Constrcutive Solid Geometry (CSG) systems enable the definition of complex three-dimensional objects by combining simpler objects. A CSG system typically includes a set of solid primitives such as cubes, cylinders, cones etc. that can be defined in a variety of sizes and shapes. These primitives are combined by “set operations” or “Boolean operations” that allow the shape of an object to be described as an intersection of other objects or as a union of objects. Inverse sets (or subtractions) are used to make holes in the overall shape. The resulting shapes can in turn be joined or intersected with other shapes to describe three-dimensional shapes of any complexity. CSG systems often include facilities for defining new primitives through sweeping operations such as the generalized cylinder method. Two well-known research CAD systems that use CSG methods are GMSolid and PADL-2.
CSG systems are more powerful than boundary representations, just as boundary representations are more powerful than wireframes. A boundary representation can be automatically derived from a CSG description (often for graphical representation purposes). Objects constructed with CSG are guaranteed to be valid three-dimensional objects such a representation cannot be ambiguous. CSG representations have been enhanced in several ways. Examples for it are the inclusion of tolerance or uncertainty data or object-oriented method knowledge for the enterprise at defined objects.
Newly developed systems contain combinations of the methods mentioned above. Gossard, Zuffante and Sakurai describe the form with a combination of CSG and boundary representations. Their “Object Graph” can include both solid elements and boundary surfaces in the description of an object.
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