Volume 19 (2000)

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Physically-Based Patination for Underground Objects
(Blackwell Publishers Ltd and the Eurographics Association, 2000) Chang, Yao-Xun; Shih, Zen-Chung
Although current photorealistic rendering techniques can produce very impressive images, the rendered objects are often too clean and shiny. Thus, the resulting images look unnatural. This paper proposes a physically-based model to simulate the appearance of patinas on ancient Chinese bronzes. Buried in the soil for thousands of years, many patinas are found on the surface of ancient bronzes as a result of the aging process and the physical and chemical conditions of the soil environment. The development of patinas is modulated herein by L-systems according to tendencies based on the environmental factors and object geometry. The tendencies are employed to represent the accumulative effect of all factors on patination. The proposed model can be extended to simulate a variety of metallic patinas including the ancient Chinese bronzes discovered at San-hsing-tui, Sichuan, China.
Motion Balance Filtering
(Blackwell Publishers Ltd and the Eurographics Association, 2000) Tak, Seyoon; Song, Oh-young; Ko, Hyeong-Seok
This paper presents a new technique called motion balance filtering, which corrects an unbalanced motion to a balanced one while preserving the original motion characteristics as much as possible. Differently from previous approaches that deal only with the balance of static posture, we solve the problem of balancing a dynamic motion. We achieve dynamic balance by analyzing and controlling the trajectory of the zero moment point (ZMP). Our algorithm consists of three steps. First, it analyzes the ZMP trajectory to find out the duration in which dynamic balance is violated. Dynamic imbalance is identified by the ZMP trajectory segments lying out of the supporting area. Next, the algorithm modifies the ZMP trajectory by projecting it into the supporting area. Finally, it generates the balanced motion that satisfies the new ZMP constraint. This process is formulated as a constrained optimization problem so that the new motion resembles the original motion as much as possible. Experiments prove that our motion balance filtering algorithm is a useful method to add physical realism to a kinematically edited motion.
Automatic Camera Placement for Image-Based Modeling
(Blackwell Publishers Ltd and the Eurographics Association, 2000) Fleishman, Shachar; Cohen-Or, Daniel; Lischinski, Dani
We present an automatic camera placement method for generating image-based models from scenes with known geometry. Our method first approximately determines the set of surfaces visible from a given viewing area and then selects a small set of appropriate camera positions to sample the scene from. We define a quality measure for a surface as seen, or covered, from the given viewing area. Along with each camera position, we store the set of surfaces which are best covered by this camera. Next, one reference view is generated from each camera position by rendering the scene. Pixels in each reference view that do not belong to the selected set of polygons are masked out.The image-based model generated by our method, covers every visible surface only once, associating it with a camera position from which it is covered with quality that exceeds a user-specified quality threshold. The result is a compact non-redundant image-based model with controlled quality.The problem of covering every visible surface with a minimum number of cameras (guards) can be regarded as an extension to the well-known Art Gallery Problem. However, since the 3D polygonal model is textured, the camera-polygon visibility relation is not binary; instead, it has a weight - the quality of the polygon's coverage.
Efficient Algorithms for Computing Conservative Portal Visibility Information
(Blackwell Publishers Ltd and the Eurographics Association, 2000) Jimenez, W. F. H.; Esperanca, C.; Oliveira, A. A. F.
The number of polygons in realistic architectural models is many more than can be rendered at interactive frame rates. Typically, however, due to occlusion by opaque surfaces (e.g., walls), only small fractions of such models are visible from most viewpoints. This fact is used in many popular methods for preprocessing visibility information which assume a scene model subdivided into convex cells connected through convex portals. These methods need to establish which cells or parts thereof are visible to a generalized observer located within each cell. The geometry of this information is termed a 'visibility volume' and its computation is usually quite complex. Conservative approximations of viewing volumes, however, are simpler and less expensive to compute. In this paper we present techniques and algorithms which permit the computation of conservative viewing volumes incrementally. In particular, we describe an algorithm for computing the viewing volumes for a given cell through a sequence of 'm' portals containing a total of 'n' edges in Omn time.
