SOLID MODELING 265MFE +1 +1 +1(Right face made)MFE +1+1+ 1(Back face made)MFE +1 +1 +1(Left face made)MFE +1 +1 +1(Front face made)MEV +1 +1VEFL SG8.9 Constructive Solid Geometry (CSG)
This scheme is another way of representing solids which can be generated by combining primitives
using Boolean operations. Primitives can include solids like block, cone, cylinder, sphere, triangular
prism, torus and many others. Solids participating in CSG need not only be bounded by analytical
surfaces but also by generic surface patches developed in previous chapters. Some primitive solids
are shown in Figure 8.20. Boolean operators include those used in set operations, for instance, union,
intersection and difference.
Solids used in the CSG approach are first instantiated, transformed and then combined to form
more complex solids. Instantiation involves making available a copy of the primitive (if existing)
from the database. Transformation of a primitive is then required to scale or position (translate and/
or rotate) itself with respect to others (or their Boolean result) as desired in design. The primitive in
its resulting size and/or position may then be joined with, cut from or intersected with an existing
solid to get the desired features. Consider, for instance, the design of an L-shaped bracket which may
be treated as a union of two blocks shown in Figure 8.21(a). The blocks can be instantiated, scaled,
transformed relatively and then joined to form the bracket as shown in Figure 8.21(b).
For computer modeling, the block primitives above may be treated as objects named Block 1 and