Module coplanar 

Coplanar triangle overlap — the constraints two coplanar overlapping faces impose on each other.

When triangle tb is coplanar with ta, each of tb’s edges is clipped to ta and the portion inside becomes a constraint for ta (and symmetrically). A clipped endpoint is either an Explicit vertex (inside ta) or a coplanar edge×edge crossing — represented EXACTLY as an auxiliary-plane LPI: the line q0q1 ∩ the plane through ta-edge (e0,e1) extruded perpendicular to the shared plane (e0 + n). Within the shared plane that plane reads as the line e0e1, so the LPI is exactly the 2D crossing — and it lies on the shared plane, keeping everything exact and conforming.

Functions

coplanar_clip

Constraints tb (coplanar with ta) imposes on ta: each tb edge clipped to ta. Empty if ta is degenerate or the triangles don’t overlap.