Neat stuff. It's worth noting that shadow rays don't entirely avoid precision issues either. You still need to handle self-intersections. One option is to add a bias epsilon (which introduces other artifacts, depending on the scale); another is to reject local self-shadowing (which means you can't get finely shadowed surface cracks); another is to give artists explicit control over which sets of objects shadow which others (which adds artist time). Offline rendered movies use all of these.