Recursive midpoint refinement smooths a polyhedral control mesh into a C² bicubic B-spline surface. Drag to rotate.
Catmull–Clark (1978): At each subdivision step, insert (1) a face point at each face centroid, (2) an edge point using the weighted average of edge endpoints and adjacent face points, (3) update each vertex using its valence and surrounding face/edge points. Iteration converges to a C² surface everywhere except at extraordinary vertices (valence ≠ 4) where it is C¹. Used in Pixar's RenderMan for nearly every film since Toy Story.