I do not understand. NURBs are just one class of splines, Non Uniform Rational B-splines, with some interesting math to let them approach quadratics more accuratly. So a NURBs modeller is a spline modeller. What sort of splines does Hash use? Here's a really nice pointer to some of the math issues: http://www.csl.mtu.edu/cs390-2/www/NOTES/notes.html