SOLID MODELS

Generally, there are three accepted types of solid modeling techniques The CSG system uses a variety of (generally simple) primitive solid shapes, such as cones, cylinders, spheres, ellipsoids, boxes, and so forth, as the basis for the construction of more complex objects by employing combining operators such as union, intersection, and subtraction. Often the combining operators are called booleans. Sometimes the familiar combining operations are hidden from the user, but they are still there nevertheless. For example, in a feature-based CSG system, the user can drill a hole through an object by specifying a hole command, rather than by explicitly defining the hole as a subtraction of the appropriate cylinder from the object. However, the subtraction operator and the operands are stored by the CSG system (as a form of boolean tree ) to define the new object.

The BREP systems create solid objects by defining a surface (or a collection of surfaces) that completely encloses the volume of the object. The shape of the surface(s) defines the shape of the object. These surface definitions often include a variety of forms such as B-splines, Bezier patches, Coons patches, simple planes, or merely faceted (tesselated) surfaces.

Hybrid systems are a combination of both CSG and BREP. The user, typically, can choose from a palette of simple shapes and then use combining operations to produce a model; or the user may select from a variety of surfaces and trim, cut, or otherwise manipulate them to define the model. However, nearly all hybrid systems evaluate their CSG constructs to produce a final BREP form of the model. The current release of the Army Research Laboratory's (ARL) Computer-Aided Design package (BRL-CAD 4.0) is a hybrid system, although its CSG capabilities are most visible and available to the user in the interactive editor mged, and BRL-CAD does not generally produce the final model as a BREP. The spline-surfaced solid, the ARS solid, the polysolid, and the new NMG solid are all examples of BREP-type objects.

As designers became more dependent on CAD systems, there was a logical progression into solid modeling because of its useful potential. Remember, human beings invented engineering drawings to capture and communicate design intent among humans. But now humans are coming to realize that machines can recognize design intent more easily in forms other than three-view engineering drawings. Today's emphasis is on machine-to-machine communication, often linking the design of a part (on one machine) to ultimately the computer-driven machine tool (on another machine). Thus, the solid model is emerging as the specification of the part (the product definition). Accordingly, many CAD vendors include solid modeling capabilities in their systems and some are including additional applications capabilities such as finite element analysis based on the solid model. (The term computer-aided engineering (CAE) is evolving as the descriptor used to distinguish the workstation with these new capabilities from the traditional CAD faculty.) As the engineers in the 1970s realized that some method was needed for exchanging CAD-generated engineering drawings between CAD systems, now they realize that the same need exists for CAD-generated solid models.

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