Volume 18 (1999)

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Fast Polyhedral Cell Sorting for Interactive Rendering of Unstructured Grids
(Blackwell Publishers Ltd and the Eurographics Association, 1999) Comba, Joao; Klosowsk, James T.; Max, Nelson; Mitchell, Joseph S. B.; Silva, Claudio T.; Williams, Peter L.
Direct volume rendering based on projective methods works by projecting, in visibility order, the polyhedral cells of a mesh onto the image plane, and incrementally compositing the cell's color and opacity into the final image. Crucial to this method is the computation of a visibility ordering of the cells. If the mesh is "well-behaved" (acyclic and convex), then the MPVO method of Williams provides a very fast sorting algorithm; however, this method only computes an approximate ordering in general datasets, resulting in visual artifacts when rendered. A recent method of Silva et al. removed the assumption that the mesh is convex, by means of a sweep algorithm used in conjunction with the MPVO method; their algorithm is substantially faster than previous exact methods for general meshes.In this paper we propose a new technique, which we call BSP-XMPVO, which is based on a fast and simple way of using binary space partitions on the boundary elements of the mesh to augment the ordering produced by MPVO. Our results are shown to be orders of magnitude better than previous exact methods of sorting cells.
Hermitian B-Splines
(Blackwell Publishers Ltd and the Eurographics Association, 1999) Grisoni, Laurent; Blanc, Carole; Schlick, Christophe
This paper proposes to study a spline model, called HB-splines, that is in fact a B-spline representation of Hermite splines, combined with some restriction on the differential values at segment boundaries. Although this model does not appear able to offer something new to the computer graphics community, we think that HB-splines deserve to be considered for themselves because they embed many interesting features. First, they include all the classical properties required in a geometric modeling environment (convex hull, local control, arbitrary orders of parametric or geometric continuity). Second, they have a nice aptitude for direct manipulation (i.e. manipulation without using control points). For this purpose, we propose a new graphic widget, called control sails, that offers the user an intuitive way to specify local properties (position, tangent, curvature) of a curve or a surface. Finally, they provide an elegant formulation of a biorthogonal wavelet family, that permits multiresolution manipulations of the resulting curves or surfaces, in a very efficient way.
A Free Form Feature Taxonomy
(Blackwell Publishers Ltd and the Eurographics Association, 1999) Fontana, M.; Giannini, F.; Meirana, M.
In this paper the notion of free form feature for aesthetic design is presented. The design of industrial products constituted by free form surfaces is done by using CAD systems representing curves and surfaces by means of NURBS functions, which are usually defined by low level entities that are not intuitive and require some knowledge of the mathematical language. Similarly to the feature-based approach adopted by CAD systems for classical mechanical design, a set of high level modelling entities which provides commonly performed shape modifications has been identified. Particularly, the paper suggests a classification of the so-called detail features for an aesthetic and/or functional characterization of predefined free form surfaces. Feature types are formally described by means of an analytical definition of the surface modification through deformation and elimination laws. A topological classification is then given according to the application domain of such laws. A further sub-classification of morphological types is then suggested according to geometric properties of weak convexity and concavity for the resulting modified shape, leading to a taxonomy of simple free form features meaningful for aesthetic design.
An Efficient and Flexible Perception Pipeline for Autonomous Agents
(Blackwell Publishers Ltd and the Eurographics Association, 1999) Bordeux, Christophe; Boulic, Ronan; Thalmann, Daniel
Agents in virtual environments require a combination of perception and action to behave in an autonomous way. We extend a software architecture for the management of actions blending, called AGENTlib, with a perception mechanism. The perception system provides a uniform interface to various techniques in the field of virtual perception, including synthetic vision, database access and perception persistency. We describe the framework we designed to efficiently filter valuable information from the scene and we address concerns about computation redundancy and data propagation through multiple filtering modules.
Nonphotorealistic Rendering by Q-mapping
(Blackwell Publishers Ltd and the Eurographics Association, 1999) Hall, P.
We present Q-mapping which is a technique for rendering three-dimensional objects using nonphotorealistic cues, by applying Q-maps. Q-maps are three-dimensional textures that make marks on objects, and thus provide visual cues for shape, shade, and texture. Q-maps adapt to light intensity, typically by making more marks in darker areas. Q-maps can produce images with a very wide range of visual styles (e.g. half tone shading, and pen-and-ink colour wash). The primary contribution is that these styles reside in a single parametric space. Importantly this space includes photorealism as a style, which is therefore regarded as a special case of nonphotorealistic image rendering in general. We illustrate our explanation of Q-mapping using examples from scientific visualisation and computer graphics - and provide a gallery of images to show the versatility of the approach.
An Informed Environment Dedicated to the Simulation of Virtual Humans in Urban Context
(Blackwell Publishers Ltd and the Eurographics Association, 1999) Farenc, Nathalie; Boulic, Ronan; Thalmann, Daniel
In this paper, we outline the creation of an Informed Environment, dedicated to urban life simulation.We propose methods and tools for creating and providing the information necessary for animating virtual humans in a city using an Informed Environment. The Informed Environment is based on a hierarchical decomposition of a urban scene into Environment Entities providing geometrical information as well as semantic notions, thus allowing a more realistic simulation of human behaviour. In this manner, virtual humans can integrate with a certain kind of urban knowledge.
