Computers are increasingly being seen not only as computing tools but more so as communication tools, thus placing special emphasis on human-computer interaction (HCI). In this article, the focus is on visual HCI, where the messages exchanged between human and computer are images appearing on the computer screen, as usual in current popular user interfaces. We formalize interactive sessions of a human-computer dialogue as a structured set of legal visual sentences, i.e., as a visual language, and show how rewriting systems can be generalized to specify both the pictorial and the computational aspects of visual languages. To this end, Visual Conditional Attributed Rewriting (VCARW) systems are introduced, and used for specification of visual languages. These specifications are given as inputs to a procedure illustrated in the article as a system of algorithms, which automatically generates control mechanisms of the interaction, thus favoring the design of more reliable and usable systems.

We refine our design methodology for visual interactive systems based on the visual sentence theory. We explore the dimensions of time and space in visual interaction and discuss the importance of the notions of scaffold and frame of reference. Usable visual systems are realised by designing an augmented version of the user task visual language and a scaffold visual language supporting user navigation and action execution. However, the frame effect, i.e. user's perception of persistent characteristic structures in different images, must be explicitly considered, to avoid user disorientation. Some examples of positive and negative use of the frame of reference are included and some design guidelines are derived.

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The need of a precise definition of the concepts of metaphor and visual representation in the database realm was widely discussed in the VDB3 Panel. Also, the issue of singling out the relationships existing between the above two concepts and the data model was addressed. An introduction by the panel organizer together with a short position statement by each panelist are reported in the following document. KeywordsData model, metaphor, visual representation. INTRODUCTIONThe need of a better interaction between users and database systems has been widely recognized and discussed. It is generally accepted that the quality of the interaction mainly depends on the interface characteristics. Thus, the crucial point is to develop good interfaces, where the word "good" is to be interpreted from the user's perspective. Now, the point is to understand which parts of this interaction belong to the interface and which to the database system. A database is typically defined in terms of a data model, but the abstract concepts of such a model need to be expressed through a representation in order to be perceived by a user. Any representation of a model aims at stressing some basic features of the model itself, and obviously a many-to-many correspondence exists between data models and representations (visual or not). The representation should be part of the interface, while the data model should be part of the database system. On the other hand, there are data models, the so called semantic models, which S. Spaccapietra et al. (eds.), Visual Database Systems 3