Fig. 7.

A bivariate basis determined by ${\mathit{\Xi }}_I=\{1,1,1,2,3,4,4,4\}$ and ${\mathit{\Xi }}_J=\{1,1,1,2.5,2.5,4,4,4\}$ for $u$ and $v,$ respectively: a shows the bivariate basis spanned by ${\mathit{\Xi }}_I$ and ${\mathit{\Xi }}_J,$ whereas b illustrates the construction of a corresponding bivariate B-spline