Direct graphic user interaction with modelers based on constructive solid geometry

Boolean operations provide convenient methods for defining solid objects, and are supported by most of the solid modelers available today. Graphic User Interfaces (GUIs) for solid modelers typically are based on picking edges and vertices on a display, and correlating them with entities in the boundary representation (Brep) of a solid. The graphic operations involvend are fast, but they require that a Brep be computed, and this is computationally expensive. This paper discusses an alternative approach, which does not require the existence of a Brep, and is based on correlating pick operations directly with the surfaces of Constructive Solid Geometry (CSG) primitives. The new approach, in conjunction with recently-developed, special-purpose, modeling hardware, should lead to higher performance user interfaces than those currently available. The initial implementation of a GUI based on these concepts is presented. The interface uses the X/Motif tool kit, and interacts with the PADL-2 CSG modeler through Unix interprocess communication techniques.