17.2.4. PN Triangles
Given an input triangle mesh with normals at each vertex, the goal of the PN triangle scheme by Vlachos et al.  is to construct a better-looking surface compared to using just triangles.
The letters “PN” are short for “point and normal,” since that is all the data you need to generate the surfaces.
They are also called N-patches. This scheme attempts to improve the triangle mesh’s shading and silhouettes by creating a curved surface to replace each triangle. Tessellation hardware is able to make each surface on the fly because the tessellation is generated from each triangle’s points and normals, with no neighbor information needed. See Figure 17.27 for an example. The algorithm presented here builds upon work by van ...