How many vertices does a triangle fan use opengl

Question

I am learning openGL, and i have come across triangle fans using vertex buffer objects. If given an array of vertices to render, how does openGL decide how many of those ve

Answer 1

Visually, this is how a triangle fan works:

Each triangle shares the central vertex A, and re-uses the last vertex addressed. Thus after defining ABC each following triangle only requires 1 point (e.g. D, E, F).

Indices:     A,B,C,D,E,F     [Count: 6]
Triangles:  (A,B,C)
            (A) (C,D)
            (A)   (D,E)
            (A)     (E,F)    [N=4]  -->  4+2 = 6

Another way of thinking about this is that each triangle shares an edge radiating from a central vertex with the prior triangle; literally like a folding a paper fan.

      

You need N+2 vertices, where N is the number of triangles in your fan.