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.

Once Use Chord Height Ratio is turned off for all the patches, with the Per Span settings still at 1 for U and V, we can see the the rendering is now free of edge artifacts. The only remaining problem is that the ears look a little chunky, as the geometry is only being tesselated once between each span.

So finally, after setting the U and V settings to 3/3, we have a clean rendering.

A useful tool to make sure your model's tesselation is matching up at the edges, is to turn on Display Object Tesselation for each patch. In the above image, we are seeing the tesselation for the patches when we reached the desired settings.

As a final note, setting these parameters can be time consuming. The above character has about 400 patches from head to toe, so setting the tesselation patch by patch is no fun. A useful tool for this situation, therefore, is located at Window/General Editors/Attribute Spread Sheet. Select all the patches for the character and click on the tesselation tab. By selecting an entire column by clicking on the column's tab, you can change values for multiples nodes at the same time. Very handy.