Secondly, let’s examine more details about the “Boundary” triangles.
Bd(Cone A, Roof* A) is the set of all the slant triangles partially covered by Roof* A.
And Bd(Cone A, Roof* A) is empty in the above example.
As shown on the right, Bd(Cone A, Roof* A) consists of the slant triangles whose top vertex is above and bottom vertex is under the surface of Roof* A. The figure shows the fourth triangle (colored red) from the bottom, whose top vertex is above the surface and bottom vertex is under the surface.