[arghbleargh]

Isn't it simple enough to make three bounds checks to essentially do the same thing you did with the zero padding? You can even make it a function

    is_in_bounds(x, y, z)

[jordigh]

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. :-)