Calculating and Applying normals to a triangle mesh

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 triangl开发者_开发百科e (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).

If I'm correct you're still only computing a normal per triangle? This is correct, but after that you should computed what the normal is per vertex. This is simply the normalized sum of all triangle normals that the specific vertex is attached to. Once completed you can proceed with your immediate mode drawing, specifying a normal per vertex.