Rational biangle surface patches

The concept of the rational biangle surface patch of the degree 2n is introduced. The construction is mostly close to (n, n) tensor product surface case because it has (n+1)2 control points and the implicit degree 2n2 in general. The biangle has many similar properties: a convex hull property; boundary Bézier curves can be easy calculated; a subdivision and degree elevation algorithms are available. The quadratic biangle (when n = 1) is a patch on an oval quadric surface with four control points. In particular, one can realize any biangle with two cyclic arcs on the sphere