Modify SurfaceTrim/Extend

The Trim/Extend Surface command enables you to trim or extend a surface using one of two methods, to be chosen in the Mode drop-down list:

• Parameter
  by changing the range of the parameters of the selected Surfaces so that the limits of the range differ from the original limits (this mode does not apply to surface boundaries deriving from a trimming operation: in such cases selection is not enabled)
• Tangency
  by indicating a numeric value corresponding to the length of the arc to be added to the specified boundary (Boundary) of the surface while maintaining tangency continuity.

Using the Parameter mode, as each surface is described by parametric equations as a function of two parameters, U and V, varying in the range [0,1], if you modify the limits for one or both parameter ranges, you can change the extension of the surface. Thus, if you indicate ranges contained in the original ranges, the surface is trimmed while if you indicate ranges containing the original ranges, the surface is extended according to the new values.

Once you have set the new limits for the parameter ranges, the program reparameterizes the surface (i.e.: it recalculates the parametric equation of the modified surface so that the limits of the U and V parameters are once again 0 and 1). Therefore, surfaces obtained by changing the parameter ranges are not surfaces with parameter limitations and cannot be "reconstructed" using the Untrim command to obtain the original surface again.

Notice that the parametric extent of very flexible NURBS surfaces (i.e., those with a high number of control points) may give rise to unwanted results. Therefore, it is not a good idea to modify the parameters by more than 20%.

The Boundaries drop-down list is available in the Parameter mode: the Select value enables you to select any surface boundary. In this case, the parametric modification of the surface is performed by modifying its trimming loops.

Try to ...

• Trim or extend a surface