How we detect collision in d3d. Do we detect it using untransformed vertices or transformed. How can we reach the transformed vertices if directx has performed the transformation. One way can be locking the vertex buffer do we always use that (for detecting collision and determining co-ordinate of rectangle that is to be redrawn). I have tried D3DXVec3Project function but that give strange results. Its one parameter is viewport. We do not always manually set a viewport as in directx 8 samples. Then how can we call that function.