Volume 06 (1987)

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The Clockworks: An Object-Oriented Computer Animation System
(Eurographics Association, 1987) Breen, David E.; Getto, Phillip H.; Apodaca, Anthony A.; Schmidt, Daniel G.; Sarachan, Brion D.
The Clockworks is an object-oriented computer animation system developed at RPl's Center for Interactive Computer Graphics (CICG). The Clockworks has the ability to model and graphically simulate complex 3-D engineering processes, Its interactive capabilities also allow it be used as a design tool. Object-oriented concepts have been utilized in developing its high level architecture and its low level implementation. The Clockworks is defined as a collection of objects which communicate with the user and each other via messages. The actual implementation involved the creation of an object-oriented programming methodology using C and Unix. The complete system provides a rich research environment for exploring modelling, scripting and rendering. It also provides an interactive environment for design and a simulation environment for visual analysis of complex interacting structures.
GKS Review Workshop
(Blackwell Publishing Ltd and the Eurographics Association, 1987)
A Simple and Efficient Haloed Line Algorithm for Hidden Line Elimination
(Blackwell Publishing Ltd and the Eurographics Association, 1987) Franklin, W.R; Akman, V.
An efficient algorithm, HALO, is given to compute haloed line drawings of wire frame objects. Haloed line drawings are described by Appel et al.1HALO has two parts: CUT and DRAW. CUT uses an adaptive grid to find all edge intersections. It overlays a square grid, whose fineness is a function of the number and length of the edges, on the Scene. It determines the cells that each edge passes through, sorts these by cell to obtain the edges in each cell, and then, in each cell, tests each pair of edges in that cell for intersection. For broad classes of input this takes time linear in the number of edges plus the number of intersections. CUT writes a file containing all the locations where each edge is crossed in front by another. Given a halo width, DRAW reads this file edge by edge. For each edge, it subtracts and adds the halo width to each intersection to get the locations where the edge becomes invisible and visible. It sorts these along the edge, and then traverses the edge, plotting those portions where the number of"Visible" transitions is equal to the number of"invisible" transitions. DRAW takes time hear in the number of edge segments. Dividing HALO into two parts means that redrawing a plot with a different halo width is fast, since only DRAW need to be rerun.CR Categories and Subject Descriptions: I.3.5 [Computer Graphics]: Computational Geometry and Object Modeling - geometric algorithm, languages, and systems- F.2.2 [Analysis of Algorithms and Problem Complexity]: Nonnumerid Algorithms and Problems -geometrical problem and computationsGeneral Terms: Algorithms, design.
Calendar of Events
(Blackwell Publishing Ltd and the Eurographics Association, 1987)
Parallel Polygon Rendering with Precomputed Surface Patches
(Eurographics Association, 1987) Theoharis, Theoharis; Page, Ian
We describe an algorithm for rendering a restricted class of trapeziums, and hence arbitrary polygons, in parallel on an NxN SIMD array processor. The algorithm achieves good performance by precomputing “surface patches” (finite portions of half -planes), thus trading storage for increased speed. The number of surface patches that are precomputed grows only linearly with the size of the array processor but as the square of the subpixel accuracy desired. NxN texture patterns can be added at very little extra cost additional to filling a trapezium with a particular colour.
Splines for Engineers
(Eurographics Association, 1987) Ohlin, S.C.
This paper introduces the concept of interpolation consistency. It is claimed that this property is an essential one for splines that are intended for use in Computer Aided Design. Consistency is defined as the property of an interpolation algorithm that, if the given set of points is extended by any point on the interpolated curve, the algorithm applied to the extended set of points will yield exactly the same curve as before. A clear distinction is made between function interpolation, where y is a function of x, and curve interpolation, where x and y are Cartesian coordinates in the geometric plane. The class of consistent curve interpolating planear splines that corresponds to cubic function interpolation is discussed in some detail and includes the Cornu spiral, the lemniscate, and the tension-free mechanical spline. Next the class that corresponds to pentic function interpolation is considered, and an efficient algorithm is presented for the calculation of one such spline, a planar spline where the curvature is a cubic polynomial in the arclength. This algorithm produces curves where the curvature is, loosely speaking, as constant as possible. Some examples illustrating the desirable properties of this spline are given.
The Reconstruction of Neurone Morphology from Thin Optical Sections - Graphical Aspects
(Eurographics Association, 1987) Brown, A. D.; Wheal, H. V.; Stockley, E. W.
The aims of the project are two-fold: firstly, to create an accurate three-dimensional model of a neurone from the mammalian nervous system, which can be used to obtain quantitative geometric and topological data on the neurone, and secondly, to create a software simulation of an active neurone, that accurately mirrors the electrochemical responses of a real, living mammalian neurone to external excitations and to changes in its electrochemical ambient. The focus of this paper is the first of these two goals. Portions of brain tissue are prepared, containing stained neurones. Using an optical sectioning technique, these neurones can be photographed in two-dimensional slices, down to about 10um thick. These photographs are digitised, and input to a computer. The system reconstructs the three-dimensional structure of the neurone, and provides a number of ways of displaying the reconstruction.
