256 AutoCAD and Its Applications—Advanced 256 AutoCAD and Its Applications—Advanced Untrimming Surfaces If you need to restore a trimmed surface back to its original shape, use the SURFUNTRIM command. After selecting this command, select the edge of the surface area to untrim. If the surface has multiple trimmed edges, you can use the Surface option. The SURFUNTRIM command untrims surfaces trimmed by the SURFTRIM command. It does not untrim surfaces trimmed using the PROJECTGEOMETRY command. PROFESSIONAL TIP PROFESSIONAL TIP Open the Properties palette if you are unsure if a surface has been trimmed. The Trimmed surface property setting indicates the surface trim status. NOTE NOTE Exercise caution if you are trimming an associative surface using another surface as a cutting object. When using another surface as the cutting object, turn off surface associativity (set the SURFACEASSOCIATIVITY system variable to 0) before trimming to avoid potential problems in future edits. NURBS Surfaces NURBS Surfaces When creating a NURBS surface, you use many of the same commands that you would use to create a procedural surface. You can use splines and various curve shapes to create NURBS surfaces. In addition, you can convert procedural surfaces into NURBS surfaces. As discussed earlier in this chapter, a NURBS surface is created when the SURFACEMODELINGMODE system variable is set to 1. In addition, NURBS surfaces are non-associative. The setting of the SURFACEASSOCIATIVITY system variable has no effect when creating a NURBS surface. The advantage of working with NURBS surfaces is that you use control vertices to control or infl uence the shape of the surface. The ability to edit control vertices provides signifi cant fl exibility in creating and sculpting freeform, organic shapes. For example, in computer animation work, NURBS surface modeling techniques are commonly used for modeling characters to produce the organic shape desired. In AutoCAD, you can create highly sophisticated, freeform shapes using NURBS surface models. This chapter introduces the NURBS surface modeling tools available in AutoCAD. NURBS Surface Modeling Workfl ows There are two common workfl ows used in NURBS surface modeling. You can begin by creating procedural surfaces and then convert them to NURBS surfaces, or you can create the initial surfaces as NURBS surfaces. When you start the modeling process by working from procedural surfaces, the following workfl ow is common: • Create procedural surfaces using commands such as EXTRUDE, REVOLVE, SWEEP, LOFT, PLANESURF, and SURFNETWORK. The SURFACEMODELINGMODE system variable should be set to 0. • Create other surfaces, such as blend surfaces, patches, fi llets, and offset surfaces. Use the commands presented in this chapter. • Convert the surfaces into NURBS surfaces. • Edit the NURBS surfaces as needed to create the desired sculpted shape. SURFUNTRIM Ribbon Surface Edit Surface Untrim Type SURFUNTRIM