Triangular subpatches of rectangular Bézier surfaces

A formula is presented for describing triangular subpatches of rectangular Bézier surfaces. Calculations using it are numerically stable, since they are based on de Casteljau recursions and convex combinations of combinatorial constants. Several examples of quadratic, cubic and quartic subpatches are given, and the bi- and the quadripartition of a rectangular Bézier surface are discussed.