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# Background

Since the MGED system is presently based on the COMGEOM solid modeling technique, a brief overview of the COMGEOM technique is required to effectively use MGED. For more detailed information on the COMGEOM technique see [KBR79][BR75].

    Symbol        Name

    ARS           Arbitrary Triangular Surfaced Polyhedron
    ARB           Arbitrary Convex Polyhedron
    ELLG          General Ellipsoid
    POLY          Polygonal Faceted Solid
    SPL           Non-Uniform Rational B-Spline (NURB)
    TGC           Truncated General Cone
    TOR           Torus
    HALF          Half Space (Plane)

        Figure 2.1: Basic Solid Types

    Symbol        Name

    RPP           Rectangular Parallelpiped
    BOX           Box
    RAW           Right Angle Wedge
    SPH           Sphere
    RCC           Right Circular Cylinder
    REC           Right Elliptical Cylinder
    TRC           Truncated Right Cylinder
    TEC           Truncated Elliptical Cylinder

        Figure 2.2: Special-Case Solid Types

The COMGEOM technique utilizes two basic entities - a solid and a region. A solid is defined as one of fifteen basic geometric shapes or primitives. Figure 2.1 lists the basic solid types, and Figure 2.2 lists special cases of the basic solid types for which support exists. The individual parameters of each solid define the solid's location, size, and orientation. A region is a combination of one or more solids and is defined as the volume occupied by the resulting combination of solids. Solids are combined into regions using any of three logic operations: union (OR), intersection (+), or difference (-). The union of two solids is defined as the volume in either of the solids. The difference of two solids is defined as the volume of the first solid minus the volume of the second solid. The intersection of two solids is defined as the volume common to both solids.

Any number of solids may be combined to produce a region. As far as the COMGEOM technique is concerned, only a region can represent an actual component of the model. Regions are homogeneous; they are composed of a single material. Each region represents a single object in the model; the solids are only building blocks which are combined to define the shape of the regions. Since regions represent the components of the model, they are further identified by code numbers. These code numbers either identify the region as a model component (nonzero item code) or as air (nonzero air code). Any volume not defined as a region is assumed to be ``universal air'' and is given an air code of ``01''. If it is necessary to distinguish between universal ``01'' air and any other kind of air, then that volume must be defined as a region and given an air code other than ``01''. Normally, regions cannot occupy the same volume (overlap), but regions identified with air codes can overlap with any region identified as a component (i.e. one that has a nonzero item code). Regions identified with different air codes, however, can not overlap.

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