Volume 11 (1992)

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The Visualization Input Pipeline - Enabling Semantic Interaction in Scientific Visualization
(Blackwell Science Ltd and the Eurographics Association, 1992) Felger, W.; Schroder, F.
Scientific Visualization systems are primarily output-oriented, Users can specify and change parameters that are controlling the visualization process, which will result in different data representations or images respectively. But no mechanism is provided to really interact with the application data (semantic interaction) that has been changed step by step by the process of visualization.In this paper general concepts are elaborated and presented to achieve semantic interaction in dataflow environments for Scientific Visualization.
A New Two Dimensional Line Clipping Algorithm for Small Windows
(Blackwell Science Ltd and the Eurographics Association, 1992) Day, J. D.
A new algorithm for clipping lines against rectangular windows is described. It is suitable for computations in both object space (floating point arithmetic) and image space (integer arithmetic). The algorithm is compared with other object and image space algorithms and shown to be superior for small windows.
X: Why Z?
(Blackwell Science Ltd and the Eurographics Association, 1992) Bowen, Jonathan P.
Window management systems are now used extensively for user interfaces to computer systems. In particular, X11 has come to dominate the workstation market as a widely accepted industry standard on many different hardware platforms. However, no formal standard currently exists for this window system, both in terms of an international standards body (although this is being addressed), and in terms of a precise (mathematical) specification of what the interface is intended to do. This paper advocates the use of a formal notation to describe such an important system to avoid ambiguity and undesired or unintended variations between different implementations of the same system.Theformal notation used for demonstration purposes, Z, is based on set theory, and has been developed at the Programming Research Group in Oxford.
Pandora: An Experiment in Distributed Multimedia
(Blackwell Science Ltd and the Eurographics Association, 1992) King, Tony
An experimental workstation is described which supports digital video and audio in a distributed environment, and which presents this functionality to the user through the medium of a video-extended implementation of the X Window System. The Pandora Workstation is built out of two quite separate parts. A standard UNIX machine (the Pandora Host) brings standard hardware and software computing resources to the system- a highly-specialised processing engine (Pandora s Box) handles the high-bandwidth, time-critical, and device-dependent processing. A 50 Mbit/second ATM network provides for real-time data communication within the system. Nineteen Pandora Workstations have been deployed within Olivetti Research Ltd and the Cambridge University Computer Laboratory, and are used routinely to run distributed applications including video mail, video conferencing, and real-time media delivery services.
SX/Tools - An Open Design Environment for Adaptable Multimedia User Interfaces
(Blackwell Science Ltd and the Eurographics Association, 1992) Kuhme, Thomas; Schneider-Hufschmidt, Matthias
In this paper a brief review of user interface management systems (UIMSs) and their advantages is given. Shortcomings of current UIMSs and open requirements imposed by industrial application areas are outlined. Among the most important requirements are uniform design mechanisms independent of interaction media, techniques, and styles as well as a straight forward end-user adaptability of user interfaces.A new approach to interactive interface construction, called"direct composition", is introduced. In this approach all interface objects contain the means for their own modification and design and therefore offer consistent interaction techniques for both the construction and usage of user interfaces. Moreover, end-user adaptability is an inherent feature of direct composition interfaces.The user interface design environment SX/Tools is described to exemplify specific properties of user interface management systems following the principle of direct composition. The impacts of direct composition on user interface design include a uniform interface development process covering tool development, interface development, and"on-usage" interface adaptation. It is argued that the direct composition approach can overcome many of the problems with traditional UIMSs.
Computing Dynamic Changes to BSP Trees
(Blackwell Science Ltd and the Eurographics Association, 1992) Chrysanthou, Y.; Slater, M.
This paper investigates a new method for dynamically changing Binary Space Partition (BSP) trees. A BSP tree representation of a 3D polygonal scene provides an ideal data structure for rapidly performing the hidden surface computations involved in changing the viewpoint. However, BSP trees have generally been thought to be unsuitable for applications where the geometry of objects in the scene changes dynamically. The purpose of this paper is to introduce a dynamic BSP tree algorithm which does allow for such changes, and which maintains the simplicity and integrity of the BSP tree representation. The algorithm is extended to include dynamic changes to shadows. We calibrate the algorithms by transforming a range of objects in a scene, and reporting on the observed timing results.
Visual Composition and Multimedia
(Blackwell Science Ltd and the Eurographics Association, 1992) de Mey, Vicki; Breiteneder, Christian; Dami, Laurent; Gibbs, Simon and Tsichritzis, Dennis
Multimedia applications can be seen as sets of autonomous parts, independent yet capable of working together. This assumption allows for the construction of multimedia applications in a"connectionist" fashion. In this paper we describe mechanisms for the visual composition of multimedia applications. These mechanisms allow for quick and flexible application construction by direct manipulation with reference to multiple composition models. An object-oriented framework for multimedia systems will be introduced throughout the paper.
