# projects

## Piecewise-smooth Subdivision

Below are some examples from our paper Piecewise-smooth subdivision with normal control (H. Biermann, A. Levin, D. Zorin, SIGGRAPH 2000). In this paper, we present a complete set of subdivision rules for piecewise-smooth surfaces.

**Examples**

Quite often, standard subdivision rules don't produce satisfying results: In this example, we want to interpolate the boundary of the control mesh (left). We would expect a smooth surface of similar shape as the control mesh (middle), but the standard corner rules produce the surface on the right.

Rather than adding ad-hoc rules for various special cases, we define
the class of **piecewise-smooth surfaces**, which is sufficiently
general for modeling applications. The class contains surfaces with
boundaries and corners. There are two corner types, which are not
equivalent: convex (left) and concave (middle). Combining convex and
concave sectors results in complex corners (right).

We present a uniform framework to generate piecewise-smooth surfaces. Moreover, the same algorithm allows to prescribe normal directions on the surface. By manipulating the normals, one can bend the surface in an intuitive way without changing the control mesh.

In our reference implementation, the user applies features to the surface by tagging the control mesh. This example shows the surface without tags (left), convex and concave corners (center), and a complex corner of three sectors (right).