An Advanced Color Representation for Lossy Compression of CMYK Prepress Images
(Blackwell Publishers Ltd and the Eurographics Association, 2000) De Neve, P.; Denecker, K.; Philips, W.; Lemahieu, I.
CMYK color images are used extensively in prepress applications. When compressing those color images one has to deal with four different color channels. Usually compression algorithms only take into account the spatial redundancy that is present in the image data. This approach does not yield an optimal data reduction since there also exists a high correlation between the different colors in natural images.This paper shows that a significant gain in data reduction can be achieved by exploiting this color redundancy. Some popular transform coders, including DCT-based JPEG and the SPIHT wavelet coder, were used for reducing the spatial redundancy. The performance of the algorithms was evaluated using a quality criterion based on human perception like the mean CIEL*a*b*?E error.
Realistic Collision Avoidance of Upper Limbs Based on Neuroscience Models
(Blackwell Publishers Ltd and the Eurographics Association, 2000) Nebel, Jean-Christophe
Dynamic Polygon Visibility Ordering for Head-Slaved Viewing in Virtual Environments
(Blackwell Publishers Ltd and the Eurographics Association, 2000) Sadagic, Amela; Slater, Mel
This paper presents an approach to visibility called the Viewpoint Movement Space (VpMS) algorithm which supports the concept of dynamic polygon visibility orderings for head-slaved viewing in virtual environments (VE). The central idea of the approach is that the visibility, in terms of back-to-front polygon visibility ordering, does not change dramatically as the viewpoint moves. Moreover, it is possible to construct a partition of the space into cells, where for each cell the ordering is invariant. As the viewpoint moves across a cell boundary typically only a small and predictable change is made to the visibility ordering. The cost to perform this operation represents a notable reduction when compared with the cost of resolving the visibility information from the BSP tree where the classification of the viewpoint with every node plane has to be performed. The paper demonstrates how the subdivision into such cells can represent the basic source for an acceleration of the rendering process. We also discuss how the same supportive data structure can be exploited to solve other tasks in the graphics pipeline.
Morphing the BlobTree
(Blackwell Publishers Ltd and the Eurographics Association, 2000) Galin, Eric; Leclercq, Antoine; Akkouche, Samir
Implicit surfaces have proved to be a particularly well suited and efficient model for animating and morphing shapes of arbitrary topologies. The BlobTree model is characterized as a hierarchical combination of skeletal primitives organized in a tree. The nodes hold blending, boolean and warping operators, which allows the design of complex objects.In this paper, we address the metamorphosis of the BlobTree. This appears a difficult task as the tree data-structures of the initial and final shapes are completely different in the general case, and consequently cannot be matched easily. We propose an original technique that solves the correspondence process and creates an intermediate generic BlobTree model whose instances interpolate the initial and final shapes.The animator may control the correspondence between features and can specify both the speed of transformation and the trajectory of the nodes and the leaves of the generic BlobTree model. This provides the end user with a tight control over the transformation so as to achieve good visual effects.
An Adaptive Spectral Rendering with a Perceptual Control
(Blackwell Publishers Ltd and the Eurographics Association, 2000) Claude Iehl, Jean; Peroche, Bernard
In this paper, we present a spectral rendering method based on a ray tracing algorithm and guided by a perceptual control of the error made. An adaptive representation of spectral data for light sources and materials is used and induces, for each pixel, the evaluation of an algebraic expression. For each visible wavelength, the computations of complex spread in refractive and dispersive materials, based on several photon maps, are locally restricted by an adaptive evaluation of the expression. This method allows to simulate high quality physically-based pictures and, in particular, some specific phenomena like dispersion in transparent objects, scattered caustics,. . .
