### How to change the mesh of a dolphin::Function in C++?

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Hi all,

this issue maybe is simple, but I am dealing with a lot of problems.

I have a dolfin::Function, say rho, on a dolfin::Mesh Th. I want to interpolate, or to "change" the mesh of rho with another mesh, say Th1 (the result of an adaption, same geometry). Consider that this new mesh is "good" or "usable", because I can correctly solve linear problems on it (redefining everything).

Firstly I tried to use dolfin::Function::interpolate in the following way:

but this gives the following run-time error:

*** Error: Unable to successfully call PETSc function 'VecSetValuesLocal'.

*** Reason: PETSc error code is: 63 (Argument out of range).

when the dolphin::Function::interpolate function is called (last line).

Then, I tried to use LagrangeInterpolator in the following way:

but this gives a segmentation fault when dolfin::LagrangeInterpolator::interpolate is called.

Do you have any suggestion in order to solve this problem?

Thank you in advance.
For non-matching meshes, you def want the LagrangeInterpolator. A little MWE with Python is handy.

this issue maybe is simple, but I am dealing with a lot of problems.

I have a dolfin::Function, say rho, on a dolfin::Mesh Th. I want to interpolate, or to "change" the mesh of rho with another mesh, say Th1 (the result of an adaption, same geometry). Consider that this new mesh is "good" or "usable", because I can correctly solve linear problems on it (redefining everything).

Firstly I tried to use dolfin::Function::interpolate in the following way:

```
Th=Th1; // substitute the old mesh with the new one
Vh =std::make_shared<varfs::CoefficientSpace_rho>(Th); // recompute the fespace
auto temp=rho; // create a temporary function
rho=std::make_shared<Function>(Vh); // clear the old function
temp->interpolate(*rho); // interpolate
```

but this gives the following run-time error:

*** Error: Unable to successfully call PETSc function 'VecSetValuesLocal'.

*** Reason: PETSc error code is: 63 (Argument out of range).

when the dolphin::Function::interpolate function is called (last line).

Then, I tried to use LagrangeInterpolator in the following way:

```
Th=Th1;
Vh =std::make_shared<varfs::CoefficientSpace_rho>(Th);
auto temp=rho;
rho=std::make_shared<Function>(Vh);
LagrangeInterpolator funInterpolator;
funInterpolator.interpolate(*rho,*temp);
```

but this gives a segmentation fault when dolfin::LagrangeInterpolator::interpolate is called.

Do you have any suggestion in order to solve this problem?

Thank you in advance.

Community: FEniCS Project

written
7 months ago by
pf4d

### 1 Answer

0

Hi Francesco,

I included APIs to copy dolfin::Function objects to and from Omega_h so that Omega_h can do the interpolation. The idea is to use from_dolfin(dolfin::Function) before adaptation and to_dolfin(dolfin::Function) after the adaptation. I think you should still create a new dolfin::Function using the new dolfin::Mesh. Note that I have not yet tested to_dolfin(dolfin::Function). Feel free to discuss this via email with me.

Edit: I don't know how to format inline code on this site.

I included APIs to copy dolfin::Function objects to and from Omega_h so that Omega_h can do the interpolation. The idea is to use from_dolfin(dolfin::Function) before adaptation and to_dolfin(dolfin::Function) after the adaptation. I think you should still create a new dolfin::Function using the new dolfin::Mesh. Note that I have not yet tested to_dolfin(dolfin::Function). Feel free to discuss this via email with me.

Edit: I don't know how to format inline code on this site.

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