We developed an approach to editable free-form features. A piecewise polynomial (B-Spline) surface follows, for instance, the changes of a curve, maintaining a set of additional predefined constraints (e.g. incidence on one or several other curves, tangencies).

The main research issues have been:

• Efficient set-up of the equations which express the constraint problem, using blossoming techniques, and a multi-index tree data structure.
• Robust and efficient equation solving for under- and over-determined systems of linear equations, based on sparse singular value decomposition and regularization.
• Expressing incidence, derivative, and integral constraints. Defining the so-called aliasing problem, and providing alternatives, such as local surface reparameterization, and smoothing.
• Application for sketch-based surface sculpting, and editing of reverse-engineered models.