The simplest solid objects used for the representation are called primitives. Typically they are the objects of simple shape: cuboids, cylinders, prisms, pyramids, spheres, cones. The set of allowable primitives is limited by each software package. Some software packages allow CSG on curved objects while other packages do not.
A primitive can typically be described by a procedure which accepts some number of parameters; for example, a sphere may be described by the coordinates of its center point, along with a radius value. These primitives can be combined into compound objects using operations like these:
|Boolean union||Boolean difference||Boolean intersection|
|The merger of two objects into one.||The subtraction of one object from another.||The portion common to both objects.|
Combining these elementary operations, it is possible to build up objects with high complexity starting from simple ones.
Constructive solid geometry has a number of practical uses. It is used in cases where simple geometric objects are desired, or where mathematical accuracy is important. The Unreal engine uses this system, as does Hammer (the native Source engine level editor), Quake Lawmaker Editor (editor for Lawmaker Engine), and Torque Game Engine/Torque Game Engine Advanced. BRL-CAD is a solid modeling CAD package that is fundamentally based on CSG modeling techniques. CSG is popular because a modeler can use a set of relatively simple objects to create very complicated geometry. When CSG is procedural or parametric, the user can revise their complex geometry by changing the position of objects or by changing the Boolean operation used to combine those objects.
One of the advantages of CSG is that it can easily assure that objects are "solid" or water-tight if all of the primitive shapes are water-tight. This can be important for some manufacturing or engineering computation applications. By comparison when creating geometry based upon boundary-representation additional topological data is required, or consistency checks performed to assure a "solid" object.
A convenient property of CSG shapes is that it can be relatively easy to classify arbitrary points as being either inside or outside the shape created by CSG. The point is simply classified against all the underlying primitives and the boolean equation is evaluated. This can be a desirable quality for some applications such as collision detection.
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