Presenting data graphically can often increase its understandability--well-designed graphics can be more effective than a tabular display of numbers. It is much easier to get an understanding of the relationships and groupings in data by looking at a pictorial representation than at raw numbers. Most visualization systems to date, however, have allowed users to only choose from a small number of pre-defined display methods. This does not allow the easy development of new and innovative display techniques. These systems also present a static display--users cannot interact with and explore the data. More innovative displays, and the systems that implement them, tend to be extremely specialised, and closely associated with an underlying application. We propose techniques and a system where the user can specify most kinds of displays. It provides facilities to integrate user-input devices into the display, so that users can interact and experiment with the data. This encourages an exploratory approach to data understanding. Most users of such systems have the sophistication to use advanced techniques, but conventional programming languages are too hard to learn just for occasional use. It is well known that direct manipulation is a powerful technique for novice users; systems that use it are much easier to learn and remember for occasional use. We provide a system that uses these techniques to provide a visualization tool. Extensions to the WYSIWYG (What You See Is What You Get) metaphor are provided to handle its shortcomings, the difficulty of specifying deferred actions and abstract objects. In the data graphics domain, the main drawbacks of WYSIWYG systems are the difficulty of allowing a variable number of data items, and specifying conditional structures. This system also encourages re-use and sharing of commonly used display idioms. Pre-existing displays can be easily incorporated into new displays, and also modified to suit the users' specific needs. This allows novices and unsophisticated users to modify and effectively use display techniques that advanced users have designed.

Chapter 1 introduces the need for rapid solutions of hidden surface elimination (HSE) problems in the interactive display of objects and scenes, as used in many application areas such as flight and driving simulators and CAD systems. It reviews the existing approaches to high-performance computer graphics and to parallel computing. It then introduces the central tenet of this thesis: that general purpose parallel computers may be usefully applied to the solution of HSE problems. Finally it introduces a set of metrics for describing sets of scene data, and applies them to the test scenes used in this thesis. Chapter 2 describes variants of several common image space hidden surface elimination algorithms, which solve the HSE problem for scenes described as collections of polygons. Implementations of these HSE algorithms on a traditional, serial, single microprocessor computer are introduced and theoretical estimates of their performance are derived. The algorithms are compared under identical conditions for various sets of test data. The results of this comparison are then placed in context with existing historical results. Chapter 3 examines the application of MIMD style parallelism to accelerate the solution of HSE problems. MIMD parallel implementations of the previously considered HSE algorithms are introduced. Their behaviour under various system configurations and for various data sets is investigated and compared with theoretical estimates. The theoretical estimates are found to match closely the experimental findings. Chapter 4 summarises the conclusions of this thesis, finding that HSE algorithms can be implemented to use an MIMD parallel computer effectively, and that of the HSE algorithms examined the z-buffer algorithm generally proves to be a good compromise solution.

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Pre-visualisation is an important tool for planning films during the pre-production phase of filmmaking. Existing pre-visualisation authoring tools do not effectively support the user in authoring pre-visualisations without impairing software usability. These tools require the user to either have programming skills, be experienced in modelling and animation, or use drag-and-drop style interfaces. These interaction methods do not intuitively fit with pre-production activities such as floor planning and storyboarding, and existing tools that apply a storyboarding metaphor do not automatically interpret user sketches. The goal of this research was to investigate how sketch-based user interfaces and methods from computer vision could be used for supporting pre-visualisation authoring using a storyboarding approach. The requirements for such a sketch-based storyboarding tool were determined from literature and an interview with Triggerfish Animation Studios. A framework was developed to support sketch-based pre-visualisation authoring using a storyboarding approach. Algorithms for describing user sketches, recognising objects and performing pose estimation were designed to automatically interpret user sketches. A proof of concept prototype implementation of this framework was evaluated in order to assess its usability benefit. It was found that the participants could author pre-visualisations effectively, efficiently and easily. The results of the usability evaluation also showed that the participants were satisfied with the overall design and usability of the prototype tool. The positive and negative findings of the evaluation were interpreted and combined with existing heuristics in order to create a set of guidelines for designing similar sketch-based pre-visualisation authoring tools that apply the storyboarding approach. The successful implementation of the proof of concept prototype tool provides practical evidence of the feasibility of sketch-based pre-visualisation authoring. The positive results from the usability evaluation established that sketch-based interfacing techniques can be used effectively with a storyboarding approach for authoring pre-visualisations without impairing software usability.

