Exception generated when trying to get the face indices on a specific element

[i3s93]

Hello,

I'm working with an application where I need to loop over faces associated with a particular element. My initial approach for this was to generate the element-to-face table using elem_face_tbl = mesh.ElementToFaceTable(), but it seems that this method is not currently supported by PyMFEM. Instead, for each element, I use the method faces, orientations = mesh.GetElementFaces(i) to return the face indices and their respective orientations as separate Python lists. When I do this, I see the following exception:

Verification failed: (el_to_face != NULL) is false:
 --> el_to_face not generated
 ... in function: void mfem::Mesh::GetElementFaces(int, mfem::Array<int>&, mfem::Array<int>&) const
 ... in file: /PyMFEM/external/mfem/mesh/mesh.cpp:6150

The relevant part of the traceback is here:

  File "/PyMFEM_env/lib/python3.9/site-packages/mfem/_ser/mesh.py", line 670, in GetElementFaces
    _mesh.Mesh_GetElementFaces(self, i, ia, ib)
RuntimeError: PyMFEM error (mfem::ErrorException): Verification failed: (el_to_face != NULL) is false:
 --> el_to_face not generated
 ... in function: void mfem::Mesh::GetElementFaces(int, mfem::Array<int>&, mfem::Array<int>&) const
 ... in file: /PyMFEM/external/mfem/mesh/mesh.cpp:6150

Internally, the call to MFEM uses the element to face table that I was originally trying to use, and I think the two problems might be related. I've checked other methods, such as mesh.GetElementVertices(i), and they work just fine, but I need access to the faces in my particular application. I'm following the API provided in docs/manual.txt (see 4-4). I'm looking to see if there are any recommendations for fixing this.

Thank you, in advance, for your help and suggestions!

[sshiraiwa]

@i3s93
In my environment, both work just fine. See below. Actually, I do recall that I encountered this issue before.
Reading mesh.cpp, this table is created from GetElementToFaceTable. And this method is, generally, called
when dim>2 (3D mesh). How are you creating the mesh object? Maybe there is a specific pattern in which the table
is not create.

Python 3.8.10 (default, Nov 14 2022, 12:59:47) 
[GCC 9.4.0] on linux
Type "help", "copyright", "credits" or "license" for more information.
>>> import mfem
>>> mfem.__version__
'4.5.0.2rc1'
>>> import mfem.ser as mfem
>>> mesh = mfem.Mesh('beam-tet.mesh')
>>> faces, orientations = mesh.GetElementFaces(2)
>>> faces
[7, 8, 6, 9]
>>> mesh.ElementToFaceTable()
<mfem._ser.table.Table; proxy of <Swig Object of type 'mfem::Table *' at 0x37a88b0> >

[i3s93]

Hi @sshiraiwa, thank you for your response. Actually, the mesh I used in my example is a 2-D Cartesian grid, which I create using mesh = mfem.Mesh(N_x, N_y, elem_type). As a workaround, I manually created this lookup table, which works fine, but I was hoping to just use the existing functionality. By any chance, do you recall what caused this issue before?

[i3s93]

@sshiraiwa Just to follow up, the issue only seems to happen when I use a Cartesian mesh. If I read in the mesh, as you have done, the error goes away. It looks like the table is not being generated for Cartesian meshes.

[v-dobrev]

Mesh::GetElementFaces is a 3D-only method. In 2D you probably want to call Mesh::GetElementEdges.

One has to be cautious -- sometimes mfem uses "faces" to refer to (dim-1)-dimensional mesh entities and sometimes to refer specifically to 2-dimensional mesh entities.

[sshiraiwa]

@v-dobrev , Thank you for the clarification. @i3s93, closing this for now. Feel free to open if your issue is not resolved.