EG1991 Proceedings (Technical Papers)

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An Algorithm of hidden Surface Removal based on Frame-To- Frame Coherence
(Eurographics Association, 1991) Tost, Daniele
Frame-to-frame coherence capitalizes on the continuity existing between successive frames in order to predict the visibility of the scene at a given instant on the basis of its visibility at the previous instant. It is shown that all the algorithms based on frame-to-frame coherence share a common model and involve similar data structures. This model is used as a framework for a discussion of possible frame-to-frame coherence algorithms. A specific solution based on a partition of the image space into several 2-D regions is then presented. A temporal graph of the depth priority relationship of the objects is first computed in a pre-process along with a temporal BSP tree of each object. In the first frame a total traversal of both data structures is required in order to establish the priority list of the faces of the scenes. In successive frames, the priority list is easily updated with partial traversals and with no geometrical computations. The complexity of the proposed algorithm is discussed. Some examples are presented with comments on the results of their implementation.
Animation of Landscapes Using Satellite Imagery
(Eurographics Association, 1991) Geymayer, Barbara; Prantl, Manfred; Müller-Seelich, Heimo; Tabatabai, Behnam
The paper presents methods to animate synthetic landscapes. The modeling of the landscape, the interactive specification of the flight path and the simulation of an explosion are described. The landscape is visualized by superimposing texture information computed from different satellite data onto a digital elevation model. A pyramid data structure is used to store the texture and digital elevation model data in different resolutions. This approach allows the generation of pictures with constant data density in an efficient way. The flight path is modeled interactively using a wireframe representation of the landscape. An explosion is simulated using physically modeled trajectories of the exploding parts. The approach is explained in the context of the “ERZBERG” animation in which an impressive flight through a valley towards the exploding Erzberg mountain has been generated.
C 2 Gregory patch
(Eurographics Association, 1991) Miuraa, Kenjiro Takai; Wangb, Kuo-King
G² continuity of free-form surfaces is sometimes very important in engineering applications. The conditions for G2 continuity between two Bezier patches has been studied and methods developed to ensure such continuity. However, certain restrictions on the shapes of such patches arise within the Bezier-patch formulation. The Gregory patch is a kind of free-form surface patch which is an extension of the Bezier patch such that cross-boundary first derivatives can be specified without restrictions on the compatibility condition. In this paper, we extend the idea of the Gregory patch and develop a formulation for the C² Gregory patch. The properties of the C² Gregory patch are discussed as well as its connection with a Bezier patch and a G² continuous interpolation method based upon such patches.
Colour Illustrations
(Eurographics Association, 1991) -
Combining Volume Rendering with Line and Surface Rendering
(Eurographics Association, 1991) Frühauf, Martin
Volume data is discrete sampled data in the three-dimensional space. Volume rendering is defined as volume visualization directly from volume primitives and not via surface primitives. Geometric objects are represented as a list of vertices and connecting lines or surface patches. Independent algorithms for rendering the two different categories of data are used. The system, proposed here, combines the results of the different algorithms in one image. A set of common parameters influencing both rendering algorithms and ensuring the consistency of the resulting merged image is identified. A volume rendering algorithm capable to produce the information to be merged is described in detail. The system is able to handle opaque and translucent objects by merging lists of image space elements. The independence of the both rendering modules allows to employ a wide range of algorithms for rendering of geometric objects, even rendering in hardware.
Computational Physics : A Modeler - Simulator for animated physical Objects
(Eurographics Association, 1991) Luciani, Annie; Jimenez, Stephane; Florens, Jean Loup; Cadoz, Claude; Raoult, Olivier
Physical modeling for animation is now firmly established. The present aim is to design and build a structured and well-defined tool rather than merely specific algorithms to simulate physical knowledge. We will first define the basic functions of a modeler - simulator for physical modeling which enables operator gestural control, and where the simulation processes are real time oriented. These constraints cannot be achieved by any kind of models and algorithms. Concerning the theoretical elements, the choice of the underlying Physics and the categorization of interactions will be presented. We will then introduce the Cordis-Anima system, its constructive langage, its real time simulator and its force feedback gestural transducers. Finally, we will describe, with some examples, how we model, simulate and manipulate a large variety of physical objects and scenes with our system.
Configurable Representations in Feature-based Modelling
(Eurographics Association, 1991) Falcidieno, B.; Giannini, F.; Porzia, C.; Spagnuolo, M.
A double description is proposed for the representation of a feature-based model which supports the distinction between form and functionality. This description consists of a primary representation in terms of geometric features ( geometric feature graph) and a set of feature-based representations which are created by transformations that are viewpoint specific and apply to the geometric feature graph.
A constraint-based UIMS using graph unification
(Eurographics Association, 1991) Samuel, J. F.
