NURBS Curves: Knot Insertion

Since NURBS curves are the projections of 4D B-spline curves to 3D, knot insertion for NURBS curves is easy. In fact, knot insertion for NURBS curves is done in three steps: (1) converting the given NURBS curve in 3D to a B-spline curve in 4D, (2) performing knot insertion to this four dimensional B-spline curve, and (3) projecting the new set of control points back to 3D to form the the new set of control points for the given NURBS curve.

Suppose we have n + 1 control points P₀, P₁, ..., P_n with associated weights w₀, w₁, ..., w_n, a knot vector U and a degree p. Let P_i = (x_i, y_i, z_i). Then, control points P^w_i = ( w_ix_i, w_iy_i, w_iz_i, w_i ), 0 <= i <= n, and knot vector U define a four dimensional B-spline of degree p. The insertion of a new knot t to this four dimensional B-spline yields a new set of control points Q^w_i = ( X_i, Y_i, Z_i, W_i ), 0 <= i <= n. Projecting them back to the three-dimensional space by dividing the first three components with the fourth yields the new set of control points of the given NURBS curve.

Let us take a look at an example. Suppose we have 9 knots

and a NURBS curve of degree 3 defined by the following 5 control points in the xy-plane:

The following shows the curve and its basis functions.

Let us insert a new knot t = 0.4. Since t is in knot span [u₃, u₄) and the degree of the NURBS curve is 3, the affected control points are P₃, P₂, P₁ and P₀. Since this is a NURBS curve, we shall use homogeneous coordinates by multiplying all control points with their corresponding weights. Call these new control points P^w_i:

Note that since P₄ is not affected, it is not computed in the above table. Then, we shall compute a₃, a₂ and a₁ as follows:

The new control points Q^w₃, Q^w₂ and Q^w₁ are:

Projecting these control points back to 2D by dividing the first two components with the third (the weight), we have

The following is the resulting NURBS curve and its basis functions: