# Triangle Problem

when i was reading an artical, i do not understand why the size of edge array is 3f/2 , f is number of the triangles.
for example,2 triangles connected with each other have 4 edges not 3*2/2=2.
Fellowing is part of the artical:

Struct Model:
Triangle triangle[f]
Edge edge[3 f / 2]
Vector3 vertex[v]
Box boundingBox
Sphere boundingSphere

A model must be closed so that every edge connects exactly two triangles and there are no Tjunctions.
A model with f triangles that meets this definition is geometrically constrained so that
the size of the edge array is 3 f / 2.

Only one edge of the two triangles is shared, not alls.

Two triangles doesn’t make an closed model …

oh thank you very much . i ignore the word “close”

Happy to have help you to learn a world today If you want to extend your two triangles example, you can make a “recto/verso” version of it, cf. two triangles for the recto and two triangles for the verso

=> it’s a closed model where you have 4 triangles and 6 edges that are alls shared (the four edges of the quadrilateral + the diagonale / in the recto and the diagonale \ in the verso because not Tjonction …)

==> (3x4)/2 = (12)/2 = 6 CQFD

For another simple closed model, you can test with a pyramide for example