Stochastic Iteration for Non-diffuse Global Illumination
(Blackwell Publishers Ltd and the Eurographics Association, 1999) Szirmay-Kalos, Laszlo
This paper presents a single-pass, view-dependent method to solve the rendering equation, using a stochastic iterational scheme where the transport operator is selected randomly in each iteration. The requirements of convergence are given for the general case. To demonstrate the basic idea, a very simple,continuous random transport operator is examined, which gives back the light tracing algorithm incorporating Russian roulette. Then, a new mixed continuous and finite-element based iteration method is proposed, which uses ray-bundles to transfer the radiance in a single random direction. The resulting algorithm is fast, it provides initial results in seconds and accurate solutions in minutes and does not suffer from the error accumulation problem and the high memory demand of other finite-element and hierarchical approaches.
Modelling and Rendering Graphics Scenes Composed of Multiple Volumetric Datasets
(Blackwell Publishers Ltd and the Eurographics Association, 1999) Leu, Adrian; Chen, Min
This paper presents a method for modelling graphics scenes consisting of multiple volumetric objects. A two-level hierarchical representation is employed, which enables the reduction of the overall storage consumption as well as rendering time. With this approach, different objects can be derived from the same volumetric dataset, and 2D images can be trivially integrated into a scene. The paper also describes an efficient algorithm for rendering such scenes on ordinary workstations, and addresses issues concerning memory requirements and disk swapping.
Virtual Dunhuang Art Cave: A Cave within a CAVE
(Blackwell Publishers Ltd and the Eurographics Association, 1999) Lutz, B.; Weintke, M.
Virtual Reality can present historical places in a three-dimensional and interactive way, giving visitors a photorealistic impression of objects. Not only existing scenarios can be shown, but VR can also be used to rebuild scenarios that were damaged or destroyed a long time ago, giving new life to the cultural heritage. We used Virtual Reality to present the Mogao Grottos in Dunhuang. This cave site is one of the most important cultural and religious places by the ancient Silk Road. The presentation is to give visitors the impression of visiting the cave site and provide information about the caves, paintings and statues in an interesting way. To achieve this, we developed a new, intuitive interaction paradigm, which enables the user to explore the caves. To give observers a photorealistic impression of the caves and to create a feeling of immersion, innovative rendering techniques were integrated.The resulting presentation combines Virtual Reality and archaeology to give tourists a realistic experience of this cave site and to support scientists in their research work.
Improved Laplacian Smoothing of Noisy Surface Meshes
(Blackwell Publishers Ltd and the Eurographics Association, 1999) Vollmer, J.; Mencl, R.; Muller, H.
This paper presents a technique for smoothing polygonal surface meshes that avoids the well-known problem of deformation and shrinkage caused by many smoothing methods, like e.g. the Laplacian algorithm. The basic idea is to push the vertices of the smoothed mesh back towards their previous locations. This technique can be also used in order to smooth unstructured point sets, by reconstructing a surface mesh to which the smoothing technique is applied. The key observation is that a surface mesh which is not necessarily topologically correct, but which can efficiently be reconstructed, is sufficient for that purpose.
Texturing 3D Models of Real World Objects from Multiple Unregistered Photographic Views
(Blackwell Publishers Ltd and the Eurographics Association, 1999) Neugebauer, Peter J.; Klein, Konrad
As the efficiency of computer graphic rendering methods is increasing, generating realistic models is now becoming a limiting factor. In this paper we present a new technique to enhance already existing geometry models of real world objects with textures reconstructed from a sparse set of unregistered still photographs. The aim of the proposed technique is the generation of nearly photo-realistic models of arbitrarily shaped objects with minimal effort. In our approach, we require neither a prior calibration of the camera nor a high precision of the user's interaction. Two main problems have to be addressed of which the first is the recovery of the unknown positions and parameters of the camera. An initial estimate of the orientation is calculated from interactively selected point correspondences. Subsequently, the unknown parameters are accurately calculated by minimising a blend of objective functions in a 3D-2D projective registration approach. The key point of the proposed method of registration is a novel filtering approach which utilises the spatial information provided by the geometry model. Second, the individual images have to be combined yielding a set of consistent texture maps. We present a robust method to recover the texture from the photographs thereby preserving high spatial frequencies and eliminating artifacts, particularly specular highlights. Parts of the object not seen in any of the photographs are interpolated in the textured model. Results are shown for three complex example objects with different materials and numerous self-occlusions.
Modeling and Rendering Escher-Like Impossible Scenes
(Blackwell Publishers Ltd and the Eurographics Association, 1999) Savransky, Guillermo; Dimerman, Dan; Gotsman, Craig
Inspired by the drawings of "impossible" objects by artists such as M.C. Escher, we describe a mathematical theory which captures some of the underlying principles of their work. Using this theory, we show how impossible three-dimensional scenes may be modeled and rendered synthetically.
