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Between the two ears in the following bunny images, there are some degenerate triangles I want to detect. It looks like a volume-less thin slits.

If the question is not clear, please let me know.

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enter image description here

enter image description here

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What about this:

Step 1: For each edge $P_iP_j$, look at the two triangles that share it: $P_iP_jP_k$ and $P_lP_jP_i$.

Step 2: Compute the counter-clockwise normal on each of the two triangle: $$\vec{n}_{ijk} = \frac{\vec{P_iP_j} \times \vec{P_jP_k}}{\|\vec{P_iP_j} \times \vec{P_jP_k}\|} \,\,\, \text{ and }\,\,\, \vec{n}_{lji} = \frac{\vec{P_lP_j} \times \vec{P_jP_i}}{\|\vec{P_lP_j} \times \vec{P_jP_i}\|}$$

Step 3: Check the angle between the two normals: $\big(\vec{n}_{ijk} \cdot \vec{n}_{lji}\big) \approx -1$ (e.g. something like $\big(\vec{n}_{ijk} \cdot \vec{n}_{lji}\big) \leq -1 + \varepsilon$) means it is close to being $180^{\circ}$.

Step 4: In this case, you can remove the edge $P_iP_j$ and merge the two vertices $P_K$ and $P_l$: Replace $P_k$ and $P_l$ by the midpoint $P_m$ of the straight segment $P_kP_l$: $$P_m = \frac{1}{2}(P_k + P_l)$$

Step 5: Connect all remaining vertices, adjacent to the former vertices $P_k$ or $P_l$, to the new vertex $P_m$. Thus, the new triangulation has now three fewer edges and two fewer triangular faces.

Continue to iterate this procedure until there are no edges whose normals have the property $\big(\vec{n}_{ijk} \cdot \vec{n}_{lji}\big) \approx -1$.

Maybe this algorithm will remove the triangles between the bunny's years.

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