Abstract
The trend towards systems that support a rich variety of interaction techniques has consequences for the traditional understanding of software design practice. Formal approaches to software development have mainly focused on design expressions that avoid explicit discussion of interactive behaviour. The emphasis in rigorous development is on transforming a mathematical specification into an implementation through a series of valid refinement steps. However, any design process for interactive systems must also add user oriented requirements. It is suggested that a suitable concern for human factors can be folded into a rigorous development process by generalizing from functional models and refinement to models that provide multiple viewpoints onto the design artefacts. The paper provides an overview of the relevant perspectives and describes work aimed at integrating them within the specification phase of software development.

Abstract
Interaction objects (interactors) are abstract structures that provide a link between rigorous approaches to software development and user-oriented aspects of interaction. The focus of the paper is on the second role of interactors; in particular, how a formal model of interaction can be used to classify and clarify various properties of interactive behaviour that are important from a user-oriented viewpoint. The formal model used in this discussion abstracts away from distinctions between states, events, and renderings; when needed, these concepts are introduced as various projections on the basic model.

Abstract
One of the most-used rendering algorithms in Computer Graphics is the Ray-Tracing. The ''standard'' (Whited like) Ray-Tracing(1) is a good rendering algorithm but with a drawback: the time necessary to produce an image is too large (several hours of CPU time are necessary to make a good picture of a moderately sophisticated 3D scene) and the image is only ready to be observed at the end of processing. This kind of situation is difficult to accept in systems where interactivity is the first goal. ''Increasing Realism'' In Ray-Tracing tries to avoid the problem by supplying the user with a preview of the final image. This preview can be calculated in a considerably shorter time but permits that, with some margin of error, the user can imagine (even see, sometimes) some final effects. With more processing time the image quality continues improving without loss of previous results. The user can, at any time, interrupt the session if the image does not match what he wants. Simultaneously with the above idea, it is necessary to accelerate image production. Parallelism is then justified by the need of more processing power. The aim of this text is to describe the Interactive Ray-Tracing Algorithm implementation, using a parallel architecture based on Transputers. An overview of the architecture used is presented and the main parallel processes and related problems are discussed.

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