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But how do we know which edge collapse to perform first? We want to make sure not to modify the topology of the object, or of the materials within the object. We ensure this be classifying the edges in the model. In particular a spanning edge should never be collapsed as this would create a point of zero thickness in a layer.

Edges are also given a collapse weight which indicates how much the boundary will be affected by the operation. Many weighting functions are possible.

Finally the collapse should not produce any negative volume elements or significantly worsen the local element quality.