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The first texture applied to a slice is the data texture. It is a 3D texture with three values per sample point: one for primary data value V, one for the gradient magnitude G, and one for the second derivative. Since we measure this quantity using a Hessian, we call it H.

Next is the transfer function. Since we are dealing with 3D transfer functions, it is represented either as a 3D texture where each axis maps a variable from the data texture, or it can also be composed as a 2D and a 1D texture.

Normals for shading are stored in a separate texture. They are derived from the normalized gradient vectors at each sample. The normal texture is a 3D texture with three values at each sample point.