Creates segments on the convex hull of the triangulation. If you are triangulating a vertex set, this switch causes a .poly file to be written, containing all edges of the convex hull. If you are triangulating a PSLG, this switch specifies that the whole convex hull of the PSLG should be triangulated, regardless of what segments the PSLG has. If you do not use this switch when triangulating a PSLG, Triangle assumes that you have identified the region to be triangulated by surrounding it with segments of the input PSLG. Beware: if you are not careful, this switch can cause the introduction of an extremely thin angle between a PSLG segment and a convex hull segment, which can cause overrefinement (and possibly failure if Triangle runs out of precision). If you are refining a mesh, the -c switch works differently: it causes a .poly file to be written containing the boundary edges of the mesh (useful if no .poly file was read).