EG1994

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A Median Cut Algorithm for Efficient Sampling of Radiosity Functions
(Blackwell Science Ltd and the Eurographics Association, 1994) Feda, Martin; Purgathofer, Werner
This paper presents an efficient method for sampling the illumination functions in higher order radiosity algorithms. In such algorithms, the illumination function is not assumed to be constant across each patch, but it is approximated by a function which is at least C1 continuous. Our median cut sampling algorithm is inspired by the observation that many form factors are computed at higher precision than is necessary. While a high sampling rate is necessary in regions of high illumination, dark areas can be sampled at a much lower rate to compute the received radiosity within a given precision. We adaptively subdivide the emitter into regions of approximately equal influence on the result. Form factors are evaluated by the disk approximation and a ray tracing based test for occlusion detection. The implementation of a higher order radiosity system using B-splines as radiosity function is described. The median cut algorithm can also be used for radiosity algorithms based on the constant radiosity assumption.
Fast Dynamic Tessellation of Trimmed NURBS Surfaced1
(Blackwell Science Ltd and the Eurographics Association, 1994) Abi-Ezzi, Salim S.; Subramaniam, Srikanth
Trimmed NURBS (non-uniform rational B-splines) surfaces are being increasingly used and standardized in geometric modeling applications. Fast graphical processing of trimmed NURBS at interactive speeds is absolutely essential to enable these applications. which poses some unique challenges in software, hardware, and algorithm design. This paper presents a technique that uses graphical compilation to enable fast dynamic tessellation of trimmed NURBS surfaces under highly varying transforms.We use the concept of graphical data compilation. through which we preprocess the NURBS surface into a compact, view-independent form amenable for fast per-frame extraction of triangles. Much of the complexity of processing is absorbed during compilation. Arbitrarily complex trimming regions are broken down into simple regions that are specially designed to facilitate tessellation before rendering. Potentially troublesome cases of degeneracies in the surface are detected and dealt with during compilation. Compilation enables a clean separation of algorithm-intensive and compute-intensive operations, and provides for parallel implementations of the latter. Also, we exercise a classification technique while processing trimming loops. which robustly takes care of geometric ambiguities and deals with special cases while keeping the compilation code simple and concise.
A Robust and Consistent Algorithm for Intersecting Convex Polyhedra
(Blackwell Science Ltd and the Eurographics Association, 1994) Sugihara, Kokichi
This paper presents a numerically robust and topologically consistent algorithm for intersecting convex polyhedra. This algorithm is new in the sense that the consistency issue is completely separated from the numerical error issue. The intersection operation is combinatorially abstracted as the operation of changing the vertex-edge graphs associated with the input polyhedra, and numerical computation is employed only for choosing the branch of processing which is most likely to lead to the correct solution of the problem. Hence, the resultant algorithm is completely free from topological inconsistency.
Real Time Fitting of Hand-Sketched Pressure Brushstrokes
(Blackwell Science Ltd and the Eurographics Association, 1994) Pudet, Thierry
A method is described for fitting the outline of hand-sketched pressure brushstrokes with Bezier curves. It combines the brush-trajectory model, in which a stroke is generated by dragging a brush along a given trajectory, with a fast curve fitting algorithm.The method has been implemented for a vector-based drawing program in which the user draws with a cordless pressure-sensitive stylus on a digitizing tablet. From the trajectory followed by the stylus, its associated pressure data, and a specified brush, a stroke of variable width is computed and displayed in real time.First, the digitized trajectory is fitted, thus removing noise. Then, from polygonal approximations of the fitted trajectory and the brush outline, a polygonal approximation of the stroke outline is computed. Working with polygonal approximations reduces computations to simple geometric operations and greatly simplifies the treatment of dynamic, pressure-controlled brushes. Last, the polygonal approximation of the stroke outline is fitted. The result is a closed piecewise Bezier curve approximating the brushstroke outline to within an arbitrary error tolerance.Several examples of hand-sketched drawings realized with this method are presented.
