ObjectARX Reference Guide > AcDb Classes > AcDbSpline Class
Class Hierarchy
class AcDbSpline : public AcDbCurve;



This class implements the AutoCAD SPLINE entity. 

Objects of the AcDbSpline class use an embedded gelib object to maintain the actual spline information. The spline itself may either be a simple curve fit (within a specified tolerance which may be 0) through a set of "fit points," or it may be a NURBS spline (that is, a set of control points, knots, and weights used to define the spline path). Internally, a curve-fit spline still has NURBS data; however the reverse is not true. 

The following books are a good place to start to get a basic understanding of spline curves: 

  • Curves and Surfaces for CAGD by Gerald Farin
  • Mathematical Elements for Computer Graphics by David Rogers and Alan Adams
  • An Introduction To Splines For Use In Computer Graphics & Geometric Modeling by Richard H. Bartels, John C. Beatty, and Brian A Barsky


The AcDbSpline class provides functions to query and set a curve-fit spline and a set of functions to query and set a NURBS spline. It is not recommended that the curve fit functions be called on a NURBS spline or that the NURBS functions be called on a curve fit spline. Invalid data, or worse, may result. The AcDbSpline::hasFitData() method can be used to determine which type of spline is in hand. 

Creating a NURBS spline with a sequential series of identical control points results in an unusable spline due to continuity problems.

See Also

SPLINE in the AutoCAD Customization Guide and AutoCAD Command Reference