NURBS Curve Manager
Tools to manage (N)URBS, (non)rational Bezier and freehand curves, arcs, circles and lines
NURBS Curve Manager

Once a Non-Uniform Rational Basis Spline(NURBS) curve from this extension, Uniform Rational B-Spline Curve, Bezier curve(non-rational or rational), freehand curve with up to 20 segments, arc, circle or line is selected, the tools in this extension, demonstrated in the animations above and described below, are available under the Tools Menu, in the Right Mouse Button Context Menu and in View > Toolbars under NURBS Curve Manager.

Inferencing is available in all the tools when a control point, control line or point on the curve itself is moved, along with tooltips about the inference. Also, the movement can be axis constrained with the Shift key. If the Shift key is held down for more than half a second, then the constraint is unlocked.  Otherwise, pressing Shift once locks and a second time unlocks the constraint.  The lock direction inference clue (the heavy, by-axis color) is also shown. Finally, hitting the Esc key during a move will reset the curve to the way it was before the move started.

Manipulate curve - Other than editing the selected curve by moving its control points or control lines, you can grab a point on the curve itself and designate a new location for it, intending that the new curve should pass through the changed location. This is useful for designers who may not like the concept of control points and lines and would prefer to manipulate points on the curve directly.

You can also influence the shape of the curve by changing the weights of the control points. To enter weights explicitly, hover the cursor over the non-end control point you want to change the weight of so that it is highlighted in green, type the new weight you want for it, without moving the cursor away from the control point, and press Enter or Return. Or grab a weight point, marked by triangles, and move it along its control line as a shape parameter to have the weights recomputed for you. It may be preferable for a designer to use these geometric handles rather than requiring inputting numbers for the weights. Note that the effect of changing a weight is different from that of moving the control point.

Manipulate Knots – To add knots or to increase the multiplicity of existing knots, enter the knot value you want and press Enter or Return. You can also add knots by clicking at the locations where you want them on the selected curve. To delete knots, click on the knots you feel are no longer necessary.

Change Degree - The current degree of the selected curve is displayed in the Value Control Box(VCB). To change the degree, type the new value you want and press Enter or Return.

(i) Degree elevation - After modifying the control polygon, curve and weights a few times, it may turn out that a degree n curve does not possess sufficient flexibility to model the desired shape. This tool lets the user increase the degree of the selected curve, yet leaves the shape of the curve unchanged.

(ii) Degree reduction – Reducing the degree of the curve will result in the curve being converted into a Uniform Rational Basis Spline curve with the given control points and specified degree.

Change Number of Segments - The number of segments used for displaying and representing the selected curve is shown in the Value Control Box(VCB). To adjust it, type the new value you want and press Enter or Return.

Subdivide - Subdivide the selected curve by picking the point(s) you want to subdivide it at. You can also enter the parameter value(s) you want to subdivide at and press Enter or Return. Use this to generate shapes that are too complex for a single curve to handle or to trim off parts of the curve you no longer need.