Gets the next half-edge, adjacent to edge0 in a
counter-clockwise direction, which lies
along the boundary of a region of triangles marked
VISIBLE or COPLANAR
Gets the next half-edge, adjacent to edge0 in a
counter-clockwise direction, which lies along the boundary
of a region of triangles marked with the value mark.
Gets the next half-edge, adjacent to edge0 in a clockwise
direction, which lies along the boundary of a region of
triangles which are marked
VISIBLE or COPLANAR