Maya Rendering Guides

NURBS Surfaces

NURBS curves are visual representations of parametric equations. NURBS surfaces extend those equations into a second dimension and add shading to the mix. Curves are parameterized in the U direction, and surfaces add the V direction. Therefore, NURBS surfaces are said to have a U and a V direction. UVs define points along a specific surface; they are Maya's way of mapping 2D coordinates to a 3D surface.

NURBS surfaces are tessellated at render time. This means that when you render a NURBS surface, Maya converts it to polygons. NURBS surfaces were originally conceived as stand-ins for large polygonal datasets. They were much more efficient because you could manipulate a surface using simpler controls and have the patches tessellate to completely smooth shapes at rendering time. In effect, you could manipulate huge numbers of polygons with a few control points.

Figure 4.4 Before and after rebuilding curves. Note how the EPs shift position.

Figure 4.5 A NURBS sphere is really only a rectangular patch that has been automatically formed and closed by choosing Create * NURBS Primitive * Sphere.

All NURBS surfaces and shapes are composed of one or more four-sided patches. Figure 4.5 shows how even a NURBS sphere breaks down into a rectangular patch. There is no actual way of changing this; when modeling with NURBS you must create and manipulate four-sided patches. As you will see, there are various quite powerful tricks and techniques for working around this limitation, but it exists nonetheless.

Each NURBS surface is composed of one or more patches. Each patch is formed by the intersection of two spans that are demarked by lines called isoparms. These isoparms are the 3D equivalent of knots, or EPs, on a curve. Unlike EPs, however, they cannot be moved in 3D space. You can and will insert new isoparms during the course of modeling.

Inserting isoparms increases the number of CVs available to manipulate the surface of the shape. This fact reveals a disadvantage of NURBS surfaces: it is impossible to manipulate the actual surface of a NURBS shape directly. The only way to edit a shape is to manipulate a CV or group of CVs. Maya offers another topology called hulls to make selecting rows or rings of CVs automatic.

The best way to learn to use NURBS surfaces is to build something using this type of surface.