The purpose of requirements validation is to determine whether a large requirements set will lead to the achievement of system-related goals under different conditions — a task that needs automation if it is to be performed quickly and accurately. One reason for the current lack of software tools to undertake such validation is the absence of the computational mechanisms needed to associate scenario, system specification and goal analysis tools. Therefore, in this paper, we report first research experiments in developing these new capabilities, and demonstrate them with a non-trivial example associated with a Rolls Royce aircraft engine software component.

The development of complex systems frequently in-valves extensive work to elicit, document and review stakeholder requirements. Stakeholder requirements are usually written in unconstrained natural language, which is inherently imprecise. During system development, problems in stakeholder requirements inevitably propagate to lower levels. This creates unnecessary volatility and risk, which impact programme schedule and cost. Some experts advocate the use of other notations to in-crease precision and minimise problems such as ambiguity. However, use of non-textual notations requires translation of the source requirements, which can introduce further errors. There is also a training overhead associated with the introduction of new notations. A small set of structural rules was developed to address eight common requirement problems including ambiguity, complexity and vagueness. The ruleset allows all natural language requirements to be expressed in one of five simple templates. The ruleset was applied whilst extracting aero engine control system requirements from an airworthiness regulation document. The results of this case study show qualitative and quantitative improvements compared with a conventional textual requirements specification.

Rolls-Royce control systems are complex, safety critical and developed in ever-compressed timescales. Scenario techniques are utilised during systems design, safety analysis and systems verification. Scenarios can be used to improve requirements quality and to ensure greater confidence in requirements coverage for both normal and exception behaviour. A study was undertaken to investigate whether the ART-SCENE process and tool could enable engineers to identify exception behaviours earlier in the system design process, thus reducing cost and improving quality. ART-SCENE provides automatic generation of scenarios and alternative course events through the Scenario Presenter. These recognition cues are used to prompt engineers to identify deviations that may otherwise be missed. This paper describes a comparative evaluation between ART-SCENE and a standard hazard identification technique to assess the effectiveness of this approach.

The midpoint method for deriving efficient scan-conversion algorithms to draw geometric curves on raster displays in described. The method is general and is used to transform the nonparametric equation f(x,y) = 0, which describes the curve, into an algorithms that draws the curve. Floating point arithmetic and time-consuming operations such as multiplies are avoided. The maximum error of the digital approximation produced by the algorithm is one-half the distance between two adjacent pixels on the display grid. The midpoint method is compared with the two-point method used by Bresenham, and is seen to be more accurate (in terms of the linear error) in the general case, without increasing the amount of computation required. The use of the midpoint method is illustrated with examples of lines, circles, and ellipses. The considerzations involved in using the method to derive algorithms for drawing more general classes of curves are discussed.