William T. Reeves

Rendering antialiased shadows with depth maps

We present a solution to the aliasing problem for shadow algorithms that use depth maps. The solution is based on a new filtering technique called percentage closer filtering. In addition to antialiasing, the improved algorithm provides soft shadow boundaries that resemble penumbrae. We describe the new algorithm in detail, demonstrate the effects of its parameters, and analyze its performance.

Approximate and probabilistic algorithms for shading and rendering structured particle systems

Detail enhances the visual richness and realism of computer-generated images. Our stochastic modelling approach, called particle systems, builds complex pictures from sets of simple, volume-filling primitives. For example, structured particle systems have been used to generate trees and a grass-covered forest floor. Particle systems can produce so much irregular, three-dimensional detail that exact shading and visible surface calculations become infeasible. We describe approximate and probabilistic algorithms for shading and the visible surface problem. Because particle systems algorithms generate richly-detailed images, it is hard to detect any deviation from an exact rendering. Recent work in stochastic modelling also enables us to model complex motions with random variation, such as a field of grass blowing in the breeze. We analyze the performance of our current algorithms to understand the costs of our stochastic modelling approach.

Inbetweening for computer animation utilizing moving point constraints

This paper presents an approach to computerized inbetweening which allows the animator more control over an interpolation sequence than existing keyframe techniques. In our approach, the animator specifies in addition to a set of keyframe constraints, a set of new constraints called moving points. Moving points are curves in space and time which constrain both the trajectory and dynamics of certain points on the keyframes. The sets of keyframes and moving points form a constraint or patch network specification of the desired dynamics. Several algorithms are presented for inbetweening or completing such a patch network. By measuring these algorithms with respect to a set of evaluation criteria, the algorithm which best meets our interpolation needs is selected.

A Microcomputer-Based Conducting System

The subject of this paper is a portable, microcomputer-based performance system that we call the conduct system.1 This system was developed at the University of Toronto as part of the research of the Structured Sound Synthesis Project (SSSP) (Buxton, Fogels, Fedorkow and Smith 1978). The system, built around a microprocessor controlling a digital sound synthesizer, had its genesis in the earlier conductor system of Mathews (Mathews 1976). That is, it is a system that allows the performer to interpret or conduct precomposed score material, rather than a system on which the prime performance task is to articulate each individual note, as is the case with conventional instruments. In the remainder of this paper, we shall provide a description of the microcomputer-based conducting system, concentrating on aspects of the human interface and the control structure. In addition, we shall discuss what we see as the relative strengths and weaknesses of the system, based on our experience with it in both the laboratory and the concert hall. We shall begin, however, by relating our work to that of others in the field, and by discussing the general issues of the problem.

Towards a laboratory instrument for motion analysis

Motion analysis is the systematic and usually quantitative study of the movements of humans, animals, organisms, cells, or other entities as recorded on movie film or video tape. Despite the utility of computer-aided motion analysis to many biological, social, and physical sciences, its role has been limited because it is so time-consuming and so expensive. Automated techniques can only be used on real images in very special cases; interactive techniques have involved laborious frame by frame operations. In recent years, Futrelle and Potel have revolutionized the process of interactive motion analysis by demonstrating how to digitize entire motions with a single sketch rather than with a sequence of operations on each constituent frame. However, their GALA-TEA system is applicable only to film and not to video tape records, and consists of equipment which cannot easily be engineered into a reliable and mass-producible laboratory instrument. The paper describes a prototype laboratory instrument for the interactive motion analysis of video tape records. The prototype includes a host PDP 11/45 computer, an experimental display processor called SPIWRIT, and a microprocessor-controlled video disk. SPIWRIT generates a computer-animated video representation of the phenomenon being analysed, and superimposes it on the actual video record being streamed from the video disk. SPIWRITs bit-slice microprogrammable display processor produces true animation on a colour raster display by decoding segmented display file images into alternate halves of a double frame buffer.

The simulation of natural phenomena (Panel Session)

This panel will discuss the issues and the problems associated with the simulation of natural phenomena. This is a difficult area of research since it generally involves complex data bases and in many instances time variant phenomena. The computational loads can become enormous as one considers the physics or the mathematical modeling of structures. Most items in nature, trees, clouds, fire and comets being some examples, have not been displayed realistically in computer graphics. This lack stems from a few different problems, all of which are significant. The first is the fact that realistic portrayals require large amounts of storage and consequently large compute time. Nature is able to create diverse detail at the most minute levels within an object of grandoise scale. The second problem is that of diversity of design within a given framework. For example, if a scene requires two dozen poplar trees, how does the designer construct trees that look different but are undeniably poplars? Humans typically become tired after the first few iterations of such a design process, with a resulting degradation in the subsequent models. Clearly, this problem applies to all of the phenomena mentioned above. Finally, there is a lack of models. First, second and third order representations are commonly used in computer graphics to model various kinds of surfaces and their boolean combinations. However, their applications to objects, which do not lend themselves well to being described as surfaces has not been addressed sufficiently. Previous attempts at realism have dealt with the appearances of the surfaces being modeled, in terms of their illumination or relief. More recently, fractal methods have introduced a new degree of realism into terrain modeling systems. However, it appears that natural phenomena will require more research into the fundamental way things occur in nature, and in terms of computer graphics, their representation will build on previous work, but will still require new modeling techniques.