2D geometry meshing fails for difficult geometries

I reported this issue on the github project page of NetGen, but didn’t receive an answer for it. That’s why I am reposting the question here.

This is the link to the Github issue:

Here is the original text:

We are using an older NetGen version (5.3.1) with great success. Thank you for this great library! Recently we wanted to upgrade to the newest version, but now some of our unit tests fail. The failing unit test creates many partly random geometries and tries to mesh them. Some geometries however fail to mesh, and the message is printed:

 give up with qualclass 201
 number of frontlines = 3

One geometry which fails with the default settings of the distributed NGSolve/NetGen 6.2.2301 is attached (errgeom2.in2d in the ZIP file). By slightly changing the parameters (for example mesh-size grading, hmin/hmax, Elements per edge) the geometry meshes fine, but using our unit test we can find failing geometries for almost every combination of parameters.


I am not sure this is actually a bug. If it isn’t, I would like to know how to work around the meshing failure, for example a set of parameters which works for all generated geometries.
I know that the geometries are very special, since some lines are very close to each other, but the old version of NetGen was able to mesh them.

Thanks in advance!