Technical details about crease/smooth/corner vertices and edges

To understand why "Creasing vertices" is tricky, you need to understand these technical limitations about edges and vertices in SubDs:

• There are two types of edges in a SubD: Smooth and Crease.

• There are four types of vertices in a SubD: Smooth, Dart, Crease, Corner.

  Smooth edge Crease edge Smooth vertex Dart vertex Crease vertex Corner vertex

• There is one type of SubD face. A face can have 3 or more boundary edges.
• A Smooth edge must have a Smooth vertex at at least one end.
• A Smooth edge must have exactly two attached faces.
• A Crease edge must have both end vertices be Dart, Crease, or Corner.
• A Crease edge can have any number of attached faces; it can be a wire (0 faces), a boundary (1 face), an interior (2 faces), or a nonmanifold (3 or more faces) edge.
• A Smooth vertex must be attached to two or more smooth edges.
• A Smooth vertex has the same number of faces and edges; every edge is smooth and is attached to exactly two faces
• A Dart vertex must be attached to two or more edges; exactly one of these edges must be an interior Crease edge and the other edges must be Smooth edges.
• A Crease vertex must have exactly two crease edges and may have zero or more Smooth edges. Either both of these Crease edges are boundaries (1 face), or both of the Crease edges are interior (2 faces).
• A Crease vertex must be attached to at least two Crease edges.
• Every non-manifold edge (attached to 3 or more faces) must be a Crease edge and every vertex touching a nonmanifold edge must be a Corner vertex.