Independent sampling intervals along all the major axes are always defined {unit_x, unit_y, unit_z}.
The origin of any reference object is always considered to be {0,0,0} in real space.
The sampling unit is angstrom/voxel edge length.
The origin of this object within the image is therefore given as the location in voxel units, so that the real space coordinates in angstroms are always given by: real_x = ( x - origin_x ) * unit_x.
Independent sampling intervals are supported because assumptions such as isotropic sampling are restrictive and does not allow a sensible treatment of cases where for instance the z-direction is not as well sampled as the other directions.