Читать книгу Computer Aided Design and Manufacturing - Zhuming Bi - Страница 51

A solid object has a geometric shape. A solid model is a digital representation of the geometry of an existing or envisioned physical object. A solid can be modelled in two basic methods: (i) designers may specify points, curves, and surfaces to define the mathematical representation of boundary surfaces and knit them together as a finite volume of the object; (ii) designers may select the models of simple shapes, such as blocks or cylinders, with specified dimensions, positions, and orientations, and combine them using logic operations such as union, intersection, and difference. The resulting representation is an unambiguous, complete, and detailed digital approximation of the object geometry or an assembly of objects (such as a car engine or an entire aeroplane).

Figure 2.15 shows that a solid object must be a watertight fine volume (B), which is defined by a number of boundary faces (F). In turn, a face F consists of a number of edges (E) and an edge E consists of a number of vertices (V). The information about vertices, edges, faces, and volumes is at different levels. In order to work with the solid models in the computer systems, they should be stored in the computer memory, which allows processing, converting, and ultimately displaying them. Different data structures can be used to represent solids.