AutoCAD solids are boundary representations (often referred to as B-rep models), consisting of a collection of topological connectivity objects and associated geometric boundary objects. The topological objects are defined in the AcBr library and are described later in this section, whereas the geometric objects are defined in the AcGe library.
Objects defined or generated by the AcBr library reside in three-dimensional Euclidean model space (E 3 ). The only exceptions are geometric objects defined in the two-dimensional parameter space of a surface (such as parameter curves and parameter points).
In general, only the 2-manifold topological domain is supported by the AcBr library. Singularities (which are geometric degeneracies) are supported in order to represent the apex of a cone, but wire bodies and mixed dimensionality solids (which may include dangling wires and faces) are not supported; nor can they be realized in AutoCAD.
The general nonmanifold domain is a superset of the 2-manifold domain, and allows distinct solid volumes to touch at single points, curves, or faces; and allows any combination of wireframe, sheet, and solid objects. The following nonmanifold objects are supported by AutoCAD and the AcBr library:
- Two 2-manifold solids united along a shared edge or vertex
- An AcDbBody object containing a single face
A topological object may be unbounded (that is, it may have no lower dimensional bounding topology) only in the following cases:
- A closed surface, which is intrinsically bounded in both the u and v directions (such as a full torus or sphere), is represented by a face that has no loop boundaries.
- A closed curve, which is intrinsically bounded (such as a full circle or ellipse), is represented by an edge that has coincident start and end vertices.