HA!
The problem was which way the quads were facing.
To determine this, you take another vertex from one of the triangles, a vertex which does not form part of the edge. That point needs to lie behind the quad. Do a plane distance test with the plane equation you get from the first three vertices of the projected quad. If the distance is less than zero, reverse the order of the quad vertices.
Thanks to all that contributed.
The problem was which way the quads were facing.
To determine this, you take another vertex from one of the triangles, a vertex which does not form part of the edge. That point needs to lie behind the quad. Do a plane distance test with the plane equation you get from the first three vertices of the projected quad. If the distance is less than zero, reverse the order of the quad vertices.
Thanks to all that contributed.