topic Re: AcGeNurbSurface - control points in ObjectARX Forum

AcGeNurbSurface - control points

Anonymous — Thu, 19 Apr 2018 07:53:55 GMT

Hello everyone,

I am trying to get the control points from an AcGeNurbSurface entity. What I found from the documentation is that I can use the method getControlPoints. However, this method does not mention how the control points are arranged, if they vary most quickly in the u direction and then in the v direction or vice versa?

In addition, I was expecting to find a method to get a single control point at specific location. So, I can revert by myself.

Could anyone explain me how the control points are arranged and if there is any other way to obtain more specific information for them?

Respectfully,

Evangelos

Re: AcGeNurbSurface - control points

tbrammer — Thu, 19 Apr 2018 15:43:31 GMT

I use the follwing code to convert some 3rd party NURBS surface data to AcGeNurbSurface:

bool setNurbSurface(AcGeNurbSurface &acSurf, const ExternalNurbSurface &extNurbSurf)
{
	int degreeU = extNurbSurf.DegreeU(); 
	int degreeV = extNurbSurf.DegreeV();
	//props: See AcGe::NurbSurfaceProperties 
        //enum NurbSurfaceProperties {kOpen=0x01, kClosed=0x02, kPeriodic=0x04,
        //                            kRational=0x08, kNoPoles=0x10, kPoleAtMin=0x20,
        //                            kPoleAtMax=0x40, kPoleAtBoth=0x80};

	const ExternalNurbSurfNet surfNet=extNurbSurf.SurfNet();
	bool bIsReversed = extNurbSurf.IsReversed();

	int propsInU = AcGe::kOpen; 
	int propsInV = AcGe::kOpen; 

	AcGeDoubleArray weights;
	AcGePoint3dArray controlPoints;
	AcGePoint3d acPt;
	double w, f;
	int numControlPointsInU = (int)surfNet.rows();
	int numControlPointsInV = (int)surfNet.columns();
	int u,v;

	for (u=0; u<numControlPointsInU; u++)
	for (v=0; v<numControlPointsInV; v++)
	{
		const ExternalPoint &extPt = surfNet[u][v];
		w = Weight(extPt.w());
		f = (fabs(w) > 1e-10)?  (1.0 / w) : 1.0;

		acPt.x = f*extPt.x();
		acPt.y = f*extPt.y();
		acPt.z = f*extPt.z();

		if ((u>0) && (w != Weight(surfNet[u][0].w())) )
			propsInU |= AcGe::kRational; 
		if ((v>0) && (w != Weight(surfNet[0][v].w())) )
			propsInV |= AcGe::kRational; 

		controlPoints.append(acPt);
		weights.append(w);
	}

	ExternalKnotVectors extKnots = extNurbSurf.GetKnotVectors();
	size_t nUknots = extKnots.UKnots();
	size_t nVknots = extKnots.VKnots();
	AcGeKnotVector uKnots, vKnots;

	for (u=0; u<nUknots ; u++)
		uKnots.append(extKnots.GetUKnot(u));

	for (v=0; v<nVknots ; v++)
		vKnots.append(extKnots.GetVKnot(v));

	acSurf.set(
		degreeU, degreeV,
		propsInU, propsInV,
		numControlPointsInU, numControlPointsInV,
		controlPoints.asArrayPtr(),
		weights.asArrayPtr(),
		uKnots, vKnots
	);

	if (bIsReversed)
		acSurf.reverseNormal();

	return true;
}

As you can see in the two for-Loops the controlpoints are ordered like this:

[u0,v0], [u0,v1], [u0,v2], [u1,v0], [u1,v1], [u1,v2], [u2,v0], [u2,v1], [u2,v2], ...

Re: AcGeNurbSurface - control points

Anonymous — Fri, 20 Apr 2018 07:47:18 GMT

Thank you very much for your reply tbrammer!

So, AutoCAD assumes by default that the control points vary most quickly with v and then with u. In my opinion, it must be mentioned in the description of the control points of the AcGeNurbSurface.

Kind regards,

Evangelos