EG1991 Proceedings (Technical Papers)

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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.
A Testbed for Architectural Modeling
(Eurographics Association, 1991) Hall, Roy; Bussan, Mimi; Georgiades, Priamos; Greenberg, Donald P.
This paper describes the philosophy and implementation of a modeling system that is easy to use yet addresses some of the difficulties of design. It does this by supporting concurrent schematic and geometric representations, alternative solution schemes, ambiguous and incomplete specification, and multiple levels of detail through a wide range of scale. The system treats an object as a hierarchical record of design decisions and treats geometry as an artifact of traversing a decision tree. For displaying geometry, the system incorporates fast rendering techniques for interactive use and global illumination algorithms for design evaluation and final presentation. The system is intended to serve as an extensible testbed for long-term research in modeling and design. Vigorous use by students in the Department of Architecture at Cornell is the vehicle for system evaluation and redirection of research goals.
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.
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.
Tessellation of Curved Surfaces under Highly Varying Transformations
(Eurographics Association, 1991) Abi-Ezzi, Salim S.; Shirman, Leon A.
We pursue the problem of step size determination for tessellating arbitrary degree polynomial and rational Bezier patches, under highly varying modeling and viewing transformations, to within post-viewing size and/or deviation thresholds specified in display coordinates. The technique involves the computation of derivative bounds of surfaces in modeling coordinates, and the mapping of these bounds into world coordinates (or lighting coordinates), where tessellation takes place by using norms of modeling transformations. A key result of this work is a closed form expression for the maximum scale a perspective transformation is capable of at an arbitrary point in space. This result allows the mapping of thresholds from DC into WC (LC). In practice, while the step size determination needs to take place during every traversal, the costly operations of finding derivative bounds, computing norms of modeling transformations, and factoring viewing transformations take place at creation time.
Variable-Radius Blending by Using Gregory Patches in Geo- metric Modeling
(Eurographics Association, 1991) Harada, T.; Konnoa, K.; Chiyokura, H.
Blending surfaces, which connect two curved surfaces smoothly, often appear in geometric modeling. Many of the blending surfaces are variable-radius blends. Variableradius blending surfaces are very important in the design process, but it is difficult to generate such surfaces with existing geometric modelers. This paper proposes a new method to generate variable-radius blends. Instead of the popular rolling-ball method, we adopt “sliding-circle” blending. A circle slides on two curved surfaces so that the circle is perpendicular to a specified control curve, and its trajectory defines a blending surface. A variable-radius blend can be generated if the radius of the circle changes smoothly. In our method, the shape of the variable-radius blend is represented by Gregory patches. The Gregory patch is an extension of a Bezier patch and two Gregory patches can be connected together with tangential continuity. The characteristics of the Gregory patch are suitable for representing blending surfaces with geometric modelers.
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.
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.
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.
Refinement criteria for adaptive stochastic ray tracing of textures
(Eurographics Association, 1991) van Walsum, Theo; van Nieuwenhuizen, Peter R.; Jansen, Frederik W.
Adaptive stochastic ray tracing is a rendering technique that generates high-quality anti-aliased images by sampling the image in a non-regular pattern that is adaptively refined. Image refinement can be guided by image space or object space criteria. For display of textures, additional criteria that operate in texture space can be added to further improve image quality. In this paper three texture space refinement criteria are introduced. The methods minimize the chance of sampling errors at the cost of only a small amount of preprocessing and are comparable in efficiency with existing texture prefiltering methods.
Several approaches to implement the merging step of the split and merge region segmentation
(Eurographics Association, 1991) Popovic, M.; Chantemargue, F.; Canals, R.; Bonton, P.
The purpose of this paper is to propose several approaches for the implementation of the merging step of split and merge region segmentation. The splitting step has already been studied and its parallelization has subsequently been implemented on a transputer network. First, the most widely known merging step is described. Then, two approaches which are better suited to a parallelization are presented. Next we discuss the principle behind these approaches. Finally region segmentation according to motionbased criteria has been chosen in order to provide results to evaluate the performance of each approach. We emphasize that the description is general and can be applied to all split and merge algorithms. Therefore this work is a contributory factor in the evolution of region segmentation towards its parallelization.
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.
MOVE-X: A System for Combining Video Films and Computer Animation
(Eurographics Association, 1991) Ertl, Gerhard; Müller-Seelich, Heimo; Tabatabai, Behnam
The objective of the presented project was to implement a system for the visualization of buildings. The system is used to create movies of buildings and interior rooms before they are built. For a realistic impression of a building it is very important to show its actual environment in the film. The designed software solution permits to create films where real images of the environment are overlaid with computer generated images of the building. In order to overlay the video film with computer generated images, it is necessary to compute the exact position, the viewing direction and the adjustment of the zoom for every frame of the film. These parameters can be calculated from the video images. This paper describes the algorithms used to calculate the camera parameters and to track passpoints in a sequence of video images. Some problems resulting from interlaced video and low resolution are discussed in detail. The rendering techniques used to generate images of the new building are also discussed.
Shading with Area Light Sources
(Eurographics Association, 1991) Tanaka, Toshimitsu; Takahashi, Tokiichiro
This paper derives a shading model for area light sources which covers both diffuse and specular reflection. The shading model assumes ideally diffuse polygonal light sources and uses Phong’s reflection model. The model can accurately integrate the intensities of diffuse and specular reflection without simulating an area light source as an array of point light sources. To simplify the reflection integration, each light source is transformed from the Cartesian coordinate system into the polar system. The light source is projected onto a unit sphere and then triangulated along great circles of the unit sphere. Finally, the integration value is calculated by polynomial approximation. Since our method can accurately integrate both diffuse and specular reflection, it can generate images that are more photorealistic than conventional methods. Because point light sources are not employed, highlight roughness is completely suppressed. Several images are presented that show the advantages of our method.
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 Testbed for Image Synthesis
(Eurographics Association, 1991) Trumbore, Ben; Lytle, Wayne; Greenberg, Donald P.
Image Synthesis research combines new ideas with existing techniques. A collection of software modules that provide such techniques is extremely useful for simplifying the development process. We describe the design and implementation of a new Testbed for Image Synthesis that provides such support. This Testbed differs from previous Testbeds in both its goals and its design decisions. The Testbed design addresses the problems of high model complexity, complicated global illumination algorithms and coarse grain parallel processing environments. The implementation is modular, portable and extensible. It allows for statistical comparison of algorithms and measurement of incremental image improvements, as well as quantitative comparison of Testbed images and light reflectance measured from physical models. The Testbed is designed to interface with any available modeling system. This compatibility was achieved through careful design of the data format that represents environments. The software modules of the Testbed are organized in a hierarchical fashion, simplifying application programming.
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.
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.
Timewarps: A Temporal Reparameterization Paradigm for Parametric Animation
(Eurographics Association, 1991) Smith, Jeff; Drewery, Karin
The ability to reparameterize the temporal axis in an animated sequence is a powerful and underexplored tool. Timewarps are described as extensions to previous work and the advantages of this tool are outlined. Timewarps allow an animator to progressively refine the timing of all or part of an animation. Problems inherent to this new concept were brought to light by the authors' implementation and are described in terms of both user interface and development issues. Solutions are discussed and directions for further study are outlined.
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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