Tuned ternary quad subdivision

A well-documented problem of Catmull and Clark subdivision is that, in the neighborhood of extraordinary point, the curvature is unbounded and fluctuates. In fact, since one of the eigenvalues that determines elliptic shape is too small, the limit surface can have a saddle point when the designer's input mesh suggests a convex shape. Here, we replace, near the extraordinary point, Catmull-Clark subdivision by another set of rules derived by refining each bi-cubic B-spline into nine. This provides many localized degrees of freedom for special rules so that we need not reach out to, possibly irregular, neighbor vertices when trying to improve, or tune the behavior. We illustrate a strategy how to sensibly set such degrees of freedom and exhibit tuned ternary quad subdivision that yields surfaces with bounded curvature, nonnegative weights and full contribution of elliptic and hyperbolic shape components.