Bezier Patches with Local Shape Control Parameters
(Eurographics Association, 1987) Schmitt, Francis J. M.; Du, Wen-Hui
In this paper, we introduce three types of shape control parmeters (tension, bias, and continuity) into the modelling of free-form surfaces composed of bicubic Bernstein-Bezier patches. This is an extension to surfaces of the three control parameters introduced in the local interpolatory spline curves by Kochanck and Bartels [12]. We first analyze the general conditions for guaranteeing the G' continuity of the surface at corners where three, four, or five patches meet. The analysis of the continuity constraints reveals the remaining degrees of freedom which can be exploited for the control of the surface shape. The shape control parameters arc then introduced. Experimental results are provided. They show the efficiency of these parameters for an easy and intuitive control of the surface shape.
An Intersection-Sensitive Hidden-Surface Algorithm
(Eurographics Association, 1987) Devai, Ferenc
Given a set of pairwise disjoint planar polygonal faces with altogether N edges in the three-dimensional space. Let k be the number of intersection points of the projections of the edges in a projection plane, 0 < k < N(N-1)/2. An algorithm for the elimination of hidden surfaces in O((N+k)log N) time and O(N+k) space is proposed. If k is O(N), which is often the case in practice, the algorithm has O(N log N) expected time, regardless of the probability distribution of the input data. The constants of proportionality are low enough to make this algorithm of practical use. The method is a three-dimensional generalization of a two-dimensional scan-line algorithm, called the NlogN algorithm. A total order hide of faces is introduced. By determining the intersections of the projected edges, regions are designated within which visibility is unvaried. Then each region is visited, maintaining the total order hide, from which visible surfaces and rendering of partially transparent objects can be inferred.
Generative Scene Modelling
(Eurographics Association, 1987) Beyer, Thomas; Friedell, Mark
We describe a new theory of scene modelling that includes generative processes as integral components of scene descriptions. A single, uniform treatment of generative processes replaces the variety of ad hoc mechanisms commonly in use. We discuss the implementation of the theory in an experimental modelling system and show that L-systems, fractals, and particle systems, as well as many specialized generative processes, become comparatively easy to define and use.
Making Drawings Talk: Pictures in Minds and Machines
(Blackwell Publishing Ltd and the Eurographics Association, 1987) Bijl, Aart
Interactions between human minds and machine systems will be discussed, drawing on experience of CAD and focusing on modes of expression. Free forms of text and drawings pose deep questions when interpretations have to be"meaningful" to different knowledge representations - especially in loosely defined application fields. What should we expect from technology?
Inaugural Meeting of the EUROGRAPMCS Spanish Chapter
(Blackwell Publishing Ltd and the Eurographics Association, 1987) Duce, D A
Institutional Membership Column
(Blackwell Publishing Ltd and the Eurographics Association, 1987) Encarnacao, J. L.
Evaluation of Standard Graphics Packages
(Blackwell Publishing Ltd and the Eurographics Association, 1987) Klok, Fopke
In order to compare the quality of different implementations of GKS, the ISO0 standard for computer graphics, an evaluation method for GKS implementations is presented. It is based upon several groups of criteria. One group of criteria is concerned with performance, by which we understand here the memory requirements and time requirements for programs using GKS functions. A program that measures the performance of GKS packages is presented. Results of this evaluation method with several commercially available GKS implementations are described in summary. A checklist for evaluation of standard graphics packages is added as an appendix.
An Intersecting Modification to the Bresenham Algorithm
(Blackwell Publishing Ltd and the Eurographics Association, 1987) Skala, Viclav
Call for participation
(Blackwell Publishing Ltd and the Eurographics Association, 1987)
Parallel Graphical Output from Dialogue Cells
(Eurographics Association, 1987) Ten Hagen, P.J.W.; Schouten, H.J.
A system that accepts and processes graphical output from parallel processes using a single workstation, and its use in a User Interface Management System called Dialogue Cells, is described. It allows for a programmer to easily, concisely and precisely describe pictures and operations on them in a highly interactive environment. Each parallel process can do output independently, but pictures can also be moved from one process to another. Each process has its own graphical output environment. The description and implementation of the system is based on GKS.
UltraPaint: A New Approach to a Painting System
(Blackwell Publishing Ltd and the Eurographics Association, 1987) Watters, Geoff; Willis, Philip
The ideas underlying a Painting System being developed at the University of Bath are introduced. This differs from a conventional system in that ultra high definition pictures of upto 1 gigabyte may be created. We discuss approaches to implementing a number of standard paint program utilities in a form suited to our high definition method. Finally, we describe an algorithm for the rapid extrusion of brush strokes on a raster device.
Display of Solid Models with a Multi-Processor System
(Eurographics Association, 1987) Jansen, Frederik W.; Sutherland, Robert J.
There is a growing need for fast high-quality display of solid objects. Recently-developed custom-VLSI hardware offers high-speed display but lacks the flexibility and processing power at the pixel level to sustain the calculations and filter operations needed for realistic reflection models, texture mapping and anti-aliasing. Multi-processor systems built with general-purpose components of relatively high capacity and moderate cost provide an alternative; they also offer higher performance than single processor systems but retain their flexibility. A CSG hidden-surface algorithm based on depth-buffering and image subdivision is presented which is suitable for use with such a multi-processor system.
Viz: A Production System Based User Interface Management System
(Eurographics Association, 1987) van Harmelen, Mark; Wilson, Stephanie M.
A production system based User Interface Management System (UIMS) is discussed. The UIMS, called Viz, has been designed and implemented in a windowed workstation environment, and (a) capitalises on the graphical facilities of the workstation while (b) being designed for both textual and graphical applications. The paper considers interaction, the appearance of Viz generated interfaces, and dialogue descriptions which parameterise Viz.

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