Open University Computing Department Research Report 95/16 Spatial Expressions in Design Idea Capture Languages Martin Stacey Computing Department, The Open University, Milton Keynes, UK. [email protected] Paper for the AID'96 Workshop on Visual Representation, Reasoning and Interaction in Design Convened by J.C.B. Damski and N.H. Narayanan. Abstract. Intelligent support systems for conceptual design in engineering have so far failed to support spatial thinking in conceptual design. But multimedia interface technology including techniques for recognising speech and drawn gestures offers solutions to the HCI problems involved in computer support for spatial conceptual design. Effective computer support could be made possible by the use of a design idea capture language for expressing and changing shapes and qualitative spatial and functional relationships, fluently in a machine-understandable way. The design idea capture language would serve to constrain the expression of design ideas sufficiently to enable the successful use of AI techniques for generating coherent spatial representations from sets of spatial expressions in the language. This paper discusses the design meanings required for spatial conceptual design with reference to linguistic studies of spatial expressions in natural language, which show that geometric approaches are insufficient for representing spatial relationships important in design. 1. Introduction So far spatial aspects of conceptual design have been neglected in the development of intelligent support systems for conceptual design in engineering. This paper takes the view that the goal of research on intelligent design support systems for engineering should be developing systems that provide workspaces in which designers express their ideas as they create them, so that creating computer representations of designs to support AI reasoning is coextensive with designing. Creating useful intelligent workspaces for conceptual design involves enabling designers to use visual representations and design operations that match human designers' early conceptions of their designs, which are fluid, qualitative and cut across conceptual categories, and are often essentially spatial. This paper argues that including spatial thinking in conceptual design within the scope of intelligent support systems involves solving significant human computer interaction problems as well as artificial intelligence problems, but multimedia interface technology provides the means to solve these problems. The key challenge is finding a good compromise between the human designers' need to express fluid and conceptually messy design ideas easily and fluently, and the machine's need for input that is unambiguous and well-structured enough to enable it to construct coherent internal representations of the design. This conflict can be tackled with design idea capture languages, in which designers can express qualitative spatial design ideas in linguistic or quasi-linguistic spatial expressions with a precise syntax and semantics that the machine can interpret in terms of its model of the design. Building an intelligent support system for spatial conceptual design is partly a human computer interaction problem, defining a spatial design language that is easy and natural for designers to use, and partly an AI problem, constructing mechanisms for interpreting expressions in that language to construct and modify coherent spatial representations of designs. 1.1. Spatial Thinking in Conceptual Design 2 M.K. Stacey Conceptual design is the stage at the beginning of the design process when the major decisions are made about what the product is: how it works, how big it is, what its major components are, and so on. The space of possible designs is explored at an abstract level, before a small subset of that space is selected for further exploration at more concrete and detailed levels. The output of conceptual design is a skeletal qualitative description of the product that is refined in the course of detail design. The concepts designers reason about at the conceptual design stage are characterised by informality, provisionality, incompleteness, approximate and qualitative relationships, and rapid switches between conceptual categories; this makes representing them and reasoning about them a hard challenge for artificial intelligence. Whether spatial design activities are essential to the conceptual design of a product depends on the nature of the design problem. Some design domains are clearly spatial, notably architecture and clothing design. Software development and most electronic engineering is non-spatial. Mechatronics - multi-technology engineering - is harder to characterise. It involves both spatial and non-spatial design processes. Rzevski et al (1995) argue that for large classes of engineering products, including many mechatronic systems, the critical decisions are about technologies and physical principles, and about the choice of types of components and how they are connected; and that these decisions precede thinking about spatial layout. However for another large class of products, involving complex or innovative mechanical systems, spatial thinking is central to the conceptual design process. Moreover many engineers think spatially even when spatial representations are not essential to the task. Spatial thinking in conceptual design in engineering involves the rapid creation and modification of qualitative spatial relationships between objects, and between objects and non-object concepts such as movement paths and magnetic fields. The creation and modification of spatial relationships is combined with the creation and modification of functional relationships; some important relationships are both spatial and functional, so making a clear separation between layout and behaviour is not always straightforward. Many such relationships in mechanical engineering concern movement and physical causality. Changing spatial relationships is often coextensive with adding or removing components such as gears or universal joints. Spatial conceptual design also often includes rapid qualitative changes in the shapes of individual objects. 1.2. Requirements for Tools for Creating Spatial Conceptual Designs Two objectives of research on computer support for conceptual design are to create tools that (1) only restrict the design process in planned and intended ways, and that (2) provide a seamless integration of conceptual design and detail design, by enabling the construction and progressive refinement of coherent internal product models. These objectives conflict. Any computer design tool embodies an inevitable tradeoff in its choices of primitive operations and design elements, between allowing designers freedom to express whatever human-understandable ideas they wish, and enabling the machine to interpret designs by restricting designers to machine-understandable actions. Paintboxes and three dimensional modelling systems for computer aided manufacture constitute opposite choices to meet different needs. Any design tool provides a limited range of primitive operations and design elements. The closeness of the mapping between the designers' conceptual structures and actions and the tool's primitive elements and actions, and simple combinations of them, has a profound influence on the usability of the tool (Green, 1989, 1991; Green and Petre, 1996). Different tools make different designs easy or difficult to create, and this can have a powerful biasing effect on the design process (Stacey, 1995). For instance, pencils are effective tools because primitive operations incur very low cost, but they make it very hard to create precise angles and projections of three dimensional shapes, as well as perform duplication, mirroring or rotation. Three dimensional modelling tools make it easy to create designs comprising regular geometric shapes, but not with subtleties of shading or the appearance of roughness and provisionality, for which inaccurate or seemingly inaccurate pencil lines are ideal. Design tools can also influence design by favouring mental representations that match the tools, especially in complex design fields like engineering, in which designers can have many different mental representations of the design, none encompassing every aspect of it. Stacey et al (1996b) argue that design tools should be designed to both exploit and mitigate these biasing effects, by supporting visual representations and operations that Spatial Expressions in Design Idea Capture Languages 3 encourage the use of effective mental representations, and by enabling designers to escape the biasing effects of any one formalism; the FACADE system is designed with this in mind. In order to be worth having, a tool for creating conceptual designs must be easy and natural to use, both as an external memory and communication tool for rough ideas, and as a workspace for designers who create by sketching and reacting to their own sketches (see for instance, Schön and Wiggins, 1992). (If this is an unattainable goal, the alternative objective of design tool development is a tool that enables the efficient encoding of a design worked out on paper or in the head of the designer.) Thus an effective tool for creating conceptual designs in engineering must provide easy ways to create design elements and relations between them that correspond to engineers' mental representations of designs; the operations for manipulating the design elements must correspond to the transformations