Editorial
(Blackwell Publishers Ltd and the Eurographics Association, 2000) Coquillart, Sabine; Duke, David
A Hybrid Approach for Stroke-Based Letterform Composition Including Outline-Based Methods
(Blackwell Publishers Ltd and the Eurographics Association, 2000) Schneider, Uwe
For three decades a number of computer-aided systems have been developed in order to assist in the design of digital type. Even though some of them are used by typographers in commercial type design, they are not yet widely accepted. One reason is the lack of appropriate design metaphors in systems which provide low-level operations (e.g. the manipulation of outlines). Another reason is the lack of essential functionality in high-level approaches (not all characters can be modeled). While these two reasons correspond with the underlying paradigms of those systems, namely the outline and the stroke approach, the presented model provides a synthesis of both. By exploring the high-level semantics of the stroke-based paradigm, letterforms can be composed by individual strokes. Properties like round corners at stroke intersections, as they typically appear in the design of Western type, can be modeled via outline segments attached to the associated stroke elements. As a consequence, Latin characters as well as characters incorporating hand-written characters, like Kanji, can be expressed using a single model. These two classes of types are considered by the typographic community to be fundamentaly different.
Perceptual Principles and Computer Graphics
(Blackwell Publishers Ltd and the Eurographics Association, 2000) May, Jon
Now that technology allows us to present photorealistic animations of scenically lit objects acting in real-time, the problem of computer graphics has changed from making displays recognisable, to ensuring that users notice what they are intended to see, without being distracted by irrelevant information. Worse than that, the use of veridical displays that are intended to be lifelike runs the risk of introducing unpredictable sources of information, that can lead users to infer all sorts of unwanted details. Traditional visual theory, based upon bottom-up models of feature extraction from the retinal image, cannot inform us about these aspects of perception. Broader based cognitive theories are required that integrate visual perception with attention, memory, emotion and inference. Theories such as Barnard's Interacting Cognitive Subsystems enable phenomena such as change blindness and the craft principles of film editing to be interpreted within a common framework, supporting extrapolation to computer graphics.
Computation of Irradiance from Triangles by Adaptive Sampling
(Blackwell Publishers Ltd and the Eurographics Association, 2000) Urena, C.
We introduce an algorithm for sample positioning in a planar triangle, which can be used to make numerical integration of an arbitrary function defined on it. This algorithm has some interesting properties which make it suitable for applications in the context of realistic rendering. We use an adaptive triangle partitioning procedure, driven by an appropriate measure of the error. The underlying variance is shown to be bounded, and in fact it can be controlled, so that it approaches the minimum possible value. We show results obtained when applying the method to irradiance computation, in the context of final-gather algorithms. We also describe a C++ class which offers all required functionality, and we made available its source code.
Direct Manipulation and Interactive Sculpting of PDE Surfaces
(Blackwell Publishers Ltd and the Eurographics Association, 2000) Du, Haixia; Qin, Hong
This paper presents an integrated approach and a unified algorithm that combine the benefits of PDE surfaces and powerful physics-based modeling techniques within one single modeling framework, in order to realize the full potential of PDE surfaces. We have developed a novel system that allows direct manipulation and interactive sculpting of PDE surfaces at arbitrary location, hence supporting various interactive techniques beyond the conventional boundary control. Our prototype software affords users to interactively modify point, normal, curvature, and arbitrary region of PDE surfaces in a predictable way. We employ several simple, yet effective numerical techniques including the finite-difference discretization of the PDE surface, the multigrid-like subdivision on the PDE surface, the mass-spring approximation of the elastic PDE surface, etc. to achieve real-time performance. In addition, our dynamic PDE surfaces can also be approximated using standard bivariate B-spline finite elements, which can subsequently be sculpted and deformed directly in real-time subject to intrinsic PDE constraints. Our experiments demonstrate many attractive advantages of our dynamic PDE formulation such as intuitive control, real-time feedback, and usability to the general public.
Wavelet Radiative Transfer and Surface Interaction
(Blackwell Publishers Ltd and the Eurographics Association, 2000) Lewis, Robert R.; Fournier, Alain
Recently, there has been considerable interest in the representation of radiance in terms of wavelet basis functions. We will present a coordinate system called Nusselt coordinates which, when combined with wavelets, considerably simplifies computation of radiative transport and surface interaction. It also provides straightforward computation of the physical quantities involved.We show how to construct a discrete representation of the radiative transport operator ? involving inner products of smoothing functions, discuss the possible numerical integration techniques, and present an application. We also show how surface interaction can be represented as a kind of matrix product of the wavelet projections of an incident radiance and a bidirectional reflectance distribution function (BRDF).