This paper describes an experimental UIMS developed to investigate methods of combining and reusing objects within a constraint-based system. A novel approach based on graph-unification facilitated the definition of composite objects and allowed behaviours to be transferred easily from one object to another. A very simple form of local propagation was used as the constraint satisfaction mechanism, which, as well as being fast, could also cope with cyclic dependencies. This method used a time-stamping method which made it possible to express time-dependency in the constraints. The system has features from both class-based and prototype-based object-oriented languages. Dynamic modification of objects can be performed rapidly enough for interactive control. Interfaces built with the system include a variety of 'standard' user interface objects such as sliders as well as a number of physical simulations utilising time-dependent constraints.
Construction Techniques of Graphic, Direct-Manipulation User Interfaces
(Eurographics Association, 1991) Preea, Wolfgang; Pornberger, Gustav; Sikora, Hermann
This paper deals with human-computer interaction in several ways. On the one hand it presents the roots of interactive, graphic user interfaces and how such interfaces are implemented on the abstraction level of programming languages: The construction of graphic, direct-manipulation interfaces with conventional programming techniques is compared with an object-oriented approach based on powerful class libraries (called user interface application frameworks). Although application frameworks substantially ease the building of highly interactive applications the abstraction level is considered to be too low to support prototyping such interfaces in a comfortable way. Hence we portray DICE1 (Dynamic Interface Creation Environment), a tool for prototyping graphic user interfaces implemented itself in an object-oriented manner. In particular this paper discusses the question of how dynamic behavior can be added to a user interface prototype. It also presents a useful and powerful way to combine conventionally developed and object-oriented software systems.
Constructive Cubes: CSG Evaluation For Display Using Discrete 3-D Scalar Data Sets
(Eurographics Association, 1991) Breen, David E.
The algorithm presented in this paper converts a CSG model into a representation for interactive display on an engineering workstation. Called Constructive Cubes, the algorithm extends the standard CSG-point classification algorithm and then employs a popular isosurface generation algorithm, Marching Cubes, to generate a list of polygons that approximates the surface of a CSG model. The polygons may then be interactively displayed, shaded and inspected on a workstation. The algorithm has many advantages over other CSG algorithms. It is straightforward to implement, requiring no complex surface intersection calculations. The algorithm provides an inherent flexibility that allows a user to balance the time/quality trade-off. It is designed to take advantage of current and future advances in both visualization and engineering workstation design.
Constructive Page Description Opening Up the Prepress World
(Eurographics Association, 1991) Samara, Veronika; Wiedling, Hans-Peter
Constructive Page Description (CPD) is an overall approach allowing different kinds of data to be exchanged between a variety of systems and manipulated in arbitrary system environments. Fully changeable pages, which keep information for modification as long as necessary, as well as fully assembled pages, ready for the printing process, can be constructed by the use of CPD. Moreover, descriptions of data as well as operations can be distributed, and so allow the use of networking facilities. CPD is thereby very flexible in handling, combining, and exchanging data and operations used in the construction of pages. In sum, CPD helps bridge the gap between the printing and the computer graphics world; it is an approach to lead prepress towards an open system architecture.
Continuation Methods for Approximating Isovalued Complex Surfaces
(Eurographics Association, 1991) Zahlten, Cornelia; Jürgens, Hartmut
Basically there are two different approaches for rendering isovalued surfaces in 3D space: projection methods and surface reconstruction. We are discussing two algorithms of the second kind. Both use continuation methods for efficiently scanning an isovalued surface. A simplicial pivoting algorithm by Ralf Widmann which continues earlier work of E. L. Allgower et al., is compared to an approach which is based on subdividing space into cubes. The algorithms determine all simplices or cubes intersecting the surface and then generate an oriented polygonal approximation. For demonstration and comparison we use several fractal and some smooth surfaces. These surfaces are implicitely defined by a function, but it is also possible to apply both methods to 3D volume data.
A DDA Octree Traversal Algorithm for Ray Tracing
(Eurographics Association, 1991) Sung, Kelvin
A spatial traversal algorithm for ray tracing that combines the memory efficiency of an octree and the traversal speed of a uniform voxel space is described. A new octree representation is proposed and an implementation of the algorithm based on that representation is presented. Performance of the implementation and other spatial structure traversal algorithms are examined.
Declarative Graphics And Dynamic Interaction
(Eurographics Association, 1991) Johnson, C.W.; Harrison, M.D.
First order logic provides a means of integrating the specification and prototyping of interactive systems. It can describe graphical images in a declarative and order independent manner. It supports the definition of abstract devices which avoid the complexity of representing ‘raw’ input from a variety of physical devices. The following pages show how such techniques must be extended in order to prototype and reason about dynamic interaction with graphical interfaces. The incorporation of a temporal ordering into logical specifications provides a means of describing changes in the structure of graphical images. It can also identify the sequencing which may be implicit within specifications of interactive dialogues. This paper describes how PRELOG, a tool for Presenting and REndering LOGic specifications of interactive systems, has been extended to include a temporal logic interpreter.
