Copyright Goodheart-Willcox Co., Inc. Chapter 10 Advanced Surface Modeling 257 When you start the modeling process by creating NURBS surfaces, the following workfl ow is common: • Set the SURFACEMODELINGMODE system variable to 1 (on). With this setting, NURBS surfaces are created. • Create the surfaces needed to create the desired model shape. When using this approach, you use splines or curves to defi ne the surface profi le. Splines are created using the SPLINE command. Splines used for NURBS surface models are typically created with the Method option of the SPLINE command set to CV. This creates splines with control vertices (CVs), also known as CV splines. See Figure 10-23. Control vertices play a major role in editing NURBS surfaces. • Edit the NURBS surfaces as needed to create the desired shape. There are several important points to keep in mind when you are working with NURBS surfaces. Once a procedural surface is converted to a NURBS surface, the NURBS surface cannot be converted back to a procedural surface. In addition, once a NURBS surface is created, it cannot be converted to a procedural surface. The design workfl ow you use is important. Make sure to plan ahead so that your modeling process is suit- able for the design. Creating and Editing NURBS Surfaces If the surface model you are creating will have only slight modifi cations, a simple approach is to create a procedural surface and then convert it to a NURBS surface. The CONVTONURBS command is used to convert a procedural surface to a NURBS surface. After entering this command, select the surface to convert. When you press [Enter], the surface is converted. CONVTONURBS Ribbon Surface Control Vertices Convert to NURBS Type CONVTONURBS A B C Control vertices Control frame Start point End point Pick grip to access control points menu Figure 10-23. Splines used to create NURBS surfaces are typically created as CV splines. A—A CV spline used for creating an extruded NURBS surface. B—The spline shown selected with control vertices displayed. C—A NURBS surface model created by using the EXTRUDE command.