Nice thingy. But I see a small flaw in capabilities.
You cannot draw not-aliased X crosses, having horizontal lines. You have only cubic voxels. Each square pixel can be divided by 4 triangles with diagonals (from center), so every voxel could be divided by 4*6 tetrahedrons (6 pyramids, each splitted by diagonals, resulting in 24 tetrahedrons).
Cubic voxels can be automatically smoothed with these tetrahedrons, or these "subvoxels" can be set manually where needed.