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.
Keywords Subdivision - ternary - bounded curvature - convex hull