Volume 10 (1991)

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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.
Inverse Displacement Mapping
(Blackwell Science Ltd and the Eurographics Association, 1991) Patterson, J.W.; Hoggar, S.G.; Logie, J.R.
Inverse displacement mapping is a variant of displacement mapping which does not actually perturb the geometry of the surface being mapped. It is thus a true texture mapping technique which can be applied during rendering without breaking viewing pipeline discipline. The method works by first projecting probing rays into texture space and solving for a ray-texture intersection there. Shadows can also be determined by mapping a probe from the intersection point towards the light source into texture space and seeing if an intersection results. Our implementation uses as much knowledge about the base surface as possible to speed up the ray-surface intersection calculation. We have limited our treatment to spheres, cones, cylinders and planes, and our rendering method to ray casting, in order to contain the scope of this work up to the present. The inverse displacement mapping technique can, however, be applied more widely, for example as part of a full ray-tracer, and also as part of the rendering pipeline for a wider class of smooth surfaces.
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.
Control Points for Multivariate B-Spline Surfaces over Arbitrary Triangulations
(Blackwell Science Ltd and the Eurographics Association, 1991) Fong, Philip; Seidel, Hans-Peter
This paper describes first results of a test implementation that implements the new multivariate B-splines as recently developed by Dahmen et al. 10for quadratics and cubics. The surface scheme is based on blending functions and control points and allows the modelling of Ck? 1 -continuous piecewise polynomial surfaces of degree k over arbitrary triangulations of the parameter plane. The surface scheme exhibits both affine invariance and the convex hull property, and the control points can be used to manipulate the shape of the surface locally. Additional degrees of freedom in the underlying knot net allow for the modelling of discontinuities. Explicit formulas are given for the representation of polynomials and piecewise polynomials as linear combinations of B-splines.
Development of an Intelligent Wheelchair Using Computer Graphics Animation and Simulation
(Blackwell Science Ltd and the Eurographics Association, 1991) Ojala, Jari; Inoue, Kenji; Sasaki, Ken; Takano, Masaharu
A new robot simulator JC-1 is used as a control software development tool in a project in progress where an intelligent wheelchair for a blind user is being developed. The intelligent wheelchair is planned to be able to fulfill simple symbolic commands like"follow wall" or"follow object" and using the JC-1 simulator an evaluation team which includes e.g. the user, a rehabilitation engineer and a software engineer, can check control algorithms and user interface routines before constructing a real wheelchair prototype. The JC-1 simulator models the environment using simplified boundary- representation where objects, robot sensors and actuators are presented as symbolic objects in the graphics data-base of the simulator. In the JC-1 simulator a robot controller under development controls the motion of the graphical model of the robot while simulator commands or other robot controllers can be used to control the movement of disturbing obstacles. Computer graphics animation and simulation help to find fundamental design errors at an early design stage and as this paper suggests, enable the user of the final product to take part in to the designing process of the robot controller. Benefits and difficulties of using computer graphics simulation in the wheelchair development process are discussed.
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.
A Methodology And Tool Set For Supporting The Development Of Graphical User Interfaces
(Blackwell Science Ltd and the Eurographics Association, 1991) Neelamkavil, F.; Mullarney, O.
A methodology and tool set for building application (assumed to be inherently non-graphical) software with graphical user interface is described. Initially, pure application software is built from a set of basic building blocks- subsequently, graphical representations for application objects are defined without direct coding and then the graphical user interface is generated automatically. This paper concentrates on the graphical representation aspects of the user interface. Portability, configurability and sound software engineering principles are major considerations in the design of the overall system architecture. The prototype implementation is based on VDM (Vienna Development Method), Object-based Design, GKS (Graphical Kernel System) and the programming language ADA. An example from CIM (Computer Integrated Manufacturing) is used to illustrate the methodology presented here.
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.
The Shortest Way to Draw a Connected Picture
(Blackwell Science Ltd and the Eurographics Association, 1991) Seebold, Patrice; Slowinski, Karine
With any word over the alphabet ?=r, r?, u, u, we associate a connected picture in the following manner: the reading of each letter of this word induces a unit line: r (r?, u, u respectively) stands for a right (left, up, down respectively) move. We present a rewriting system which can yield, from any word over ?, all the words describing the same picture. Particularly, we give an algorithm to find a minimal word describing a given picture: this word represents the shortest way to draw this picture without penup .
Computer Art from Numerical Methods
(Blackwell Science Ltd and the Eurographics Association, 1991) Szyszkowicz, Mieczyslaw
Zero-finding methods, Euler s method and the numerical algorithms elaborated as package subroutines are studied by using a computer graphics technique. The patterns obtained show chaotic behaviour (science) and beauty of numerical algorithms (art).
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.

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