In legacy (old) dolfin, there’s a parameter:
parameters["refinement_algorithm"]
which has the options
[plaza,plaza_with_parent_facets,regular_cut]
You can read the source to find out exactly what these do. But I believe the most popular is plaza refinement with parent facets since it preserves parent-child information between refinement levels.
Two reasons for bisecting into 4 rather than 2 simplices:
- Old dolfin doesn’t support non-conforming meshes, you cannot have hanging nodes.
- Following the previous point, it’s important to preserve mesh quality. This is extremely difficult if you restrict yourself to bisection of triangles only.