For NeRF this problems exists. However, in the past it was already solved for gaussian splatting. Usually you define a normal field over the (2D) splat, This a

For NeRF this problems exists. However, in the past it was already solved for gaussian splatting. Usually you define a normal field over the (2D) splat, This allows you to have phong shading at least. It is not too difficult to go to a 2D normal field over the 3D gaussians..