I'm just getting my headround using OpenGL ES for Android... I'm wanting to use it for 2d graphics so have a produced a simple example program which puts a square on the screen with a texture on it... After a long time I got this to work fine but I'm having problems understanding how to know where like the screen edges are if that makes any sense?

So I draw the texture in the middle of the screen, but as I see I'm moving in 3d space, how can I know where the edge of the screen is? I obviously need to know this for 2d games. So if you've got a square with 4 points in 3d space, if I moved the square, how would I know when it hits the edge of the screen? Or indeed the dimensions of the screen reltive to the squares position?

Thanks a lot, I hope that makes sense.