When Maya renders a NURBS surface, the object is tesselated into triangles (polygons). Understanding tesselation is crucial to clean silhouettes and patch boundary relationships. There are several options for tesellation, but this quick tutorial is dealing with multi-patch models.



Above we have a head built out of 198 patches as seen in hardware shading. All of the patches have positional and tangential continuity as well as matching parameterization, being '0 to # spans', although '0 to 1' works as well.



When rendered with the default tesselation settings, however, you can see the problem. Even though the model is clean, the rendering is displaying edge artifacts. This is due to the tesselation settings. The default settings are tesselating each patch with no regard to adjacent surfaces, based on curvature tolerence. (ie the more curvature, the higher the degree of tesselation). But the defaults are clearly no good. What we must do is, for each patch, turn on 'Explicit Tesselation'. This allows the user to have manual control over the tesselation in a much more intuitive fashion.

With explicit tesselation on, we are given more options for how the U and V directions of the surface are tesselated. The default settings for U and V mode are fine, being 'Per Span # of Isoparms'. This means that the specified # is how that direction is tesselated, where 1 for U and 1 for V should have the geometry render just as it looks when in shaded mode with the resolution set to 1. But with the settings at 1 and 1, in the above image, we are still seeing edge artifacts. This is because 'Use Chord Height Ratio' in on by default in the Explicit Control options. This again uses curvature to determine tesselation, therefore meaning that the parameterization of the surface is pretty much ignored. But with a multi-patch model, much care is taken to make sure the edge parameterization lines up, for a reason.