Overlapping constraints

[kenohori]

The CGAL constraint hierarchy doesn't always behave how we want. When we remove a part of a constraint, the number_of_enclosing_constraints() is still 1. This can cause issues when 3 or more constraints overlap.

[kenohori]

Implementing the odd/even rule on constraints is difficult without a great deal of work, so our solution right now only works for some cases. Specific problems to be solved:

• Detecting whether a constraint is already there (identical constraints): CGAL ignores them, we need to count them
• Detecting whether a constraint is already there (sub constraint != any input constraint): hackish solution by adding then removing, but count doesn't drop
• Removing (part of) a constraint: hackish solution by adding then removing, but count doesn't drop

This could be solved by correct odd/even counting on constraints, but how?

[kenohori]

The solution: instead of using Constrained_triangulation_plus_2, use a custom insert_constraint() that:

• checks which edges intersect the new constraint using find_intersected_faces()
• removes all unconstrained ones
• inserts the new constraint, computing intersections with other constraints if necessary and removing overlapping parts of the constraints
• triangulates the holes produced

[kenohori]

Needs testing, but implemented in branch improvements-with-ogr