Sliding Constraints on Tangency

There is an alternative constraint which can be applied to the tangency of the edge of a surface patch. This constraint fixes the direction of the tangency but allows the magnitude to vary in a manner consistent with increasing the fairness of the surface. The means of applying this constraint depends on having two types of constraints applied to individual CPs. If a CP is selected using the mouse it becomes a "secondary" type, if it is selected a second time then it becomes a "primary" type. Note if a CP is selected a third time then it reverts to being unconstrained. If an edge of a surface patch is constrained using constraints of the secondary type and the adjacent row/column of CPs are constraint using constraints of the primary type, these interior CPs are allowed to slide in the direction of their adjacent boundary CP. It is this restricting the CPs to sliding in the direction of the boundary CPs, which maintains the direction of the tangency.

A complication arises if there are sliding constraints on adjacent boundaries. This is because there will be an internal CP which has conflicting sliding constraints placed upon it. This CP will be internal and positioned next to a corner CP. A compromise is reached by sliding this CP in the direction of the corner CP.