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[Mael Rouxel-Labbé <>]

In addition to what Maxime said :

You should use Euler::remove_face() and not Surface_mesh::remove_face() because the former function will take care of removing elements that should not exist anymore (edges whose two incident faces have been removed, vertices whose star has been removed, ...).

Once that is done, your problem is indeed the presence of non manifold vertices, which violates a precondition of smooth_mesh(). You can remove non-manifold vertices using Polygon_mesh_processing::duplicate_non_manifold_vertices(), and your code will then run fine.

Best,
Mael

On 15/07/2020 10:17, Maxime Gimeno ( via cgal-discuss Mailing List) wrote:

Hello,

From what I see, removing faces creates some non-manifold vertices. It usually means that Polygon Mesh Processing algorithms won't work.

For some reason I could not repair the mesh after the faces removal, but replacing your function with `experimental::remove_self_intersections()` worked fine. 

Le mar. 14 juil. 2020 à 03:36, Yaoyu Hu <> a écrit :

Hi Everyone, 

I am new to CGAL and I am running into an issue when I am trying to smooth a surface mesh which I just performed a face removal.

The surface mesh is loaded from a file as a CGAL::Surface_mesh object. The original mesh has some defects such as small dangling faces and self-intersected faces. I decided to deal with the self-interactions first. I collected all the self-intersections by using CGAL::Polygon_mesh_processing::self_intersections() function. And I insert the face descriptors into an std::set. Then I loop over all the face descriptors saved in the std::set and do a remove_face(). After that, a call to collect_garbage() is issued on the Surface_mesh object. 

So far so good. However, when I try to continue performing other operations on the Surface_mesh object, I get an exception from CGAL. The operation I'd like to do is a smoothing by CGAL::Polygon_mesh_processing::smooth_mesh(). The exception I got is as follows:

=== Terminal output begins. ===

terminate called after throwing an instance of 'CGAL::Assertion_exception'
  what():  CGAL ERROR: assertion violation!
Expr: _idx < data_.size()

File: /path/to/cgal/Surface_mesh/include/CGAL/Surface_mesh/Properties.h

Line: 195

=== Terminal output ends. ===

I am using Ubuntu 18.04 and CGAL 5.0.2 cloned from the Github repo.

You could find the sample code and the original mesh as the attachments.

My questions are 

(1) What is the correct procedure to remove faces from a Surface_mesh object and make the downstream process works correctly? I also tried Euler operations but got no luck. There will be the same exception.

(2) In the original mesh, there are som small faces dangling around the top and bottom borders. How can I robustly remove those dangling faces? I tried to use face area to do thresholding on all the faces. It sort of works but later I still get the above exceptions in the smoothing section.

Regards and thank you!

Yaoyu 

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