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This dissertation is concerned with the representation of visual knowledge. Image features often have many different local interpretations. As a result, visual interpretations are often ambiguous and hypothetical. In many model-based vision systems the problem of representing ambiguous and hypothetical interpretations is not very specifically addressed. Generally, specialization hierarchies are used to suppress a potential explosion in local interpretations. Such a solution has problems, as many local interpretations cannot be represented by a single hierarchy. As well, ambiguous and hypothetical interpretations tend to be represented along more than one knowledge representation dimension limiting modularity in representation and control. In this dissertation a better solution is proposed. Classes of objects which have local features with similar appearance in the image are represented by discrimination graphs. Such graphs are directed and acyclic. Their leaves represent classes of elementary objects. All other nodes represent abstract (and sometimes unnatural) classes of objects, which intensionally represent the set of elementary object classes that descend from them. Rather than interpreting each image feature as an elementary object, we use the abstract class that represents the complete set of possible (elementary) objects. Following the principle of least commitment, the interpretation of each image feature is repeatedly forced into more restrictive classes as the context for the image feature is expanded, until the image no longer provides subclassification information. This approach is called discrimination vision, and it has several attractive features. First, hypothetical and ambiguous interpretations can be represented along one knowledge representation dimension. Second, the number of hypotheses represented for a single image feature can be kept small. Third, in an interpretation graph competing hypotheses can be represented in the domain of a single variable. This often eliminates the need for restructuring the graph when a hypothesis is invalidated. Fourth, the problem of resolving ambiguity can be treated as a constraint satisfaction problem which is a well researched problem in Computational Vision. Our system has been implemented as Mapsee-3, a program for interpreting sketch maps. A hierarchical arc consistency algorithm has been used to deal with the inherently hierarchical discrimination graphs. Experimental data show that, for the domain implemented, this algorithm is more efficient than standard arc consistency algorithms. / Science, Faculty of / Computer Science, Department of / Graduate

This thesis describes and compares several methods for producing bilevel patterns to simulate grey level values for use in polygon regions as generated for computer graphics. Random distribution, ordered dither, and error diffusion methods are shown to be visually inferior for many grey levels to the proposed maxmin algorithm for producing patterns for polygon area filling procedures. Through even spatial arrangement of the pixels and taking into consideration the edges of the pattern, the number of artifacts is decreased and the accuracy in small subregions of the pattern is improved, especially at low grey levels where most pattern generators degrade. At these lower levels, the maxmin algorithm can produce pleasing patterns if given sufficient flexibility through enlarged grid sizes. At higher grey levels, the proximity of pixels does not leave sufficient room to eliminate all artifacts, but by varying the criteria of the algorithm, the patterns still appear more pleasing than other methods. / Science, Faculty of / Computer Science, Department of / Graduate

The design and implementation of an interactive graphics system for graph manipulation are discussed. The activation fcr such a system is examined, and the relevant literature is described and evaluated. A number of ways to improve and extend the system are presented. The system provides the basic graph drawing operations cf adding, deleting, labeling, and changing both vertices and edges. Also included are a number of graph manipulation operations which, among ether things, allow a user to subdivide edges, associate vertices, reverse the direction cf arcs, move vertices about the screen, cr even move whole graphs about the sc teen. A facility is provided whereby the screen can te divided into as many as four regions, thus allowing users tc display more than one graph at a time. Graphs can be saved on disk and later restored. The image on the graphics screen can te easily plotted tc obtain a hard copy of graphs. A few routines which perform graph-theoretic operations have teen implemented. Among these are a routine for finding the minimum and maximum degrees of a graph, and a routine fcr finding the blocks, cutnodes, and bridges cf a graph. Moreover, the system is designed to allow users to add their own routines. / Science, Faculty of / Computer Science, Department of / Graduate