Periodic, closed, and open geometry
 
 
 

A NURBS curve or surface can have a periodic, closed, or open form. The form affects the how the object deforms.

Curves

In contrast to a periodic curve, a closed curve can have a sharp corner at the seam because it lacks the extra spans.

The figure below shows similar closed and periodic curves, both originally circles. For each, we moved the CV next to the seam inward. The periodic curve keeps its continuity, but the closed curve’s tangency is broken.

To see whether a curve is periodic, closed, or open, select the object, display the Attribute Editor, and examine the contents of the Form text box.

Note that if you use Edit Curves > Open/Close Curves to “close” an open curve, you create a periodic rather than a closed curve. To create a closed curve, use Edit Curves > Detach Curves on any edit point of a periodic curve.

NURBS surfaces

Periodic and closed surfaces work like their curve counterparts, except they have two parametric directions (U and V) instead of one (U).

If you create a surface from a periodic curve, the surface is periodic in one direction (U or V).

To see whether a surface is periodic, closed, or open, select the object, display the Attribute Editor, and examine the contents of the Form U and V text boxes. A surface is considered periodic whether it’s periodic in U, V, or both.

If you use Edit NURBS > Open/Close Surfaces on an open surface, you create a periodic rather than a closed surface. To create a closed surface, use Edit NURBS > Detach Surfaces on an isoparm of a periodic surface.