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Item Efficient Rendering of Local Subsurface Scattering(The Eurographics Association and Blackwell Publishing Ltd., 2005) Mertens, Tom; Kautz, Jan; Bekaert, Philippe; Van Reeth, Frank; Seidel, Hans-PeterA novel approach is presented to efficiently render local subsurface scattering effects. We introduce an importance sampling scheme for a practical subsurface scattering model. It leads to a simple and efficient rendering algorithm, which operates in image space, and which is even amenable for implementation on graphics hardware. We demonstrate the applicability of our technique to the problem of skin rendering, for which the subsurface transport of light typically remains local. Our implementation shows that plausible images can be rendered interactively using hardware acceleration.Item Accelerating Path Tracing by Re-Using Paths(The Eurographics Association, 2002) Bekaert, Philippe; Sbert, Mateu; Halton, John; P. Debevec and S. GibsonThis paper describes a new acceleration technique for rendering algorithms like path tracing, that use so called gathering random walks. Usually in path tracing, each traced path is used in order to compute a contribution to only a single point on the virtual screen. We propose to combine paths traced through nearby screen points in such a way that each path contributes to multiple screen points in a provably good way. Our approach is unbiased and is not restricted to diffuse light scattering. It complements previous image noise reduction techniques for Monte Carlo ray tracing. We observe speed-ups in the computation of indirect illumination of one order of magnitude.Item Interactive Rendering of Translucent Deformable Objects(The Eurographics Association, 2003) Mertens, Tom; Kautz, Jan; Bekaert, Philippe; Seidel, Hans-Peter; Reeth, Frank Van; Philip Dutre and Frank Suykens and Per H. Christensen and Daniel Cohen-OrRealistic rendering of materials such as milk, fruits, wax, marble, and so on, requires the simulation of subsurface scattering of light. This paper presents an algorithm for plausible reproduction of subsurface scattering effects. Unlike previously proposed work, our algorithm allows to interactively change lighting, viewpoint, subsurface scattering properties, as well as object geometry. The key idea of our approach is to use a hierarchical boundary element method to solve the integral describing subsurface scattering when using a recently proposed analytical BSSRDF model. Our approach is inspired by hierarchical radiosity with clustering. The success of our approach is in part due to a semi-analytical integration method that allows to compute needed point-to-patch form-factor like transport coefficients efficiently and accurately where other methods fail. Our experiments show that high-quality renderings of translucent objects consisting of tens of thousands of polygons can be obtained from scratch in fractions of a second. An incremental update algorithm further speeds up rendering after material or geometry changes.Item Information-Theoretic Oracle Based on Kernel Smoothness for Hierarchical Radiosity(Eurographics Association, 2002) Feixas, Miquel; Rigau, Jaume; Bekaert, Philippe; Sbert, MateuOne of the main problems in the radiosity method is how to discretise the surfaces of a scene into mesh elements that allow us to accurately represent illumination. In this paper we present a robust information-theoretic refinement criterion (oracle) based on kernel smoothness for hierarchical radiosity. This oracle improves on previous ones in that at equal cost it gives a better discretisation, approaching the optimal one from an information theory point of view, and also needs less visibility computations for a similar image quality.Item Deblurring by Matching(The Eurographics Association and Blackwell Publishing Ltd, 2009) Ancuti, Cosmin; Ancuti, Codruta Orniana; Bekaert, PhilippeRestoration of the photographs damaged by the camera shake is a challenging task that manifested increasing attention in the recent period. Despite of the important progress of the blind deconvolution techniques, due to the ill-posed nature of the problem, the finest details of the kernel blur cannot be recovered entirely. Moreover, the additional constraints and prior assumptions make these approaches to be relative limited.In this paper we introduce a novel technique that removes the undesired blur artifacts from photographs taken by hand-held digital cameras. Our approach is based on the observation that in general several consecutive photographs taken by the users share image regions that project the same scene content. Therefore, we took advantage of additional sharp photographs of the same scene. Based on several invariant local feature points, filtered from the given blurred/non-blurred images, our approach matches the keypoints and estimates the blur kernel using additional statistical constraints.We also present a simple deconvolution technique that preserves edges while minimizing the ringing artifacts in the restored latent image. The experimental results prove that our technique is able to infer accurately the blur kernel while reducing significantly the artifacts of the spoilt images.Item Interactive Rendering of Translucent Objects?