Relation between required faces or elements between remeshes

Developers Help
#1

Hi

is there a way to know the relation (in terms of ids) between required faces or elements between remeshes ?
are they kept in the same order as they were given (not the same id but the same order) ?
I have faces or elements that are tagged required. I want re-evaluate fields efficiently on these faces/elements.

Best regards

#2

After some experiments, it seems that required faces/elements are not ordered the same way before and after remeshing.
Is there a way to know the id relation of required faces/elements before and after remeshing ?

#3

Hello,
Yes, required tetrahedra and triangles have different IDs after remeshing (either due to calls to Scotch renumbering or due to mesh packing).
A practical (although not sophisticated) way to retrieve their original IDs is to store them in their reference field when defining the required elements through the API functions MMG3D_Set_tetrahedron and MMG3D_Set_triangle (or their ParMmg equivalents), since the reference field is always preserved during the adaptation. In parallel, the global IDs can do the trick.

Hope this helps.
Best regards,
Luca

#4

indeed ! I should have thought about that. thanks a lot !!

1 Like
#5

One caveat is that for Mmg the interface between elements with different references is seen as an immersed surface: so you will see more triangles (in 3D) in the output mesh. Anyway this should not be a problem as these triangles will also be required, and the additional cost/memory should be OK as long as not too many tetrahedra are set as required…

Question on Lagrangian movement + relation between meshes
#6

thanks for the remark.

in fact, we may either require element or facet. The later should be enough. I just need to keep track of the interface.
In my case, it is the surface of a swimmer(some parts are actually elastic).

Best regards

#7

Hi,

I have the same question with vertices: can one keep track of the positions (in the input file) of the vertices of required triangles after remeshing ? I have tried using MMG2D_Set_vertex to set the id of the vertices as references but did not succeed… If one has an example, it would be nice to look at it ! :slight_smile:

The objective would be to refill a P1 finite element function at some dofs (vertices) that were not modified after calling mmg (since some triangles are set to “required”)

May be there is a better way to do that by giving directly the P1 function to mmg before remeshing ?

Thank you for your help.
Cheers.

#8

Hi,

Normally it works: if the vertices are marked as required, which is the case if they belong to a required triangle, their references should be preserved during the remeshing.

I have tested on a small mesh (1.1 KB). Vertices 12, 14 and 25 are marked as required and are assign different references (resp. 1, 2 and 3). The remeshing seems preserving it (in library mode too).

It is a feature that is not used much, maybe are you facing a bug: what is doing your code exactly?

Thank you by advance,
Algiane

#9

Hi Algiane,

Thank you for your answer. It’s my bad, I thought I had to change the references of the vertices inside the call to mmg. If I mark my vertices before calling mmg, they are indeed preserved.

So, everything woks fine !
Sorry for the misunderstanding and thank you very much for your explanations !
Cheers.

#10