From BlenderWiki
Trim-Compatible Tessellation
The new NURBS tessellator takes a surface and a list of trim curves and produces the following polygonal approximations for the purposes of visualization in the editor. Note that normals still aren't calculated (left) and that automatic calculation of normals (right) does not produce good results. There are good (numerically stable and efficient) techniques for calculating the tangents (and therefore normals) analytically, but the code to do this is still a WIP: the theoretically "production-ready" code from Piegl&Tiller still has edge cases that need to be hunted down.
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One of the bugs I worked on early this week involved "zombie vertices" that intermittently lingered despite having been removed from the displist. Fortunately it has been fixed now.
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