Mfem: How to obtain the face area and normal direction in a tetrahedra .

[ztdepztdep]

Dear sir:
I want to obtain the area and normal direction of each face in a tetrahedra, which function can resolve this.
Regards

[vikramsg]

I haven't used grids with tetrahedra, but I guess it should be similar to hexahedra. You go to each face and use transfomations to get the normals like this.

for (int i = 0; i < mesh->GetNumFaces();; i++)
{
 T = pmesh->GetInteriorFaceTransformations(i); 
 CalcOrtho(T->Face->Jacobian(), nor);
}

The vector nor also contains information about the surface area. If you just get the magnitude of the vector, it is the surface area.

[v-dobrev]

If you just need the face normal and area, it is cheaper to use Mesh::GetFaceTransformation; also before calling the method ElementTransformation::Jacobian you should set the point at which you want to compute the normal and the area, for example:

   Vector nor;
   const int nfaces = mesh->GetNumFaces();
   for (int i = 0; i < nfaces; i++)
   {
      ElementTransformation *T = mesh->GetFaceTransformation(i);
      // Evaluate the Jacobian at the center of the face:
      T->SetIntPoint(&Geometries.GetCenter(mesh->GetFaceBaseGeometry(i)));
      CalcOrtho(T->Jacobian(), nor);
      // Use 'nor' ...
   }

[fanronghong]

Hi @v-dobrev ,

How to get the direction of every normal vector?