Discrepancy as a Quality Measure for Sample Distributions
(Eurographics Association, 1991) Shirley, Peter
Discrepancy, a scalar measure of sample point equidistribution, is discussed in the context of Computer Graphics sampling problems. Several sampling strategies and their discrepancy characteristics are examined. The relationship between image error and the discrepancy of the sampling patterns used to generate the image is established. The definition of discrepancy is extended to non-uniform sampling patterns.
FACES: Facial Animation, Construction and Editing System
(Eurographics Association, 1991) Patel, Manjula; Willis, Philip J.
The aim of the Facial Animation, Construction and Editing System (FACES) is to provide a software simulation of the human face. Attention has focused on the face as an important means of non-verbal communication. The interactive composition and modification of the human head and its subsequent animation, have been identified as being of particular interest. The novelty of FACES is that it integrates the modelling and animation of faces using a three-layer anatomical model. FACES consists of four sub-systems: CONSTRUCT, MODIFY, ANIMATE and RENDER. The CONSTRUCT and MODIFY sub-systems enable changes to be made to the structure of the head, at both global and local levels, enabling specific faces to be created. The ANIMATE sub-system caters for motion specification and control, so that real and exaggerated facial expressions can be animated. The RENDER part of the system facilitates the generation of realistic images and their real-time playback. In this paper we consider the system from the user's point of view, examining the facilities which are provided, their appropriateness and practicality.
Fast Rendering of General Ellipses
(Eurographics Association, 1991) Fellner, Dieter W.; Helmberg, Christoph
Even though GKS did not include circles and, in a more general form, ellipses and elliptical arcs in the list of elementary graphics primitives, CGM settled this omission with its standardization in 1987. According to CGM as well as to CGI, ellipses and elliptical arcs are defined in a very general way via endpoints of conjugate diameter pairs (CDP). Based on the algorithm of Maxwell & Baker [5] this paper presents a new algorithm for the rendering of general ellipses (i.e. not aligned to the coordinate axes) and elliptical arcs which is not only fast and very well suited for implementation in hardware but also deals with all degenerate cases of ellipses at no extra cost. Furthermore, the algorithm provides all the information which is necessary for the generation of anti-aliased elliptical curves.
Geometric Modelling from Range Image Data
(Eurographics Association, 1991) Schmitt, Francis; Cken, Xin; Du, Wen-Hui
An adaptive surface fitting algorithm is proposed for modelling the digitized surface of a real object described by an array of 3D points sampled on a rectangular mesh and stored in the form of a range image. A G1 -piecewise approximation of the data is obtained by using an adaptive top-down method which combines the Delaunay triangulation technique with a triangular Gregory-Bezier patch model (tGB). The method begins with a rough approximation of the surface and progressively refines it in successive steps in the regions where the accuracy requirement of the approximation is not satisfied. The method, therefore, is essentially a local process. An optimization approach is used to obtain a G '-continuous piecewise approximation where each tGB patch is as smooth and regular as possible. Some experimental results are given to demonstrate the potential usefulness of this approach for the geometric modelling from range image data.
Integrating Inheritance and Composition in an Objective Presentation Model for Multiple Media
(Eurographics Association, 1991) Took, Roger
A formal model is presented which combines, in a single structure called a tangle, the power to express both the composition of aggregate objects, and the selective inheritance of object properties over a number of instances or manifestations. The model allows an objective implementation, that is, one in which objects can be created and updated randomly, incrementally, and dynamically. Such a model is ideal as the basis for interactive presentation. The tangle is defined as generic in its node type, and so can model the structure of multiple presentation media.
Interactive Conformance Testing for PHIGS
(Eurographics Association, 1991) Cugini, John V.
Conformance testing for the Programmer’s Hierarchical Interactive Graphics System (PHIGS) standard presents certain novel difficulties, especially the indirect effect of many functions, and the inaccessibility to the program of visual effects. The PHIGS Validation Tests (PVT) incorporate several innovative design features in order to address these difficulties. The model of deductive inference suggests ways to organize a system as logically complex as the PVT. This complexity makes the use of certain database concepts quite valuable in allowing users to navigate within the system. The problem of inaccessible effects can be addressed by careful design of the user interface, so as to minimize the subjectivity and operational difficulty inherent in testing such features. Subjectivity is minimized by posing short simple questions to the operator, in which the expected answer is randomized. Several design features enhance ease of use, including a customizable interface, self-explanatory displays, and automatic capture of results.

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