Splines

Some common questions and answers about SOLIDWORKS and splines are presented in this help topic. 

What are the different types of splines used in SOLIDWORKS?

Any curve in SOLIDWORKS that is not an analytic curve, such as a line, arc, conic, or a composite curve, is represented as a b-spline. SOLIDWORKS uses standard b-splines as defined in computer-aided design literature.
 

What are the mathematical names for b-spline and p-spline? What is the difference between ICurve::GetPCurveParams2 and ICurve::GetBCurveParams5? Do these methods return parameters for two different kinds of splines, or do they return parameters formatted in two different ways for a single spline type?

Names such as p-spline, or p-curve, are used for b-splines in a particular context. If you are unsure of the context, then you might find the names confusing. For example, p-curves, as in IModeler::CreatePCurve, are b-splines that lie on a surface and are created from b-splines in the parameter space of the surface.

Any b-spline is a piecewise polynomial curve, and a p-spline is a b-spline represented as the polynomial coefficients that define the polynomials in each segment of the b-spline.

• ICurve::GetBCurveParams5 gets the control point/knot standard b-spline representation for the input curve.

• ICurve::GetPCurveParams gets the polynomial coefficients for the segments of a b-spline for the input curve.

• ICurve::GetPCurveParams2 is a hybrid of ICurve::GetBCurveParams5 and ICurve::GetPCurveParms. You can use either the control point/knot representation, which it returns in the SplineParamData object, or the polynomial coefficient representation for the individual segments.

When can you edit the control points of a b-spline?

You can edit control points when creating a b-spline; however, you currently cannot replace control points of an existing b-spline.

What kind of spline is returned by ISketchManager::CreateSpline2, IModelDoc2::SketchSpline, and IFeatureManager::MakeStyledCurves2? 

All three methods create b-splines.

• ISketchManager::CreateSpline2 creates a b-spline using the input points as through points, or interpolation points, with some control over the curve’s end conditions.

• IModelDoc2::SketchSpline is an attempt to simulate the way you create a b-spline in the SOLIDWORKS user interface, one point at a time.

• IFeatureManager::MakeStyledCurves2 fits a b-spline that approximates a composite curve of sketch segments.

What b-spline points for the curve does ICurve::GetSplinePts return? 

ICurve::GetSplinePts gets a spline's through/interpolation points, i.e., the end points of its individual spline segments, as opposed to its representation by knots and control points.