# Transform planes

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I just tried to use User Clip Planes (XNA) to cull everything below a particular plane. Since the culling is done after the vertex shader, I must transform the plane with the same transform as the vertices. Normally, if I have a plane, I just transform the normal with the inverse-transpose of my transformation-matrix. But now that I got translations and 4-dimensional matrices, I got a little confused. How should I look at it? Cause somehow I need to take the translation into account, and multiply the vector (normalx, normaly, normalz, constant) with the inverse-transpose.

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Treat your plane as a 4D vector (Nx, Ny, Nz, d) where d = -(N dot P) for any point P in the plane. Then you can transform it by multiplying it by the inverse transpose of the full 4x4 matrix.

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