给定(n+1)×(m+1)个空间点阵r_ij(i=0,1,…,nj;j=0,1,…,m),双三次B样条曲面可分块表示为     r_l,k(u,v)=∑3i=0∑3 j=0 Ei,3(u)Ej,3(v)r(i+l)(j+k),     0≤u,v≤1,l=0,1,…,n-3,k=0,1,…,m-3(211) 其中 基函数为     E0,3(t)=(-t3+3t2-2t+1)/3!,    E1,3(t)=(3t3-6t2+4)/3!,    E2,3(t)=(-3t3+2t2+3t+1)/3!,    E3,3(t)=t3/3! 变量t可用u或v代替,这里r_ij称为deBoor点