Generating Bezier/BSpline Surface of STEP file.

Okay, so I think what I should do is this:

Use a TopExp_Explorer on the Compound to iterate over all faces (TopAbs_Face):

TopExp_Explorer exp;
for (exp.Init(shape, TopAbs_FACE); exp.More(); exp.Next())
{
	ProcessSingleFace(exp.Current());
}

Then use BRepBuilderAPI_MakeFace:

void ProcessSingleFace(const TopoDS_Shape& shape)
{
	if (!shape.IsNull())
	{
		TopoDS_Face face = TopoDS::Face(shape);

		TopLoc_Location loc;
		Handle(Geom_Surface) aSurf = BRep_Tool::Surface(face, loc);
		aSurf = Handle(Geom_Surface) { static_cast<Geom_Surface*>(aSurf->Transformed(loc.Transformation()).get()) };

		Standard_Real u0, u1, v0, v1;
		BRepTools::UVBounds(face, u0, u1, v0, v1);

		if (!aSurf.IsNull())
		{
			// Make face. Result is NURBS surface(s) (right?)
			TopoDS_Wire outerWire = BRepTools::OuterWire(face);

			BRepBuilderAPI_MakeFace faceMaker{face, outerWire};
			
			// Add other wires
			TopExp_Explorer exp;
			for (exp.Init(shape, TopAbs_WIRE); exp.More(); exp.Next())
			{
				TopoDS_Wire wire = TopoDS::Wire(exp.Current());
				if(wire != outerWire)
				{
					faceMaker.Add(wire);
				}
			}

			const TopoDS_Shape& result = faceMaker.Shape();

			if(result.IsNull())
			{
				return;
			}

			Handle(Geom_Surface) surf = BRep_Tool::Surface(TopoDS::Face(result));
			Handle(Geom_BSplineSurface) bsSurf { static_cast<Geom_BSplineSurface*>(surf.get()) };

			if(bsSurf.IsNull())
			{
				return;
			}

			int uKnots = bsSurf->NbUKnots(); // -> CRASH
		}
	}
}

The problem is, that when I try to access the bsSurf after it gets created, my application crashes.

For example, even some simple like bsSurf->NbUKnots() already crashes my application. I haven't set breakpoints to check the variables, because I'm compiling this to a DLL which I use indirectly on another thread, and I don't really want to set that up if I don't need to.

Is there anything I'm doing wrong here? Or misunderstand?

I just set up debugging anyway, and I think I cannot assume that BRepBuilderAPI_MakeFace::Shape() returns a BSplineSurface?

When I look at the bsSurf variable, the first thing I notice is a mention of CylindricalSurface. Which makes me think it is not actually a BSplineSurface.

Well. I can make it stop crashing now:

// ... Use Make Face

TopoDS_Shape result = faceMaker.Shape();

BRepBuilderAPI_NurbsConvert nc{};
nc.Perform(result);
result = nc.Shape();
				
if (result.IsNull())
{
	return;
}

for (exp.Init(result, TopAbs_FACE); exp.More(); exp.Next())
{
	face = TopoDS::Face(exp.Current());

	if (face.IsNull())
	{
		return;
	}

	Handle(Geom_Surface) surf = BRep_Tool::Surface(face);

	if (surf.IsNull())
	{
		return;
	}

	Handle(Geom_BSplineSurface) bsSurf { static_cast<Geom_BSplineSurface*>(surf.get()) };
	
	// Deal with surface. WORKS
}

Unfortunately, the surface do not use the wires like I need. They are still the full surfaces...

Does the Make Face object take these wires into account at all?

Thank you :).

The thing that still eludes me is how to create (Bezier/NURBS) faces so that they incorporate the holes and contours correctly. I know the information is available. I can get the curves, and if I triangulate the shape, it is also correct. Only I don't want a triangulation.

Ah that is interesting. I am not yet very clear yet on the differences between Shape, Topo, Geom, etcetera.  I will spend some more time reading that blog and the documentation.

Thanks

Actually, I think I noticed that for a cylinder, the uvs of the Geom_CylindricalSurface were always [0,2pi] and [-inf,inf], while the uvs of the TopoDS_Face for this cylinder were correct.

So the Face contains the wires, and the Geometry is/contains a somewhat general surface of a certain type.

But do I really need to manually trim/clip the surface? I hoped OpenCASCADE would be able to do that. Because it is also able to create a triangulated mesh for the Face through BRepMesh_IncrementalMesh.

For example: suppose you're writing a plugin for Maya, and you want to import .stp files, and create nurbs surfaces from it in Maya. You would have to somehow create the nurbs surfaces, either by using OpenCASCADE functions for a given TopoDS_Face (or better yet, for any TopoDS_Shape), or by doing it manually.

One way or another, I will be exploring OCCT in the next months, but it's actually pretty important for me to just get this nurbs-ification working asap, by any means.

Just to close this and inform others how this ended up (in case others are trying the same thing). My conversation with Roman can be read in the comments here: https://opencascade.blogspot.nl/2009/02/continued.html

Roman has been very helpful and explained a lot to me which makes me understand topology/geometry and brep a lot better. He also confirmed that OCC will not help me with splitting a complex face into "regular" nurbs patches (please correct me if I understood it wrong).

Breaking down a brep face into simple nurbs patches now seems to me not a trivial task at all in general, and I'm not sure if any software offers that kind of functionality right now.

For my own project, I'm going to try some more "creative" solutions that I have in mind :P.

Thanks Roman, and also Benjamin for the help :). It's really appreciated because this project I'm working on might actually be pretty important for us over here