A New Approach to the Construction of Surfaces from Contour Data
(Blackwell Science Ltd and the Eurographics Association, 1994) Jones, Mark W.; Chen, Min
This paper presents a new approach to the construction of a surface from a stack of contour slices. Unlike most existing methods, this new approach handles ambiguous conditions consistently without employing an algorithm to establish a correspondence between vertices on one contour and those on the next. It is easy to implement and fast to compute, requiring only basic geometric properties, namely closedness and simplicity, to be available with contour data. The advantages of this new approach have also been demonstrated with solutions to a few classical problems from the literature and some practical problems in medical imaging. It can also be applied to geographical surveying and keyframe animations.
Cross Scan Buffer and its Applications
(Blackwell Science Ltd and the Eurographics Association, 1994) Tanaka, Toshimitsu; Takahashi, Tokiichiro
We propose the Cross Scanline Buffer which preserves the result of hidden surface removal as performed by the Cross Scanline Algorithm. The Cross Scan Buffer reduces image re-generation time and eliminates aliasing artifacts even if the image is arbitrarily scaled. Perfect anti-aliasing is achieved because the Cross Scanline Algorithm analytically determines visible polygonal surfaces and divides them into sets of triangles and trapezia. The Cross Scan Buffer supports the various applications that currently use the conventional buffering methods for anti-aliasing. This paper introduces and tests three applications: image scaling, shadow creation, and texture mapping. Experimental results verify that the Cross Scan Buffer is very powerful yet efficient.
An Inexpensive BRDF Model for Physically-based Rendering
(Blackwell Science Ltd and the Eurographics Association, 1994) Schlick, Christophe
A new BRDF model is presented which can be viewed as an kind of intermediary model between empirism and theory. Main results of physics are observed (energy conservation, reciprocity rule, microfacet theory) and numerous phenomena involved in light reflection are accounted for, in a physically plausible way (incoherent and coherent reflection, spectrum modifications, anisotropy, self-shadowing, multiple surface and subsurface reflection, differences between homogeneous and heterogeneous materials). The model has been especially intended for computer graphics applications and therefore includes two main features: simplicity (a small number of intuitively understandable parameters controls the model) and efficiency (the formulation provides adequation to Monte-Carlo rendering techniques and/or hardware implementations).
Reducing The Latency In Head-Mounted Displays By A Novel Prediction Method Using Grey System Theory
(Blackwell Science Ltd and the Eurographics Association, 1994) Wu, Jiann-Rong; Ouhyoung, Ming
In this paper we propose a novel prediction method for the head motion using Grey System theory, where a 6D tracker is attached to an HMD on a user s head in virtual reality applications. Our prediction method using Grey System Model can greatly reduce the latency by at least one half and reduce image jittering. A system latency below 100 ms or even 50 ins can be achieved, even though without prediction the latency is around 200 ms. Using 6 points in prediction with Grey System Model is currently the best in tracker prediction as we tried from 2 points to 10 points.In order to measure the latency, we also propose a way to measure it in an HMD system precisely and conveniently. During the process, we have implemented four different prototypes respectively on a PC486, a SUN SparcStation10, an SGI IndigoR4000, and a high performance computer image generator. The computation complexity of our prediction method is relatively low and therefore real time requirement is easily met.
Raycasting of Nonregularly Structured Volume Data
(Blackwell Science Ltd and the Eurographics Association, 1994) Fruhauf, Thomas
Raycasting is an efficient, but computation-intensive technique for the direct rendering of volumetric data. Nonregularly structured volume data is produced in a variety of applications, such as numerical simulation, ultrasonic inspection, or seismic measurement. Unfortunately, in raycasting nonregulary structured grids, sampling data along the rays is even more computation-intensive. One idea might be to resample the data onto a regular grid for further processing. It is shown, that resampling typical grids results either in extreme sub- sampling of the most interesting areas or in data sets too huge to handle. Additionally, no better hardware will overcome the fact that all topology and context information is lost through resampling. Therefore, dedicated algorithms for the raycasting of nonregularly structured volumes are required.We present a new approach for this task, which is based on sending rays through the computational space of a nonregular grid, instead of conventionally raycasting the physical space. This technique overcomes the difficulties in sampling data along the rays, since the computational space is regular by definition. The paper outlines the mathematical operations of this technique as well as its embedding in a rendering algorithm. We have implemented the new raycasting strategy as a module of our interactive visualization system, providing the users with an additional tool for the efficient exploration of their data.
