Get mapping after remeshing

[emil-peters]

Hi all,

When remeshing the blue area in the image below, the border edges can get subdivided or merged into a longer edge.
It is simple to get the face_bitset after remeshins of the newly added faces in the requested region.

But there are faces lost and added along the region boundary.
Is there a way to easily get a mapping of these faces from oldFaceID -> newFaceID or is there another way you would manage this?
We would need to map regions to before and after remeshing, and these faces mess with our logic since the number of faces and indices no longer match

[Grantim]

Hello!

The easiest way is to use these callbacks to track topology updates during remesh:

MeshLib/source/MRMesh/MRMeshDecimate.h

Lines 276 to 279 in a838bfc

	/// this function is called each time edge (e) is split into (e1->e), but before the ring is made Delone
	std::function<void(EdgeId e1, EdgeId e)> onEdgeSplit;
	/// if valid (e1) is given then dest(e) = dest(e1) and their origins are in different ends of collapsing edge, e1 shall take the place of e
	std::function<void(EdgeId e, EdgeId e1)> onEdgeDel;

Also you can mark region boundary edges as non-flappable.

[Fedr]

If you re-mesh some region on a mesh, then you specify it in FaceBitSet * RemeshSettings::region setting. After re-meshing is done, the region will updated with all valid triangles inside the region (new triangles inside the region will be added there).

As to exact mapping, between old and new triangle, it may not always exist, because only region boundaries and manually specified edges are maintained. Other edges can move during re-meshing. So one new triangle can partially overlap several old triangles and vice versa.