall posture of the upper-body action while adding secondary motion details appropriate to the timing and configuration of the lower body. By decoupling upper-body action from lower-body locomotion, our motion synthesis technique allows example motions to be captured independently and later combined to create new natural looking motions.Categories and Subject Descriptors (according to ACM CCS): I.3.7 [Computer Graphics]: Animation
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    A Robust Two-Step Procedure for Quad-Dominant Remeshing
    (The Eurographics Association and Blackwell Publishing, Inc, 2006) Marinov, Martin; Kobbelt, Leif
    We propose a new technique for quad-dominant remeshing which separates the local regularity requirements from the global alignment requirements by working in two steps. In the first step, we apply a slight variant of variational shape approximation in order to segment the input mesh into patches which capture the global structure of the processed object. Then we compute an optimized quad-mesh for every patch by generating a finite set of candidate curves and applying a combinatorial optimization procedure. Since the optimization is performed independently for each patch, we can afford more complex operations while keeping the overall computation times at a reasonable level. Our quad-meshing technique is robust even for noisy meshes and meshes with isotropic or flat regions since it does not rely on the generation of curves by integration along estimated principal curvature directions. Instead we compute a conformal parametrization for each patch and generate the quad-mesh from curves with minimum bending energy in the 2D parameter domain. Mesh consistency between patches is guaranteed by simply using the same set of sample points along the common boundary curve. The resulting quad-meshes are of high-quality locally (shape of the quads) as well as globally (global alignment) which allows us to even generate fairly coarse quad-meshes that can be used as Catmull-Clark control meshes.Categories and Subject Descriptors (according to ACM CCS): I.3.5 [Computer Graphics]: Geometric algorithms, languages, and systems
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    Compression of Dense and Regular Point Clouds
    (The Eurographics Association and Blackwell Publishing Ltd, 2006) Merry, Bruce; Marais, Patrick; Gain, James
    We present a simple technique for single-rate compression of point clouds sampled from a surface, based on a spanning tree of the points. Unlike previous methods, we predict future vertices using both a linear predictor, which uses the previous edge as a predictor for the current edge, and lateral predictors that rotate the previous edge 90 left or right about an estimated normal.By careful construction of the spanning tree and choice of prediction rules, our method improves upon existing compression rates when applied to regularly sampled point sets, such as those produced by laser range scanning or uniform tesselation of higher-order surfaces. For less regular sets of points, the compression rate is still generally within 1.5 bits per point of other compression algorithms.
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    Translational Covering of Closed Planar Cubic B-Spline Curves
    (The Eurographics Association and Blackwell Publishing Ltd, 2006) Neacsu, Cristina; Daniels, Karen
    Spline curves are useful in a variety of geometric modeling and graphics applications and covering problems abound in practical settings. This work defines a class of covering decision problems for shapes bounded by spline curves. As a first step in addressing these problems, this paper treats translational spline covering for planar, uniform, cubic B-splines. Inner and outer polygonal approximations to the spline regions are generated using enclosures that are inside two different types of piecewise-linear envelopes. Our recent polygonal covering technique is then applied to seek translations of the covering shapes that allow them to fully cover the target shape. A feasible solution to the polygonal instance provides a feasible solution to the spline instance. We use our recent proof that 2D translational polygonal covering is NP-hard to establish NP-hardness of our planar translational spline covering problem. Our polygonal approximation strategy creates approximations that are tight, yet the number of vertices is only a linear function of the number of control points. Using recent results on B-spline curve envelopes, we bound the distance from the spline curve to its approximation. We balance the two competing objectives of tightness vs. number of points in the approximation, which is crucial given the NP-hardness of the spline problem. Examples of the results of our spline covering work are provided for instances containing as many as six covering shapes, including both convex and nonconvex regions. Our implementation uses the LEDA and CGAL C++ libraries of geometric data structures and algorithms.