Spectral Data Modeling for a Lighting Application
(Blackwell Science Ltd and the Eurographics Association, 1994) Deville, Pascal M.; Merzouk, Slimane; Cazier, Dorothee; Paul, Jean C.
In computer graphics, image generation uses the RGB colorimetric system. However, this system does not produce an accurate simulation of the spectral characteristics of both light and material, due to the fact that it is device dependent. Indeed, to get realistic images, the image calculation process must deal with spectral characteristics of lights and materials and with the problem of sampling the wavelength domain. Methods have been shown that are a good way to solve the two problems mentioned above, however, these approaches do not take into account complex spectrums (discontinuous). In this paper, we propose a method which removes the constraints imposed by the current methods. Our method is based upon an algorithm of spectrum analysis.
Magicsphere: an insight tool for 3D data visualization
(Blackwell Science Ltd and the Eurographics Association, 1994) Cignoni, P.; Montani, C.; Scopigno, R.
How to render very complex datasets, and yet maintain interactive response times, is a hot topic in computer graphics. The MagicSphere idea originated as a solution to this problem, but its potential goes much further than this original scope. In fact, it has been designed as a very generical 3D widget: it defines a spherical volume of interest in the dataset modeling space. Then, several filters can be associated with the Magicsphere, which apply different visualization modalities to the data contained in the volume of interest. The visualization of multi-resolution datasets is selected here as a case study and an ad hoc filter has been designed, the MultiRes filter. Some results of a prototipal implementation are presented and discussed.
Volume Rendering on Non-regular Grids
(Blackwell Science Ltd and the Eurographics Association, 1994) Tabatabai, Behnam; Sessarego, Emanuela A.; Mayer, Harald F.
In this article we present the principle methods for volumerendering of non-regular grids as used in the Finite Element Method or the Method of Control Volumes. Besides linear element types, the methods discussed here also cover the handling of commonly used non-linear element types but little considered in the Computer Graphics community. The very basic operation in the whole rendering process, namely the interpolation of the geometry and function values is fully based on the element type specific shape functions, ensuring a consistent mathematical treatment of the object in the sense of the underlying numerical simulation. We discuss the implications of this approach and propose optimizations justified by general assumptions on the qualify of the simulation model.
Beyond WYSIWYG: Display of Hidden Information in Graphics Editors
(Blackwell Science Ltd and the Eurographics Association, 1994) Schleich, R.; Durst, M.J.
Graphics editors such as computer aided design systems are complex and difficult to use. We attribute this to the fact that only a small part of the information is visible and therefore accessible and understandable. Only the graphics objects themselves are visible, but the relations between these objects remain hidden. We propose an effective way to visualise this information in the form of trees and graphs in so-called structure browsers. A prototype implements this proposal based on an object-oriented application framework for graphics editors.
Sketching 3D models with 2D interaction devices
(Blackwell Science Ltd and the Eurographics Association, 1994) Branco, Vasco; Costa, Antonio; Ferreira, F. Nunes
The creation of a 3D shape using CAD tools, namely solid modelers, is difficult and time-consuming. Therefore, these systems are kept away from the conceptual phases of design, perhaps the most important in any product life cycle.The sketch is considered by many people as one of the fundamental techniques during a design s conceptual stages, as it enables high level design options to be explored at low cost. In general, this technique IS under-appreciated in the computer graphics world.In this paper, we present a system called IDeS that tries to combine simplicity and intuition while drawing, with the useful features of the solid modelers, for example, CSG operators.
Improved Visualisation of Marine Data: Algae and Acid Rain
(Blackwell Science Ltd and the Eurographics Association, 1994) Day, A.M.; Turner, S.M.; Arnold, D.B.
