This menu item is for specifying and displaying curve segments. The supported curve types are Bézier, rational Bézier, B-spline and NURBS curves, where different curve types have slightly different options. Once a curve type is selected, its type is displayed in this item.
Normally, one should select a curve type followed by creating control points. If control points have already been created on the drawing canvas with a wrong curve type, one can use Change Current Curve to to change the curve type without changing the control points.
Once B-spline or NURBS is selected, this program assumes the curve has degree 3 and displays this degree in the Degree menu. If degree 3 is not suitable for a particular application, it can be changed with the Degree menu. Note that the maximum degree is 10 for B-spline and NURBS curves, which is sufficient for most applications.
For a B-spline and NURBS curve, it is required to tell this system if the curve should be displayed as an open curve, a clamped curve or a closed curve. These three types of curves correspond to with Uniform Knots, with Clamped Knots and with Closed Knots, respectively.
The above figures, from left to right, show open, clamped and closed B-spline curves of degree 4 with the same set of control points. An open B-spline/NURBS curve does not pass though the first and the last control points. This system displays an open B-spline/NURBS curve with uniformly spaced knots. A clamped B-spline/NURBS curve is tangent to the first and last legs at the first and last control points, respectively. This system displays a B-spline/NURBS curve with the first p+1 knots and the last p+1 knots clamped to 0 and 1, respectively. All internal knots are uniformly spaced. For a closed B-spline/NURBS curve, the knots are still uniformly spaced; but some knots at both ends are fixed and cannot be modified.
Note that except for closed B-spline/NURBS curves, one can always manipulate the knot vector to make an open B-spline/NURBS a clamped one and vice versa.
This item can be selected at any time to change its knot pattern after a B-spline and NURBS curves has been displayed.