Ray intersection behavior on edges and vertex

[brandonrwin]

RayMeshIntersector.intersects_location() has the following comment:

If you are counting the number of hits a ray had, this method
should be used as if only the triangle index is used on- edge hits
will be counted twice.

I'm having trouble understanding this, but I think what it's trying to say to me doesn't go far enough.

There's three possibilities for a ray hitting a "sane" mesh:

1. Hits the interior of a triangle: one hit, one triangle
2. Hits an edge: two hits, one for each triangle the edge belongs to
3. Hits a vertex: n hits, one for each triangle the vertex belongs to

So, my questions are:

1. How does trimesh.ray.raytriangle handle 3? From the comment, it's not clear what I should do in the case of 2 or 3. Maybe we can come up with something that provides some more direct guidance for edges, and a clear indication of vertex behavior.
2. As is, the comment appears to be incorrect when embree is used (see below).

For embree's handling of edge and vertex hits, a develop said:

• Vertex hits are not detected reliably, for performance reasons.
• Edge hits are only detected reliably if the RTC_SCENE_ROBUST flag is used when making the scene, which pyembree does not do.
  • It looks like the difference is whether Pleucker (wiki) or Moeller (wiki) is used when the bvh accelerator is instantiated. But, Pleucker is only watertight at edges, as explained by this paper, which does solve the problem (maybe at a performance cost).

For context, I'm making a, probably futile, attempt to quickly use this in a real world scenario, where a ray passing through without detection means real world things get smashed. 😅

[brandonrwin]

What do you think of this, for a modified comment?

Edge and vertex hits will not be detected reliably. Edges can result in 0, 1, or 2 hits and vertices can result in 0 to n hits, where n is the number of triangles that share that vertex. Multiple rays, with slight offsets, can be used to help prevent the 0 hit case.