The other method to generate NURBS geometry is to define the geometry in both directions with curves. This option has certain advantages. By using in two directions curves one could precisely define the outer edges of the geometry. If the geometry is closed in one direction the use of curves in one direction will be no problem , if the surface however is open in both directions the direction of geometry which is defined by the computer is less accurate. The extra control over the edges makes it perfect for generating geometry which will have to accurately fit between objects. Another option is the modeling of complex shapes by using a series of patches defined at the edges and connected in a later stage.
There are several methods of generating the surfaces based on defining both directions based on curves:
The BI RAIL 2 option will make a shape between two profile curves along two rail curves. First use the Bi-rail 2 Tool. Select the two profile curves, then the two rail curves. And now the shape will appear.
The Bi-rail 3 option will make a shape between three profile curves along two rail curves. First use the Bi-rail 3 Tool. Select the three profile curves, then the two rail curves. And now the shape will appear.