Nonnegative surface fitting with Powell-Sabin splines. - Karin Willemans; Paul Dierckx (Numer. Algorithms, 1995)

Smoothing scattered data with a monotone Powell-Sabin spline surface. - Karin Willemans; Paul Dierckx (Numer. Algorithms, 1996)

Smoothing scattered data with a constrained Powell-Sabin spline surface. - Karin Willemans (1996)