Graceful Degradation of Collision Handling in Physically Based Animation
(Blackwell Publishers Ltd and the Eurographics Association, 2000) Dingliana, John; O'Sullivan, Carol
Interactive simulation is made possible in many applications by simplifying or culling the finer details that would make real-time performance impossible. This paper examines detail simplification in the specific problem of collision handling for rigid body animation. We present an automated method for calculating consistent collision response at different levels of detail. The mechanism works closely with a system which uses a pre-computed hierarchical volume model for collision detection.
Multiresolution Shape Deformations for Meshes with Dynamic Vertex Connectivity
(Blackwell Publishers Ltd and the Eurographics Association, 2000) Kobbelt, Leif P.; Bareuther, Thilo; Seidel, Hans-Peter
Multiresolution shape representation is a very effective way to decompose surface geometry into several levels of detail. Geometric modeling with such representations enables flexible modifications of the global shape while preserving the detail information. Many schemes for modeling with multiresolution decompositions based on splines, polygonal meshes and subdivision surfaces have been proposed recently. In this paper we modify the classical concept of multiresolution representation by no longer requiring a global hierarchical structure that links the different levels of detail. Instead we represent the detail information implicitly by the geometric difference between independent meshes. The detail function is evaluated by shooting rays in normal direction from one surface to the other without assuming a consistent tesselation. In the context of multiresolution shape deformation, we propose a dynamic mesh representation which adapts the connectivity during the modification in order to maintain a prescribed mesh quality. Combining the two techniques leads to an efficient mechanism which enables extreme deformations of the global shape while preventing the mesh from degenerating. During the deformation, the detail is reconstructed in a natural and robust way. The key to the intuitive detail preservation is a transformation map which associates points on the original and the modified geometry with minimum distortion. We show several examples which demonstrate the effectiveness and robustness of our approach including the editing of multiresolution models and models with texture.
Virtual Dunhuang Mural Restoration System in Collaborative Network Environment
(Blackwell Publishers Ltd and the Eurographics Association, 2000) Li, Xiangyang; Lu, Dongming; Pan, Yunhe
This paper introduces a virtual Dunhuang mural restoration system in collaborative network environment. It describes the style of Dunhuang mural, analyzes the reasons of mural spoilage, and presents the necessity to develop a collaborative mural restoration GroupWare. It describes the components and the workflow of mural restoration in detail, solves some key technologies in the system. In the end, it introduces the system architecture, and presents the system interface and some restored results.
Efficient Glossy Global Illumination with Interactive Viewing
(Blackwell Publishers Ltd and the Eurographics Association, 2000) Stamminger, Marc; Scheel, Annette; Granier, Xavier; Perez-Cazorla, Frederic; Drettakis, George; Sillion, Francois
The ability to perform interactive walkthroughs of global illumination solutions including glossy effects is a challenging open problem. In this paper we overcome certain limitations of previous approaches. We first introduce a novel, memory- and compute-efficient representation of incoming illumination, in the context of a hierarchical radiance clustering algorithm. We then represent outgoing radiance with an adaptive hierarchical basis, in a manner suitable for interactive display. Using appropriate refinement and display strategies, we achieve walkthroughs of glossy solutions at interactive rates for non-trivial scenes. In addition, our implementation has been developed to be portable and easily adaptable as an extension to existing, diffuse-only, hierarchical radiosity systems. We present results of the implementation of glossy global illumination in two independent global illumination systems.
Using an Intermediate Skeleton and Inverse Kinematics for Motion Retargeting
(Blackwell Publishers Ltd and the Eurographics Association, 2000) Monzani, Jean-Sebastien; Baerlocher, Paolo; Boulic, Ronan; Thalmann, Daniel
In this paper, we present a new method for solving the Motion Retargeting Problem, by using an intermediate skeleton. This allows us to convert movements between hierarchically and geometrically different characters. An Inverse Kinematics engine is then used to enforce Cartesian constraints while staying as close as possible to the captured motion.

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