ON_NurbsSurface control vertices

openNURBS
1

Hi!

I am using opennurbs library, and I’m trying to create a simple square surface taken from an image:

https://postimg.org/image/4jvelpjn5/

I am setting the C & V vector sizes to the image height and width:

ON_NurbsSurface *surf = ON_NurbsSurface::New(3, false, 4, 4, im_height, im_width);

Then, for each white pixel in the image, I add a control point/vertex:

surf -> SetCV(x, y, ON_3dPoint(x, y, 0));

Now the result I’m getting is this:

https://postimg.org/image/lc6cy0mrr/

For some reason it is connecting all control points to the origin (0,0), I have no idea why.

Is there a fix for this please?

Kind regards.

2

Hi Elias,

Here is some useful background information on NURBS geometry.

http://developer.rhino3d.com/guides/opennurbs/nurbs_geometry_overview/

You can create a “square” NURBS surface as follows:

// The degree must be >= 1
int degree = 1;

// The order = degree + 1
int order = degree + 1;

// The number of control points must be >= (degree + 1)
int cv_count = 2;

// Create a non-rational degree 1 NURBS curve with 2x2 control points
ON_NurbsSurface nurbs(3, FALSE, order, order, cv_count, cv_count);

// Add "Euclidean",or world 3-D, locations for the control points
nurbs.SetCV(0, 0, ON_3dPoint(0, 0, 0));
nurbs.SetCV(0, 1, ON_3dPoint(0, 10, 0));
nurbs.SetCV(1, 0, ON_3dPoint(10, 0, 0));
nurbs.SetCV(1, 1, ON_3dPoint(10, 10, 0));

// The number of knots must be = degree + cv_count - 1
nurbs.SetKnot(0, 0, 0);
nurbs.SetKnot(0, 1, 10);
nurbs.SetKnot(1, 0, 0);
nurbs.SetKnot(1, 1, 10);

if (nurbs.IsValid())
  // TODO...

Does this help?

– Dale

3

Dear Dale,

I have already tried drawing simple shapes, and it works just fine.

But my problem is this: instead of a square, say we want to produce an ‘L’ shape.

The code given above introduces a problem in this special case:

https://postimg.org/image/fjnv85yx3/

To create the nurbs surface I do:

ON_NurbsSurface nurbs(3, false, 2, 2, 3, 3);

nurbs.SetCV(0, 0, ON_3dPoint(0, 0, 0));
nurbs.SetCV(0, 1, ON_3dPoint(0, 10, 0));
nurbs.SetCV(1, 0, ON_3dPoint(10, 0, 0));
nurbs.SetCV(1, 1, ON_3dPoint(10, 10, 0));
nurbs.SetCV(1, 2, ON_3dPoint(10, 20, 0));
nurbs.SetCV(2, 0, ON_3dPoint(20, 0, 0));
nurbs.SetCV(2, 1, ON_3dPoint(20, 10, 0));
nurbs.SetCV(2, 2, ON_3dPoint(20, 20, 0));

nurbs.MakeClampedUniformKnotVector(0);
nurbs.MakeClampedUniformKnotVector(1);

//Export to ply using pcl:
pcl::PolygonMesh m;
pcl::on_nurbs::Triangulation::convertSurface2PolygonMesh (nurbs, m, 256);
pcl::io::savePLYFile("m.ply", m);

The code above produces this result:

https://postimg.org/image/xdfh6sd1z/

Which is false. I hope I provided enough information.

Kind regards,
Elias.

P.S. I am not providing any knot values, which I believe get generated automatically. However, I don’t think they are the cause of this.

4

Hi Elias,

It is not possible to define a NURBS surface with that kind of shape.

What you will need to to is create the planar surface and then defined the trimmed area using an ON_Brep object.

Rolling your own breps can be challenging. But there is plenty of good information in the openNURBS headers (opennurbs_brep.h) that will help you with this.

Also, here are a couple of samples that might be worth studying. Note, these samples are from the Rhino SDK samples repository. But the use openNURBS…

SampleTrimmedPlane
SampleFaceWithHole
SampleTwistedCube

Let me know if this helps.

– Dale

5

Mr. Dale,

Now I’m using a different method to create the nurbs surface.

Thank you!