258 AutoCAD and Its Applications—Advanced 258 AutoCAD and Its Applications—Advanced If the intention is to create more complex, freeform shapes, you can create NURBS surfaces without fi rst converting procedural surfaces. In this case, set the SURFACEMODELINGMODE system variable to 1. Create profi le geometry using splines and use the appropriate surface modeling commands. The model in Figure 10-24 shows an example of editing a NURBS surface converted from a procedural surface. The original model shown in Figure 10-24A is a loft surface created from open profi les. In Figure 10-24B, the model is shown after using the CONVTONURBS command. The model is shown selected with control vertices displayed (the wireframe view is shown for reference only). When you select a NURBS surface, control vertices do not appear by default. The display of control vertices is controlled by the CVSHOW command. To display control vertices, select the CVSHOW command and then select the NURBS surface. Then, press [Enter]. You can also select the command after initially selecting the surface. To remove the display of control vertices, select the CVHIDE command. Using this command removes the display of control vertices from all objects in the drawing. Editing control vertices is similar to editing grips. Pick on the control vertex grip and pull or drag. You can press and hold the [Shift] key to select multiple control vertices. You can also use the gizmo that appears. As you pull or drag, the shape of the surface is modifi ed. In Figure 10-24C, the model is shown after editing a control vertex on the back end of the surface. A shaded view of the model after editing and using the CVHIDE command is shown in Figure 10-24D. CVSHOW Ribbon Surface Control Vertices Surface CV-Show Type CVSHOW CVHIDE Ribbon Surface Control Vertices Surface CV-Hide Type CVHIDE A B C D Select a control vertex Figure 10-24. Editing a NURBS surface converted from a procedural surface. A—The original model is a loft surface created as a procedural surface. B—The model after using the CONVTONURBS and CVSHOW commands. A wireframe view is shown for reference only. C—The model after editing one of the control vertices on the back end. D —The model after using the CVHIDE command.