Declarative Modelling of Virtual Environments, Overview of issues and Applications

This paper presents an overview of declarative modelling. It is organised according to the three main steps of the declarative design model: description, generation, and lookup steps. To illustrate the issues and possibilities of declarative modelling, representative projects are quoted in the different sections of the paper. The properties used in declarative modelling are presented in the next section, followed by a discussion about the critical steps. Key-words: Declarative modelling, Properties, Features, Constraints, Generation System, Layout. Fig 1.1.: A future district in the French city of Colomiers, near Toulouse [25] Fig 1.2.: Declarative modelling The goal of declarative modelling is to offer a designer the possibility to describe a shape or a scene by expressing a mental image of the wanted result. So, the purpose of declarative modellers is not to be a substitute, but a real improvement for imperative modellers. The scene is created from the set of properties supplied by the designer, while an imperative modeller accepts only precise numerical data or accurate interactions, via a classical graphical user interface (GUI). Properties allow to achieve many imperative elementary actions, by the way of a single abstract order. They are composed of geometric, photometric and physical rules, and spatial relations between parts of a scene (or a shape). Sketching can also be used as description of a complex shape. Recently, many projects using properties and features have been developed in different domains as architecture, urban layout, prototyping, assisted interaction, and "text-toscene" systems. Declarative modelling is a way to create incomplete models of a scene. The designer gives a vague and imprecise description, then the system fills out the missing data and computes a solution. The user can modify the description in order to obtain a better result (see Fig 1.5). So, this iterative design process allows a progressive convergence between problem understanding and produced solutions. Declarative modellers are usually decomposed in three major steps, that correspond with the three steps of a declarative design task (see Fig 1.4). This paper presents an overview of declarative modelling. It introduces its major concepts and issues. The three first sections present the three steps of a declarative modeller (description, generation, and lookup steps). Then, the next section details the properties that have been used in declarative systems. The last section presents the critical steps that have to be studied to improve declarative modelling. And Fig 1.4: The three steps of a declarative modeller(Description, Generation, and Lookup) (GEODE team of the GDR-PRC-AMI) Fig 1.5.: Spiral design process (GEODE team of the GDR-PRC-AMI) finally the paper ends with a conclusion and perspectives.