A Semantics-based Approach for the Design and Implementation of Interaction Objects
(Blackwell Science Ltd and the Eurographics Association, 1994) Paterno , F.; Leonardi, A.
Although tools for developing graphical user interfaces are becoming increasingly popular, they do not usually highlight precisely which key elements developers should take into account. This still entails considerable expertise in developing user interfaces. In this paper we present an approach to overcome these problems. Our approach is based on a model for interaction objects and a corresponding design space. This is supported by a toolkit where the available interaction objects are initially classified by their semantics whereas in most current toolkits they are investigated by their appearance. This facilitates designers and developers in identifying the interactors needed in order to obtain an Interactive System supporting user tasks.
Towards Context- Dependent Interpolation of Digital Elevation Models
(Blackwell Science Ltd and the Eurographics Association, 1994) Dixon, A.R.; Kirby, G.H.; Wills, D.P.M.
The fractional Brownian motion (fBm) model has become increasingly popular in recent years as a mechanism for the synthesis of many natural phenomena. Unfortunately, the required interdependence of neighbouring points over an unbounded area (context dependence) renders valid approximations to fBm extremely inefficient. This paper describes a technique for the interpolation of digital elevation models to an arbitrarily high spatial resolution. This is achieved using an efficient recursive subdivision algorithm which has the ability to permit some degree of context dependence within, and across the boundaries of interpolation areas. The application of this technique to flight simulation is described and the technique is evaluated based on its ability to reduce run-time storage requirements.
GKS-9x: The Design Output Primitive, an Approach to a Specification.
(Blackwell Science Ltd and the Eurographics Association, 1994) Nehlig, Ph. W.; Duce, D. A.
This paper describes an approach to the formal definition of the design primitive introduced in the revision of the ISO/IEC computer graphics standard, GKS. The paper starts with a general description of the design primitive and then describes the specification (which is given in the Z notation) and the motivation for the approach taken in some detail. The paper concludes with a reflection on the contribution of this work, and the descriptive style adopted an the GKS revision, to the role of formal description in the presentation of graphics standards.
PREMO - An Initial Approach to a Formal Definition
(Blackwell Science Ltd and the Eurographics Association, 1994) DUCE, D.A.; DUKE, D.J.; ten HAGEN, P.J.W.; REYNOLDS, G.J.
Many descriptions of multimedia programming systems are starting to appear in the literature, but little work has yet been done on how to formally specify and reason about such systems. This paper is a contribution on the use of formal description techniques for the specification of multimedia systems and explores a style in which the Object-Z notation from the University of Queensland can be used to specify some of the properties and functionality of such systems. The work has been carried out with reference to the role of formal description techniques in the presentation of ISO/IEC International Standards and in particular to the Premo project in ISO/IEC JTCl/SC24.
The Mesh Propagation Algorithm for Isosurface Construction
(Blackwell Science Ltd and the Eurographics Association, 1994) Howic, C.T.; Blake, E.H.
A new algorithm, Mesh Propagation, is presented for the generation of isosurfaces from three-dimensional discrete data sets. While producing the same surface mesh as that generated by a corrected Marching Cubes algorithm, its characteristic is that it constructs an isosurface using connected strips of dynamically triangulated polygons. This compact data structure speeds up surface construction and reduces surface storage requirements. The surface can also be displayed more quickly, particularly where there is hardware support for rendering triangle strips.With engineering as well as medical imaging applications in mind, the algorithm can be used with both irregular and rectilinear grids of data, the primitive volume elements need not be hexahedral only, and volumes of heterogeneous polyhedral elements are supported without traversal complications.The algorithm propagates through the cells in the grid and uses the same lookup table topologies as Marching Cubes to determine patches of surface-element intersection- additional tables are used for non-hexahedral elements. The surface patches are dynamically coded into triangle strips which are then concatenated and linked to construct the surface. The data structures used for propagating through the volume overcome the topological ambiguities associated with table-based methods of surface construction and no holes are generated in the final mesh.

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