Modeling with Patch Surfaces 292 You can create patch surfaces in three ways: You can use a Patch Grid primitive object, which is just a flat plane with patch attributes; you can convert other types of surfaces to an editable patch by right-clicking or applying an Edit Patch modifier; or you can create a spline cage and apply a Surface modifier to create a skin over the cage. In this chapter, you learn the last method: skinning a cage of splines. The cage can be made of vertices and segments that form three- or four-sided areas to describe the surface. There must be one or more vertices at each intersection within a specific tolerance to define the tri or quad cage element. Patch modeling from a spline cage is a technique used by modelers to create characters, soft fur- niture, or rolling landscapes and golf courses. In this chapter, you create the smooth curves of a chair cushion, starting with the four individual splines that you can think of as cross-sections through the half a cushion. Some of the techniques covered include the following: · Spline modeling--You learn to create a spline cage that can be surfaced with a patch for organic modeling. At render time, all objects are converted to polygon mesh triangles for the purpose of calculating the rendered image. · Patch surface--You learn to surface a spline with a surface that has Bézier control of the cur- vature. These are effective for organic objects. · Symmetry modifier--You learn to apply this modifier again to turn a half model into a symmetrical surface quickly and easily. note