Edit NURBS

Menu: Geometry->Edit->Edit NURBS

Edit NURBS line:
Tool to modify some NURBS geometric properties, like control points, degree, etc.

Once a NURBS line is selected (use the Pick button in the Edit NURBS Line window), you can edit its control points (see NURBS line). Select the control points as if they were regular points and enter their new positions in the usual way (see Point definition).

The Influence factor affects the movement propagation of the neighboring control points.

Available options:

• Fix boundary Check the fix boundary option if you do not want to move the boundary control points of the line.
• Insert knot: You are asked for a knot value between 0.0 and 1.0 and this is then inserted. The program checks that the knot multiplicity is not greater than the order (order=degree+1). As the number of knots increases, the number of control points also increases, so this option can be used to have more points defining the same curve.
• Knot removal : the inverse of knot insertion. Remove knots if possible without change shape with a given tolerance. (interesting to save memory)
• Elevate degree: With this option the degree of the curve is raised by one. The new curve will have the same shape but with more control points and knots.
• Reduce degree : the inverse of degree elevation. Decrease the polynomial degree if possible without change shape with a given tolerance.
• Change weight: A new positive weight can be introduced for any control point, with the exception of the end points.
• Cancel weights: All weights of the NURBS are converted to 1.0 and the curve is no longer rational.
• Reparameterize: With the same control points a new curve is calculated to get a better curve with a more uniform parameterization.
• Similar cubic: This option converts the curve to a simplified one with degree=3, which is only an approximation of the original one.

Edit NURBS surface:

Once a NURBS surface is selected (use the Pick button of the Edit NURBS Surface window), you can edit its control points interactively (see NURBS surface). Select the control points as if they were regular points and enter their new positions in the usual way (see Point definition).

Available options:

• Insert knot: You are asked for a knot value between 0.0 and 1.0 and it is then inserted. The program checks that the knot multiplicity is not greater than the order. This option can be used to have more points defining the same surface.
• knot removal: the inverse of knot insertion. Remove knots if possible without change shape with a given tolerance. (interesting to save memory)
• Elevate degree: With this option the degree of the surface is raised by one. The new surface will have the same shape but with more control points and knots.
• Reduce degree : the inverse of degree elevation. Decrease the polynomial degree if possible without change shape with a given tolerance.
• Change Weight: A new positive weight can be introduced for any control point, with the exception of the end points that must have weight=1 (to force the surface to pass over the corner control points).
• Cancel weights: This converts the weights of all the control points to 1.0
• Reparametrize: This reparametrizes the surface obtaining an optimized surface. When a Nurbs surface is not well parameterized, the mesh is of a lower quality.
• Similar cubic: This option converts the surface to a simplified one with degree=3 in both parametric directions, which is only an approximation of the original one.

All these options are available in U and V directions.


The Movement type menu of the Edit NURBS Surface window determines the way the selected knots will move. This movement can be along an axis (X-Axis, Y-Axis, Z-Axis), can describe the Normal of the surface (Normal), can follow the screen movement of the mouse (Screen), or the new location of the knot can be defined by introducing the coordinates of a point (Point).

Note: The Insert knot and Degree elevate options can be chosen for either the u or the v parameter directions.