Determining impingement between first and second bodies by determining a spatial relationship between surface points of the first body and markers of a first reference element; determining a spatial relationship between surface points of the second body and markers of a second reference element; displacing the first and second bodies relative to the other body; determining 3D coordinates of the respective pluralities of markers; and using at least (1) the 3D coordinates of the respective pluralities of markers of the reference elements, (2) the determined relationship between surface points of the first body and markers of the first reference elements, and (3) the determined relationship between surface points of the second body and markers of the second reference elements, to determine whether a surface point of one of the first and second bodies coincides with a surface point of the other body.