A fragment illustrating the mesh-expanding procedure. The red lines are boundary edges of the mesh and stored in a queue. From the boundary edges, new triangles are progressively generated. Note that the blue lines combined with one boundary edge will construct a new triangle if it satisfies two specified conditions. If two boundary edges make an angle less than 70 degrees, the three vertices on these two edges are used to produce a new triangle, see the triangle consisting of the green dashed line and the other two boundary edges.