2 An Approach for Solid Modelling in a Virtual Reality Environment

With today’s Virtual Reality (VR) systems, it is difficult to directly and precisely create and modify objects in a VR environment. This chapter presents an approach for solid modelling in a VR environment. Solid modelling in the VR environment is performed precisely in an intuitive manner through constraint-based manipulations. A hierarchically structured and constraint-based data model is developed to support solid modelling in the VR environment. The data model integrates a high-level constraintbased model for precise object definition, a midlevel CSG/Brep (Constructive Solid Geometry/Boundary representation) hybrid solid model for hierarchical geometry abstractions and object creation, and a lowlevel polygon model for real-time visualization and interaction in the VR environment. Constraints are embedded in the solid model and are organized at different levels to reflect the modelling process from features to parts. Constraintbased manipulations are accompanied with automatic constraint recognition and precise constraint satisfaction to establish the hierarchically structured constraintbased data model and are realized by allowable motions for precise 3D interactions in the VR environment. The allowable motions are represented as a mathematical matrix for conveniently deriving allowable motions from constraints. A procedure-based degree-of-freedom combination approach for 3D constraint solving is presented for deriving the allowable motions. A rule-based constraint recognition engine is developed for both constraint-based manipulations and implicitly incorporating constraints into the VR environment. A prototype system has been implemented for precise solid modelling in an intuitive manner through constraint-based manipulations.