The Parametric Differential Method: An Alternative to the Calculation of Form Factors
(Blackwell Science Ltd and the Eurographics Association, 1992) Baranoski, Gladimir V. Guimaraes
The parametric differential method calculates the form factors without using approximations by polygons. Because of this it contributes to the improvement of the realism of the images synthesised through the radiosity method. This paper presents the most important characteristics of the parametric differential method as well as the results concerning its accuracy. Comparison of the form factors found using the new algorithm has been made with those analytically found.
Realistic Generation and Real Time Animation of Images of the Human Colon
(Blackwell Science Ltd and the Eurographics Association, 1992) Haritsis, Angelo; Gillies, Duncan; Williams, Christopher
A simulator has been built to teach doctors the skill of handling a flexible endoscope for gastrointestinal investigations. Trainees use a dummy endoscope in which the control actions are transduced into voltages and sensed by the computer. The simulator computes the position and viewing direction of the endoscope within an internal model of the human colon. Then a renderer draws the view, reproducing as far as possible what would be seen during a real colonoscopy. Since the system must generate at least ten frames per second for realistic animation, standard rendering techniques, such as ray tracing, could not be used. Consequently a new method was devised, based on identifying coherent regions along each scan line which can be rendered by table lookup. The method allows shaded Lambertian surfaces to be drawn at a frame rate of 15 per second, using modest computing resources. Although several approximations were required in the analysis, the computer images of the internal surfaces of the human colon present a high degree of visual realism.
Performance of Space Subdivision Techniques in Ray Tracing
(Blackwell Science Ltd and the Eurographics Association, 1992) McNeill, M. D. J.; Shah, B. C.; Hebert, M.-P.; Lister, P. F.; Grimsdale, R. L.
Whilst providing images of excellent quality, ray tracing is a computationally intensive task. The first part of this paper compares the speed-up achieved in ray tracing using various space subdivision algorithms and discusses the implications of implementing the algorithms on parallel processing systems. The second part addresses the problem of building the data structure within the rendering process, a situation which occurs when the rendering process is parallelised and dynamic scenes are rendered. Greater performance can be achieved with dynamic structure building compared to creation of the structure prior to rendering. The dynamic building algorithm proposed reduces the building time and storage cost of space subdivision structures, and decreases the data structure creation-render cycle time, thus enhancing image parallelism performance.
A line clipping algorithm and its analysis
(Blackwell Science Ltd and the Eurographics Association, 1992) Krammer, Gergely
One of the classical problems of Computer Graphics: line clipping against a rectangle is revisited. Coordinate raster refinement and some unusual forms of the parametric equation of the line are used to develop formulae for a line clipping algorithm. The algorithm is first presented in a form, where clarity of presentation is the prime concern. It is then transformed into one big nested branch, which after optimisation is assumed to be the most efficient form with a heavy cost on size. It is assumed that any mathematical consideration of the clipping problem would after a similar optimisation lead to a branching structure of equal complexity and speed. Line clipping thus belongs to the class of problems for which after a proper mathematical and logical analysis automatic program transformations may do the rest.This work has been supported by a grant from the Hungarian Academy of Sciences, Project No. OTKA 2572/1991
Hidden Surface Elimination on Parallel Processors
(Blackwell Science Ltd and the Eurographics Association, 1992) Highfield, Julian C.; Bez, Helmut E.
With the wide availability of general purpose parallel computers, there is a need to reconsider hidden surface elimination (HSE) algorithms with respect to parallel implementation. This paper considers MIMD parallel implementations off our common image space HSE algorithms - recursive subdivision, scan line, painter s, and z-buffer. Their suitability for parallel implementation is investigated and their relative performance in multiprocessor systems is measured using polygonal scene descriptions of between 200 and 2500 polygons. Dependence on the size of scene description is measured and results are extrapolated to larger scene descriptions. It is shown that HSE algorithms may be efficiently parallelised. A distributed z-buffer is shown to be a fast and efficient method of solving HSE problems using parallel computers.
Fractals and Solid Modeling
(Blackwell Science Ltd and the Eurographics Association, 1992) Groller, Eduard
Trying to combine fractal geometry and solid modeling seems to be a contradiction in itself, In this paper a new type of 3D objects is presented that accomplishes this combination in a specific way. Objects with a fractal macro structure and a 3D solid micro structure can be specified and rendered efficiently by using context free, attribute, geometric grammars. This new object type can be incorporated into the CSG-modeling technique (Constructive Solid Geometry) in two ways: a) using CSG for the specification of the micro structure of the new object type, b) using these fractal like objects as a new type of primitive in the CSG model. Ray tracing is used for generating high quality images of these geometrically complex objects.