(Blackwell Publishers, Inc and the Eurographics Association, 2003) Lensch, Hendrik P.A.; Goesele, Michael; Bekaert, Philippe; Kautz, Jan; Magnor, Marcus A. and Lang, Jochen and Seidel, Hans-PeterThis paper presents a rendering method for translucent objects, in which viewpoint and illumination can be modified at interactive rates. In a preprocessing step, the impulse response to incoming light impinging at each surface point is computed and stored in two different ways: The local effect on close-by surface points is modeled as a per-texel filter kernel that is applied to a texture map representing the incident illumination. The global response (i.e. light shining through the object) is stored as vertex-to-vertex throughput factors for the triangle mesh of the object. During rendering, the illumination map for the object is computed according to the current lighting situation and then filtered by the precomputed kernels. The illumination map is also used to derive the incident illumination on the vertices which is distributed via the vertex-to-vertex throughput factors to the other vertices. The final image is obtained by combining the local and global response. We demonstrate the performance of our method for several models.ACM CSS:I.3.7 Computer Graphics-Three-Dimensional Graphics and Realism Color RadiosityItem Convolution Shadow Maps(The Eurographics Association, 2007) Annen, Thomas; Mertens, Tom; Bekaert, Philippe; Seidel, Hans-Peter; Kautz, Jan; Jan Kautz and Sumanta PattanaikWe present Convolution Shadow Maps, a novel shadow representation that affords efficient arbitrary linear filtering of shadows. Traditional shadow mapping is inherently non-linear w.r.t. the stored depth values, due to the binary shadow test. We linearize the problem by approximating shadow test as a weighted summation of basis terms. We demonstrate the usefulness of this representation, and show that hardware-accelerated anti-aliasing techniques, such as tri-linear filtering, can be applied naturally to Convolution Shadow Maps. Our approach can be implemented very efficiently in current generation graphics hardware, and offers real-time frame rates.Item A Self-Shadow Algorithm for Dynamic Hair using Density Clustering(The Eurographics Association, 2004) Mertens, Tom; Kautz, Jan; Bekaert, Philippe; Reeth, Frank Van; Alexander Keller and Henrik Wann JensenSelf-shadowing is an important factor in the appearance of hair and fur. In this paper we present a new rendering algorithm to accurately compute shadowed hair at interactive rates using graphics hardware. No constraint is imposed on the hair style, and its geometry can be dynamic. Similar to previously presented methods, a 1D visibility function is constructed for each line of sight of the light source view. Our approach differs from other work by treating the hair geometry as a 3D density field, which is sampled on the fly using simple rasterization. The rasterized fragments are clustered, effectively estimating the density of hair along a ray. Based hereon, the visibility function is constructed. We show that realistic selfshadowing of thousands of individual dynamic hair strands can be rendered at interactive rates using consumer graphics hardware.Item Texture Transfer Using Geometry Correlation(The Eurographics Association, 2006) Mertens, Tom; Kautz, Jan; Chen, Jiawen; Bekaert, Philippe; Durand, Frédo; Tomas Akenine-Moeller and Wolfgang HeidrichTexture variation on real-world objects often correlates with underlying geometric characteristics and creates a visually rich appearance. We present a technique to transfer such geometry-dependent texture variation from an example textured model to new geometry in a visually consistent way. It captures the correlation between a set of geometric features, such as curvature, and the observed diffuse texture. We perform dimensionality reduction on the overcomplete feature set which yields a compact guidance field that is used to drive a spatially varying texture synthesis model. In addition, we introduce a method to enrich the guidance field when the target geometry strongly differs from the example. Our method transfers elaborate texture variation that follows geometric features, which gives 3D models a compelling photorealistic appearance.Item Self-Similarity-Based Compression of Point Clouds, with Application to Ray Tracing(The Eurographics Association, 2007) Hubo, Erik; Mertens, Tom; Haber, Tom; Bekaert, Philippe; M. Botsch and R. Pajarola and B. Chen and M. ZwickerMany real-world, scanned surfaces contain repetitive structures, like bumps, ridges, creases, and so on.We present a compression technique that exploits self-similarity within a point-sampled surface. Our method replaces similar surface patches with an instance of a representative patch. We use a concise shape descriptor to identify and cluster similar patches. Decoding is achieved through simple instancing of the representative patches. Encoding is efficient, and can be applied to large datasets consisting of millions of points. Moreover, our technique offers random access to the compressed data, making it applicable to ray tracing, and easily allows for storing additional point attributes, like normals.