FA-88782 / Mesh topology invariants / Member archive

Edge split only splits the face storing a->b · case 02

The neighbour across the edge keeps a T-junction.

Member previewVariant 2 · 3 implementations · 7 checks per implementation

Case contract

Input [faces, a, b] for a triangle mesh (any non-triangle returns "non-triangle"). Insert vertex m = max index + 1 on edge {a,b}: each triangle (u,w,c) with {u,w}={a,b} along its boundary becomes [u,m,c] then [m,w,c] at its original position. No such triangle returns "no-edge". Return [m, faces, number of split triangles].

Why this case matters

Mesh processing pipelines (remeshing, export, simulation, printing) trust these topological counts and adjacency answers; a wrong invariant silently accepts broken meshes or rejects valid ones.

One recorded failure

Sample boundary fixture

This sample comes from the broken implementation of a controlled reproducer.

Boundary fixture	Actual	Expected	Outcome
gapped indices	`[8, [[1, 8, 0], [8, 5, 0], [1, 7, 5]], 1]`	`[8, [[1, 8, 0], [8, 5, 0], [5, 8, 7], [8, 1, 7]], 2]`	Failed

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