Yes, we do internal cube face calculation. What I saw with interpolator is that the one you use doesn't really change anything once you have a bicubic at least in quality. I've made a flash animation on the documentation page you can play with. I think that you really don't need something better and that's why the interpolators have been hidden from public setting. We have them ( nearest neightboor until spline 64 ), but really this give too much complication.