A voxel is no more a cube that a pixel is a square: it's a sample point. For each sample point you store material information and the value of a scalar field at

A voxel is no more a cube that a pixel is a square: it's a sample point. For each sample point you store material information and the value of a scalar field at this point. You can then compute normals to the sampled surface using something like the marching cubes. https://en.wikipedia.org/wiki/Marching_cubes https://en.wikipedia.org/wiki/Marching_cubes