Wavelet-Based 3D Compression Scheme for Interactive Visualization of Very Large Volume Data
(Blackwell Publishers Ltd and the Eurographics Association, 1999) Ihm, Insung; Park, Sanghun
Interactive visualization of very large volume data has been recognized as a task requiring great effort in a variety of science and engineering fields. In particular, such data usually places considerable demands on run-time memory space. In this paper, we present an effective 3D compression scheme for interactive visualization of very large volume data, that exploits the power of wavelet theory. In designing our method, we have compromised between two important factors: high compression ratio and fast run-time random access ability. Our experimental results on the Visual Human data sets show that our method achieves fairly good compression ratios. In addition, it minimizes the overhead caused during run-time reconstruction of voxel values. This 3D compression scheme will be useful in developing many interactive visualization systems for huge volume data, especially when they are based on personal computers or workstations with limited memory.
Seamless Integration of Databases in VR for Constructing Virtual Environments
(Blackwell Publishers Ltd and the Eurographics Association, 1999) Soetebier, Ingo; Dorner, Ralf; Braun, Norbert
An approach for authoring virtual environments within the virtual environments themselves is presented, integrating a database containing arbitrary components that are used to construct the 3D scene. The issues important to a seamless integration of the database, such as multimedia data storage and database linkage are discussed, with the focus on the human-computer interaction component. A concept for a 3D database interface is described for query, presentation and usage of query results in the virtual environment. Finally, an implementation of the concept using VRML and Java is presented.
Book Reviews
(Blackwell Publishers Ltd and the Eurographics Association, 1999)
Books reviewed:Arlov, Laura. GUI Design for DummiesLeopoldseder, Hannes and Schopf, Christine (Editors). Cyberarts98Lorentz, H. A. On the Theory of the Reflection and Refraction of LightVince, John. Essential Virtual Reality FastJolion, J.-M. and Kropatsch, W. G. Graph-Based Representations in Pattern RecognitionKokaram, Anil C. Motion Picture Restoration: Digital Algorithms for Artefact Suppression in Degraded Motion Picture Film and Video
Event Reports
(Blackwell Publishers Ltd and the Eurographics Association, 1999)
Second Eurographics Workshop on Parallel Graphics and Visualization, Rennes, France September 24-25, 1998
Generalized View-Dependent Simplification
(Blackwell Publishers Ltd and the Eurographics Association, 1999) El-Sana, Jihad; Varshney, Amitabh
We propose a technique for performing view-dependent geometry and topology simplifications for level-of-detail-based renderings of large models. The algorithm proceeds by preprocessing the input dataset into a binary tree, the view-dependence tree of general vertex-pair collapses. A subset of the Delaunay edges is used to limit the number of vertex pairs considered for topology simplification. Dependencies to avoid mesh foldovers in manifold regions of the input object are stored in the view-dependence tree in an implicit fashion. We have observed that this not only reduces the space requirements by a factor of two, it also highly localizes the memory accesses at run time. The view-dependence tree is used at run time to generate the triangles for display. We also propose a cubic-spline-based distance metric that can be used to unify the geometry and topology simplifications by considering the vertex positions and normals in an integrated manner.
Multiresolution Curve and Surface Representation: Reversing Subdivision Rules by Least-Squares Data Fitting
(Blackwell Publishers Ltd and the Eurographics Association, 1999) Samavati, Faramarz F.; Bartels, Richard H.
This work explores how three techniques for defining and representing curves and surfaces can be related efficiently. The techniques are subdivision, least-squares data fitting, and wavelets. We show how least-squares data fitting can be used to "reverse" a subdivision rule, how this reversal is related to wavelets, how this relationship can provide a multilevel representation, and how the decomposition/reconstruction process can be carried out in linear time and space through the use of a matrix factorization.Some insights that this work brings forth are that the inner product used in a multiresolution analysis in uences the support of a wavelet, that wavelets can be constructed by straightforward matrix observations, and that matrix partitioning and factorization can provide alternatives to inverses or duals for building efficient decomposition and reconstruction processes. We illustrate our findings using an example curve, grey-scale image, and tensor-product surface.
A Progressive Algorithm for Three Point Transport
(Blackwell Publishers Ltd and the Eurographics Association, 1999) Dumont, Reynald; Bouatouch, Kadi; Gosselin, Philippe
When computing global illumination in environments made up of surfaces with general Bidirectional Reflection Distribution Functions, a three point formulation of the rendering equation can be used. Brute-force algorithms can lead to a linear system of equations whose matrix is cubic, which is expensive in time and space. The hierarchical approach is more efficient. Aupperle et al. proposed a hierarchical three point algorithm to compute global illumination in the presence of glossy reflection. We present in this paper some improvements we brought to this method: shooting, "lazy" push-pull, photometric subdivision criterion, etc. Then we will show how our new method takes into account non-planar surfaces in the hierarchical resolution process.
A Practical Analysis of Clustering Strategies for Hierarchical Radiosity (Supplementary material)
(Blackwell Publishers Ltd and the Eurographics Association, 1999) Hasenfratz, Jean-Marc; Damez, Cyrille; Sillion, Francois; Drettakis, George

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