sentences:  Some would reasonably insist, in order to give intuitive geometric meaning to the word `control', that the blending functions form a partition of unity, i.e., that the formula_6 are nonnegative and sum to one.This property implies that the curve lies within the convex hull of its control points.This is the case for Bézier's representation of a polynomial curve as well as for the B-spline representation of a spline curve or tensor-product spline surface.