A nonuniform rational B-spline curve defined by C(t) = ( sum_(i = 0)^n N_(i, p)(t) w_i P_i)/( sum_(i = 0)^n N_(i, p)(t) w_i), where p is the order, N_(i, p) are the B-spline basis functions, P_i are control points, and the weight w_i of P_i is the last ordinate of the homogeneous point (P_i)^w. These curves are closed under perspective transformations and can represent conic sections exactly.

Bézier curve | B-spline | NURBS surface