It's n-dimensional, not 3-dimensional. Also, it's really slow and wasteful to be doing bounds-checking for each voxel of an image that is almost completely not

It's n-dimensional, not 3-dimensional. Also, it's really slow and wasteful to be doing bounds-checking for each voxel of an image that is almost completely not a boundary, whereas you can't avoid checking if each voxel is lit. :-)