INTEREST IN THE DESIGN and development of interactive visual applications has increased considerably over the last few years. The underlying reason for this is the need to allow the maximum number of people to access software applications for the widest number of purposes and in the widest number of contexts. There is also growing interest in the application of formal methods to visual interaction, as user-interface design and development is a rapidly emerging field, though sometimes without well-defined basic concepts and methods. This is a strong limitation, since at times designers provide only partially valid solutions which are often difficult to analyse and may contain inconsistencies. The visual modality is still a key communication channel in user interactions, despite the spread of multimedia technology, which adds voice, audio, gestures and other media. Visual interactive interfaces represent a challenging application area for formal methods. Their internal structure is becoming increasingly more complex, as there is a continuous demand to have them support more and more interaction techniques, users, tasks, environments, with more and more dialogues active at the same time. After all, the approaches in this area must consider the most complex system in the world: the human user, whose behaviour cannot be described prescriptively. The first problem in this special issue was to define the area of interest. Here we would like to understand how a formal approach can contribute to the design of interactions between users and applications through visual support. This is an area that has stimulated a good deal of effort in the past 15 years: a good example is the work of Cardelli on Squeak [1], while other early attempts were introduced in the book edited by Harrison and Thimblebly [2]. The work of Dix [3] was useful to provide precise definitions of the fundamental concepts of interactive systems. A more updated description of many approaches in this area can be found in the book on formal methods in human}computer interaction [4]. How visual environments can support formal approaches is an interesting, but rather different issue, which was considered irrelevant for this special issue. Our aim here is to understand how formal methods can be used to improve visual interaction, and not vice versa. Another problem was to determine when a method can be considered formal. The answers offered to this question differ subtly depending on the authors’ background (software engineering, human}computer interaction, artificial intelligence, etc.). For example, in cognitive sciences a formal method aims to model user behaviour. Such models can be used to make predictions and attempt to verify these predictions through empirical studies.
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