Dynamic Tessellation - Practical Rendering and Computation with Direct3D 11 - page 459

Figure 9.43. Excessive smoothed geometry loses expected sharp edges.
starting point of Figure 9.42 is an obviously low-polygon model, the improvement added
by the Curved Point Normal Triangles algorithm is clear.
One characteristic to pay attention to in the wireframe representation in Figure 9.41
is that the tessellation is uniform across the entire mesh. This doesn't pose a problem with
regard to the final visual results; but it is inefficient in some cases.
Despite generally good results, there are cases where this algorithm doesn't work
very well. In Figure 9.43, notice that the result is probably too rounded (right hand side)
and has actually lost the sharp edges that define the intended object (left side).
Encoding Sharp Edges
In this context, a sharp edge is an edge where the positional geometry between neighboring
triangles matches, but the normal vectors do not. Figure 9.44 shows a simple example of
this, in which both sides have a shared position in the centre, but the left side has differing
normal vectors, and the right side has both a shared position and a shared normal.
Figure 9.44. A simple example of a sharp edge.