Rhino 7 and Sections through Mesh objects

Rhino Developer
1

Hi Dale,
I’ve noticed a change in behavior when creating contours/sections through mesh objects between Rhino 6 and 7. Below is a simple command that cuts 9 sections through a sample mesh. When I run this in Rhino 6 I get 9 closed curves and their direction (cw vs ccw) is consistent. When I run this in Rhino 7 with the same model I get some closed curves and lots of curve pieces. Also on the closed curves the orientation is not consistent. Do you have any thoughts on what might be the cause? The mesh is valid and I’ve tried it on a few meshes with similar results. Thanks

Rhino 6 Mesh Curves
Rhino 6 Mesh Curves1259×586 43.6 KB

Rhino 7 Mesh Curves
Rhino 7 Mesh Curves1060×530 32.8 KB

SampleMesh.3dm (246.6 KB)

public class SampleMeshSectionCommand : Command
{
	public SampleMeshSectionCommand()
	{
		Instance = this;
	}

	///<summary>The only instance of the CreateFloatPlaneCommand command.</summary>
	public static SampleMeshSectionCommand Instance { get; private set; }

	public override string EnglishName
	{
		get { return "SampleMeshSection"; }
	}

	protected override Result RunCommand(RhinoDoc doc, RunMode mode)
	{
		RhinoGet.GetOneObject("Select mesh object to section", false, ObjectType.Mesh, out ObjRef rhinoObj);

		double[] locs = new double[] { 1, 2, 3, 4, 5, 6, 7, 8, 9 };
		foreach (var loc in locs)
		{
			var curves = Mesh.CreateContourCurves(rhinoObj.Mesh(), new Plane(new Point3d(loc, 0, 0), Vector3d.XAxis));
			if (null != curves)
			{
				foreach (var curve in curves)
					doc.Objects.AddCurve(curve);
			}
		}
		return Result.Success;
	}
}
2

@piac - is this something you can help with?

3

Hi @LarryL

I just want to let you know that I’ve debugged the origin of the problems, in fact, there are two:

  • some segments do not reconnect in the right order, and therefore a few pieces remain isolated. This is a problem of tolerances that are way too high when they arrive to the inner computations. RH-65835
  • the direction of the curves. Right now in V7, there was no convention for winding (ccw, right hand rule) and therefore the winding is accidental. RH-65833

I agree that the winding might be useful when intersecting planes, and therefore I am working on fixing it. Also, the tolerance issue can easily get fixed. I’m already working on these issues.

Thanks,

Giulio

–
Giulio Piacentino
for Robert McNeel & Associates
giulio@mcneel.com

4

Hi @piac , Many thanks for looking into these. They are both quite important for Orca3D as they are used to compute the contained area of a planar intersection curve, and since the +/- sense of the area can be different (e.g., a closed area with a hole in it) the winding is also important. We’ll look forward to the update.

Kind regards,
Larry

6

I don’t know how that function is implemented. Let’s ask @dale to please chime in.

7

RH-65835 is fixed in Rhino 7 Service Release 13

8

RH-65833 is fixed in Rhino 7 Service Release 15

9

@piac, @dale I think this issue related to an inconsistent winding direction on sectioning a mesh via Mesh.CreateContourCurves has re-surfaced in Rhino 8. I am attaching a slightly modified file (deck surfaces removed) but using the same sample command (making the slight change to the CreateContourCurves to add the tolerance in the non-obsolete method). In Rhino 7 if I look at the resulting curve orientations they are consistent as below.

image
image1492×775 173 KB

However, in Rhino 8 the same command produces some curves with differing directions as below.

image
image1345×741 194 KB

Would it be possible to implement the same consistent winding logic in Rhino 8/9 as you did in Rhino 7? Let me know if you are not able to reproduce this issue, thanks.

Larry

SampleMesh NoDeck.3dm (244.9 KB)

10

Hi @LarryL,

I’ve logged the issue so we can investigate.

– Dale

11

Thanks @dale

12

@dale, @piac, not sure if this issue may be related https://mcneel.myjetbrains.com/youtrack/issue/RH-85254

13

RH-89376 is fixed in Rhino 8 Service Release 25