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ACIS brings together the separate worlds of wireframes, surfaces, and solids by allowing these alternative representations to coexist in its data structure. Wireframe entities can coexist with solids and sheets, and can share edges, coedges, and vertexes. From this coexistence comes the ability to define mixed dimensionality models and a variety of nonclosed models, such as a plane with three bounding edges and one unbounded (infinite) direction.
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ACIS delimits objects with a
bounding box that is used to determine intersections efficiently. If the bounding box of two objects intersect, the objects are likely to intersect, so more precise intersection calculations are performed. If the bounding boxes do not intersect, the objects cannot intersect, so the more precise intersection calculations are not necessary.
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ACIS decomposes solid, sheet, wire, and mixed bodies into the classes shown in Figure 4-2. These classes provide the data and methods necessary for a true boundary representation solid modeler. Pointers up and down the hierarchy allow quick traversal of the data to determine such things as whether two entities share an edge or a vertex. As Figure 4-2 illustrates, the topology classes contain pointers to the corresponding geometry classes.
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Figure 4-2. Implementation of Model Objects
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