T-Spline: four types of vertices

In T-splines there are four types of vertices (or control points): T-points, star points, tangency handles, and ordinary control points.

·        T-points: end lines of detail: T-points can be created by many commands, such as insert edge, insert point, subdivide face, merging, and welding, and are used to constrain detail to part of the surface. T-points constrain detail to the edges of the T-spline surface.

·        Star points: create non-rectangular surfaces: Star points allow a T-spline to be non-rectangular. Star points can be generated by commands such as extrude, delete face, and merge. It is more difficult to control the shape of a T-spline at star points, so they should only be used where necessary.

·        When a T-spline is exported to NURBS, it will split into separate surfaces at each star point. the NURBS is split into rectangular patches at each star point. Additional isocurves are added to the NURBS to maintain surface continuity.

·        Using T-points and star points in a model: Using star points and T-points correctly is very important to modeling with T-splines effectively. T-points are used for adding local detail by isolating control to one area on the surface. Star points are used when the model require a non-rectangular topology like surfaces with holes (non-trimmed), closed surfaces, “Y” branches, surfaces with legs, etc.

·        Influence region of a star point (two faces deep). There cannot be any T-points in this area when the model is standardized.