Smooth shading with crease angle
From Solid Graphics Wiki
Smooth shading with crease angle is a technique of drawing a 3D model which targets to solve disadvantages of flat and smooth shading. Three normal vectors are calculated for each model's triangle, one for each triangle's vertex. If triangle's edge angle with neighbour triangle is below specified crease angle then normals for the edge vertices are calculated using flat shading rules, otherwise the normals are calculated using smooth shading rules. The disadvantage of this approach is that it requires calculation and storing of about three times more normal vectors than smooth or flat shading coloring models.
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