A spline is an entity that represents a non-uniform rational B-spline (NURBS)
curve. It consists of control points, fit points, and can be one of two types:
by fit points or by control points.
The spline object is represented by the OdDbSpline class, which provides users
access to a spline entity and the ability to manipulate its properties. The
OdDbSplinePtr class is the typified smart pointer to an instance of this entity
and it is used for storing and passing references to the spline objects.