The ideas of curvature can be extended to polyhedral surfaces as well, but
we only care about its sign.
For smooth surfaces:
 Positive curvature looks like a maximum for a height function
 Negative curvature looks like a saddle.
For polyhedral surfaces:
 Curvature is zero at every point other than the vertices
 A vertex has negative curvature if it lies inside the convex
hull of its neighbors
 It has positive curvature if it lies outside.
