การหาสมการ Delaunay ถือได้ว่าเป็นตัวนูนด้านล่างของจุด 2d ที่ยกขึ้นไปยังพาราโบลา ดังนั้นหากคุณตั้งค่าจุด 2d ของคุณและกำหนดให้กับทุก ๆ จุด a- z -coordinate z i = x 2 i + y 2 1ดังนั้นการฉายภาพของเปลือกนูนด้านล่างลงในx y -plane ช่วยให้คุณสมการ Delaunay(xi,yi)zzi=x2i+y21xy
Using this perspective, what does it mean for an edge (pi,pj) to be illegal? First of all, for every triangulation T we can use the parabolic map to get a 3d (triangulated) surface that projects down to T. Of course, this surface is not necessarily convex, if it would be convex, T would be the Delaunay triangulation. Simply speaking, the edge (pi,pj) is an obstruction for the convexity of the surface, a concave edge. When flipping this edge we change the situation on the lifted surface only locally. So lets look at the 4 points pi,pj,pk,plpipjpkpipjpl define the the concave edge (pi,pj), the triangles pkplpi and pkplpj define the a convex edge (pl,pk). Therefore, flipping an illegal edge corresponds to replacing a concave edge by a convex edge in the lifting. Notice that this flips might turn other convex edges to concave edges.
Remark: The image is not geometrically correct and should only be considered as a sketch.
Let T′ be the triangulation after the flip. The lifted surface of T′ "contains" the surface of T. By this I mean that if you watch the two surfaces from the xy plane you see only triangles from the surface of T′ (or triangles that are in both surfaces). You could also say that the surface of T′ encloses more volume. Also, the edge (pi,pj) lies now "behind" the lifted surface induced by T′ when watching from the xy plane.
During the flip sequence we get a sequence of surfaces with strictly increasing volume. Thus, the edge (pi,pj) lies "behind" all these surfaces. Hence, it can never reappear during the flipping process. Since there are only n choose 2 possible edges, we have at most O(n2) flips.