Intersection between a plane and a Manifold

[johnnybigert]

Hi!

I would like to find the polygons with holes that constitute the intersection between a plane and a Manifold. I understand that I could use the TrimByPlane function and retrieve the polygons from the resulting Manifold, but it seems wasteful to do a full boolean operation for this. Is there a more efficient way?

Also, I would be really interested in mapping the edges of the intersection back to the original triangles of the Manifold (i.e., for a given edge in the intersection, what triangle in the original Manifold did it originate from). Would that be possible to accomplish?

[pca006132]

This should be #426 and is yet implemented.

[johnnybigert]

Clever with the extra property on the vertices, I will try it out!

We don't have to project to infinity, it's sufficient to stop at the bounding box Z min. No normals in the upper envelope should be pointing down (except the bottom), all normals should be pointing up or be vertical.

I'd love to set up a video call to describe what we're building!

[elalish]

You know, I have too many video calls as it is 😁. I work open source, so think I'm going to take a page out of McNeel's book (they make Rhino) and say I'm not keeping anyone's secrets: I won't sign NDAs and if you'd like my opinion on something, let's talk about it publicly in writing. Any startup founder who's worried about someone stealing their idea has not yet figured out what makes success.

[johnnybigert]

Of course, no worries. It's just hard to describe without interactivity.

I created this helper in the Python bindings:

          "create_unique_vertices",
          [](Manifold self) {
            Manifold out = self.SetProperties(
              1, [](glm::vec3 pos, const float* oldProp, float* prop) {
                prop[0] = 0.5;  // change to random later
              });
            return out;
          },
          "Creates a new Manifold with unique vertices (so that each triangle"
          "becomes its own faceID).")

and run the following Python code in a Jupyter Notebook:

manifold_raw = Manifold().from_mesh(mesh)
manifold = manifold_raw.create_unique_vertices()
test = manifold.to_mesh()
trimmed_manifold = manifold.trim_by_plane((0, 0, 1), height)
trimmed_manifold.num_vert()
trimmed_mesh = trimmed_manifold.to_mesh()     # crashes

It's interesting that all operations on manifold and trimmed_manifold work out, except the last one. Do you have any ideas why to_mesh would crash? (It all works if I don't run create_unique_vertices, so the crash is related to the new prop.)

[elalish]

Hmm, that looks like a bug; we don't have nearly enough testing on our Python bindings, so we may have an error. Also, did you add the setProperties binding? I don't think anyone else has yet...

You can verify it's a binding issue by adding a similar C++ TEST and ensuring that passes.

[johnnybigert]

I will get back to this in a while and post back here with the results! Meanwhile, I'm including Manifold in some production code scenarios to put it through some real tests ;)