To work on control points of a surface, you must click this action while the surface is selected. It allows to interactively drag control points, add new ones, and perform various operations on a NURBS surface.
How to use
1. Select the object whose surface you want to edit.
2. Click the NURBS Surface Editing icon or choose the Tools >Surfaces > NURBS surface command.
3. An alert panel warns you: The selected surface is part of a Construction Tree. In order to edit with this Modeling Tool, the surface needs not to be a part of the Construction Tree. Do you want the surface to be set as hidden and a copy to be created for editing?
4. If you click Yes, a new NURBS surface with the same shape of the selected object is created.
Note: When you apply the NURBS Surface Editing action, the Construction Tree is broken. The original parametric surface is hidden from the 3D views but still displayed in the browser. The newly created NURBS surface is NOT linked in any way to the original parametric one. Thus, modifying the shape of the original parametric surface will NOT cause the NURBS surface to change its shape accordingly.
Options
Point
To modify XYZ coordinates of the control points.
Views. Choose the Edit mode. Click the point whose position you want to modify and drag it. Release the mouse button when you have reached the desired position.
Modeling Tool panel. Select the point from the scroll list that appears when you click the [&ldots;] button. Type the new values in the XYZ data fields and confirm them by pressing Enter.
Knots
To select a single or a group of knots. Knots selection is used for knot refinement and knot removal.
Views. Choose the Edit mode. If you have enabled the Show Knots option, knots are displayed in the views as dashed lines. When you select them, they turn yellow.
The standard Windows behavior is implemented: a mouse click selects a single knot. Hold the Shift key down to select all the knots from the previously selected knot to the clicked one. Hold the Ctrl key down to select/deselect sparse knots.
Modeling Tool panel. Select knots from the list. The Shift / Ctrl behavior is the same as described above.
New knots
To specify the number of knots that will be added when the button Refine is pressed.
Modeling Tool panel. Set the number of new knots to be added either by moving the slider or by typing it in the data field and pressing Enter.
Refine
To insert new knots between the first and the last selected knots. This way you add new control points to a surface.
Modeling Tool panel. Select the surface and switch to Edit Mode. Select the knots between which you want to add more control points. Set the desired number of new knots specified in the New knots data field and push the Refine button. If less than 2 knots are selected, this button turns grey.
Tolerance
To specify the maximum allowed deviation from the curve when removing knots. Knots are removed by pressing the button Remove.
Modeling Tool panel. Set the tolerance value either by moving the slider or by typing it in the data field and pressing Enter.
Remove
To remove selected knots. Should a knot removal deviate the curve from its original shape for a greater value than the one specified in the Tolerance data field, the knot would not be removed.
Modeling Tool panel. Just press the Remove button.
Insert U knot at
Insert U knot
To insert a knot along U at a specified position.
Modeling Tool panel. Move the slider to change position of the U knot, which is represented by a dark green isocurve, over the surface. Press the Insert U knot button when you have reached the desired position.
Insert V knot at
Insert V knot
To insert a knot along V at a specified position.
Modeling Tool panel. Move the slider to change position of the V knot, which is represented by a dark green isocurve, over the surface. Press the Insert V knot button when you have reached the desired position.
U order
V order
To change the surface order along U or V directions.
Modeling Tool panel. Set the value either by moving the slider or by typing it in the data field and pressing Enter.
Invert U dir.
Invert V dir.
To invert a surface along U or V directions. Inverting U and V directions can be useful when texturing a surface.
Modeling Tool panel. Just press the button.
Show knots
To visualize surface knots.
Modeling Tool panel. Switch on the check button to enable this option, which is disabled by default.
Weight
To adjust the weight of one or more control points of a surface.
Views / Modeling Tool panel. Select the control point or points whose weight factor you want to modify. Move the slider or type the weight factor in the Weight data field. Press Enter to confirm.
Default weight factor is 1. The more you increase this value, the greater the pulling effect that the control point exerts on the surface. Weight factor should not be lower than 0.0001. In fact, 0 is not a valid weight factor.
When all control points on a surface have the same weight, the surface is called Non-rational. When control points have different weight factors, the surface is called Rational.
Note: Weight modification should be used very carefully. If you plan to exchange your projects, you should remember that many CAD systems do not support modified weight values.