Question

I have triangle collection that define mesh surface of my 3D shape,
I would like to fix normal of each triangle to point outshape.

I was trying the following (pseudo):

1. define that first triangle normal direction is right direction

2. go over the mesh using kind of DFS like this:

3. triangle = first triangle

4. foreach neigbour in triangle.getNeighbours

5. if angle between neighbor and triangle greater then 180 do neighbor.flip()

6. triangle = neighbor

7. if neighbor already picked then continue to next neighbor

8. continue recursively to 4.

But step 5 in algorithm does not work because cant know if the angle is greater then 180, because i need to know in witch direction (clockwise or counter clockwise).

Could you please help me to understand how to fix the algorithm?

Answer 1

There is no need to calculate angle between neighbours. If triangle ABC is oriented (A->B->C), and if neighbour on edge AB is triangle ABD, than it has to have opposite orientation on edge AB. That means, neigbouring triangle has to be orientated B->A->D.