Modifying Curves

You can modify curves in a variety of ways using commands in the Modify Curve menu of the Model toolbar. Each of these commands modifies the topology of a curve.

Inverting Curves

Inverting a curve reverses its parameterization so that U increases in the opposite direction. The result is as if you had drawn the curve clockwise instead of counterclockwise or vice versa.

For example, if an object uses the curve as a path, it moves in the opposite direction once you invert the curve. Similarly, if a surface has been built from the curve and its operator stack was not frozen, its normals become reversed.

To invert curves

Select one or more curves.
Choose Modify Curve Inverse from the Model toolbar.

If you didn't select a curve in step 1, you can pick one now.

The curves are inverted and the Inverse Curve property editor opens.

NoteWhile drawing a curve or adding points to it, you can also invert it simply by right-clicking and choosing Invert from the menu. See Drawing Curves by Placing Control Points or Knots and Adding Points and Knots on Curves Interactively.

Opening and Closing Curves

You can open a closed curve and close an open curve. Curves are always opened at the U = 0) position.

To open/close a curve

Select one or more curves in Object mode.

Choose Modify Curve Open/Close.

If you didn't select a curve in step 1, you can pick one now.

The curve is opened if it was closed, and closed if it was open.


Open curve	Closed curve

NoteWhile drawing a curve or adding points to it, you can also open or close it simply by right-clicking and choosing Open/Close from the menu. See Drawing Curves by Placing Control Points or Knots and Adding Points and Knots on Curves Interactively.

Shifting U on Curves

On closed curves, you can shift the start point (U = 0 position) along the length of the curve.

To shift a start point

Select one or more closed curves.
Choose Modify Curve Shift U.

If you didn't select a curve in step 1, you can pick one now.

The Curve Shift property editor opens.
Adjust the Shift U parameter. As you change values, the start point jumps to the nearest knot.

TipWhen shifting, it's helpful to show NURBS boundaries in the 3D views.

Cleaning Curves

Cleaning a curve reduces the number of control points while keeping the same general shape.

To clean a curve

Select one or more curves.
Choose Modify Curve Clean from the Model toolbar.

If you didn't select a curve in step 1, you can pick one now.

The Clean Curve property editor opens.
Set the Tolerance. This controls the maximum difference between the original curve and the resampled shape.

Raising the Degree of Curves

You can raise the degree of curves. For example, you can convert a linear curve to a cubic in order to add a curved segment to it. The existing trace is preserved when you raise the degree.

To raise a curve's degree

Select one or more curves.
Choose Modify Curve Raise Degree from the Model toolbar. The Raise Crv Degree Op property editor opens.
Set Degree to the desired value: 2 for quadratic or 3 for cubic.

Reparameterizing Curves

Parameterization refers to the way that any point along the length of a curve is described in terms of the parameter U. Any given point along the curve can have different U values under different parameterizations.

When you reparameterize a curve, the new parameterization is in effect at the position of the Reparameterize operator in the operator stack. If you move points or perform other operations on the curve after reparameterizing, the result may have a different parameterization.

For a description of the available parameterization methods, see Parameterization.

To reparameterize curves

Select one or more curves.
Choose Modify Curve Reparameterize from the Model toolbar.

If you didn't select a curve in step 1, you can pick one now.

The Reparameterize Curve property editor opens.
Choose a parameterization method.

Stitching Curves

Stitching two curves glues their ends together. Unlike blending (see Blending Curves), it does not create a new curve; instead, it deforms the originals.

To stitch a curve

Select two curve boundaries.
Choose Modify Curve Stitch from the Model toolbar.

If you didn't select both curve boundaries in step 1, you can pick them now.

The Stitch Curves property editor opens.
Adjust the parameters to obtain the desired shape. For more information, see Stitch Curves Property Editor.

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