A Unified Approach for Physical and Geometric Modeling for Graphics and Animation
(Blackwell Science Ltd and the Eurographics Association, 1992) Essa, Irfan A.; Sclaroff, Stan; Pentland, Alex
We present a unified approach for geometric and physical modeling using implicit functions, for application to graphics and animation. This method extends previously proposed techniques, and allows the standard finite element method to be directly combined with geometric modeling, resulting in quick calculation of an object s mass and stiffness matrices, and its vibration modes and frequencies. Because the approach is based on an implicitfunction representation, it allows very fast collision detection and characterization. Examples of complex physical and geometric modeling are presented.
User Interface Management Systems for Engineering Applications
(Blackwell Science Ltd and the Eurographics Association, 1992) Sastry, Lakshmi
Essential features of a User Interface Management System (UIMS) from the perspective of engineering applications user interface development are described. Three commercially available UIMS are validated against this set of essential features and the results of such a categorised evaluation are reported. The proliferation of user interface design tools necessitated an initialfiltering to select these tools on the basis of their availability on a wide range of platforms currently used by engineers, supplier support for the product and the prototyping tools the UIMS offer. Graphic Modelling System (GMS), TeleUSE and USEIT were selected for their potential utility to engineering applications.An earlier version of this paper was presented at the 1991 European X User Group Annual Conference held at Queens College, Cambridge, UK.This comparative evaluation is compiled on behalf of the UK Science and Engineering Research Council, for the benefit of the UK academic engineering research community.
An Efficient Adaptive Algorithm for Constructing the Convex Differences Tree of a Simple Polygon
(Blackwell Science Ltd and the Eurographics Association, 1992) Rappoport, Ari
The convex differences tree (CDT) representation of a simple polygon is useful in computer graphics, computer vision, computer aided design and robotics. The root of the tree contains the convex hull of the polygon and there is a child node recursively representing every connectivity component of the set difference between the convex hull and the polygon. We give an O(n log K + K log2 n) time algorithm for constructing the CDT, where n is the number of polygon vertices and K is the number of nodes in the CDT. The algorithm is adaptive to a complexity measure defined on its output while still being worst case efficient. For simply shaped polygons, where K is a constant, the algorithm is linear. In the worst case K = O(n) and the complexity is O(n log2 n). We also give an O(n log n) algorithm which is an application of the recently introduced compact interval tree data structure.
An Object-Oriented View of the User Interface
(Blackwell Science Ltd and the Eurographics Association, 1992) Zhou, Tom Z.-Y.; Kubitz, William J.
The object-oriented view of the user interface presented here is a model of object responsibilities-sharing and cooperation for a typical user interface. The model regards the user interface as a three-layer architecture, with the application contributing to one of the layers called the application exposure, The model seeks to promote user interface software reuse by coupling the user interface and the application with well-defined message protocols, by properly allocating user interfacefunctions to the layers so as to reduce and discipline their dependencies, and byfactoring complex abstractions into reusable secondary components.
Adaptive Sampling of Area Light Sources in Ray Tracing Including Diffuse Interreflection
(Blackwell Science Ltd and the Eurographics Association, 1992) Kok, Arjan J. F.; Jansen, Frederik W.
Ray tracing algorithms that sample both the light received directly from light sources and the light received indirectly by diffuse reflection from other patches, can accurately render the global illumination in a scene and can display complex scenes with accurate shadowing. A drawback of these algorithms, however, is the high cost for sampling the direct light which is done by shadow ray testing. Although several strategies are available to reduce the number of shadow rays, still a large number of rays will be needed, in particular to sample large area light sources. An adaptive sampling strategy is proposed that reduces the number of shadow rays by using statistical information from the sampling process and by applying information from a radiosity preprocessing. A further reduction in shadow rays is obtained by applying shadow pattern coherence, i.e. reusing the adaptive sampling pattern for neighboring sampling points.
Integer de Casteljau Algorithm for Rasterizing NURBS Curves
(Blackwell Science Ltd and the Eurographics Association, 1992) Anantakrishnan, Narayanan; Piegl, Les A.
An integer version of the well-known de Casteljau algorithm of NURBS curves is presented here. The algorithm is used to render NURBS curves of any degree on a raster device by turning on pixels that are closest to the curve. The approximation is independent of the parametrization, that is, it is independent of the weights used. The algorithm works entirely in the screen coordinate system and produces smooth rendering of curves without oversampling. Because of the integer arithmetic used, the algorithm is easily cast in hardware.
A Computer Model For Pinscreen Simulation: A New Animation Paradigm
(Blackwell Science Ltd and the Eurographics Association, 1992) Lopes, Pedro Faria; Gomes, Mario Rui
Actual computer image and motion synthesis paradigms are presented in order to introduce a new computer animation paradigm that simulates the traditional pinscreen animation technique. Pinscreen is a very particular and difficult device that enables a"chiaroscuro" approach to image synthesis. The paper describes the traditional pinscreen technique, the way it is operated, results and related difficulties. An extended computer model, including colour and time extensions, to simulate the pinscreen is presented. Results achieved with vector and raster approaches are discussed. Advantages and possibilities of the method when compared with the traditional method and the problems that arise from this technique will also be explained. Finally current and future work is referenced.

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