I have a triangle mesh and I'm trying to calculate the normals so I can apply them when drawing the mesh. I'm using immediate mode (will probably change to vertex arrays when I get time to understand how they work) and drawing the mesh with GL_TRIANGLE_STRIP.
I am having trouble calculating the vertex normals. More precisely deciding which neighbouring vertices to use in the calculations and then deciding when to set those normals. Consider this:
1_2
|/| Supposedly a square where the numbers represent the vertex number in a
3 4 triangle strip.
I know you have to compute the cross product of 2 vectors belonging to a plane in order to get the plane normal. So in that example the top triangle's normal could be calculated by doing (2-1)x(3-1), and the second one by doing (2-4)x(3-4). How do you then apply the normals when drawing the triangle strip in immediate mode?
What I was doing was setting the first normal when vtx 1 is set, the second when vtx 4 is set, the third when vtx 5 is set, etc. This however gives issues as you obviously end up by having different normals for each of the vertices of a triangle (when they should all be the same). For instance, triangle |2,3,4| would only have vertex 4 with the correct normal (since for vertices 2 and 3 the normal would be the one of the first triangle).
So how should it be done? Is there a way, or do I need to change to GL_TRIANGLES? (I don't want to stop using immediate mode for now as I don't have time).
