The -c switch

• Creates segments on the convex hull of the triangulation.
• If you are triangulating a point set, this switch causes a .poly file to be written, containing all edges in the convex hull. (By default, a .poly file is normally written only if a .poly file is read.)
• If you are triangulating a PSLG, this switch specifies that the interior of the convex hull of the PSLG should be triangulated. If you do not use this switch when triangulating a PSLG, it is assumed 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 Triangle's quality meshing algorithm to fail to terminate.
• If you are refining a mesh, this switch works differently; it generates the set of boundary edges of the mesh, including boundaries of holes.