A 3D texture can be used with arbitrary geometry without much additional work if your OpenGL implementation supports 3D textures. However, generating a 3D noise texture requires a large amount of memory and a large number of passes, especially if you use a filter that convolves a large number of input values at a time. A 2D texture as we just described doesn't require nearly as many passes to create, but it does require knowledge of the geometry and additional computation in order to properly shape the spot.