### Author:  [@prj](https://community.freefem.org/u/prj)
#### Post date: [November 24, 2020, 6:21pm UTC](https://community.freefem.org/t/issue-with-transfer-between-two-meshes/667/2 "2020-11-24T18:21:18Z")
Dear Christophe,
Very interesting issue! The example you use, `maxwell-mg-3d-PETSc-complex.edp`, deals with _nested_ meshes. Meaning that the meshes are build by uniformly refining the one from the previous level. In that case, you don’t need to transfer anything, the interpolation is purely local on each process. You can simply write
```auto
def(u2) = def(u);
```
If you want to transfer variables between non-nested meshes, you can for example have a look at [this](https://community.freefem.org/t/copy-variables-between-different-meshes-with-petsc/390) other post.
---
### Author:  [@christof](https://community.freefem.org/u/christof)
#### Post date: [November 24, 2020, 8:39pm UTC](https://community.freefem.org/t/issue-with-transfer-between-two-meshes/667/3 "2020-11-24T20:39:46Z")
In fact I want to transfer the result in variable u from the local meshes to the global mesh. Even when I recreate the original coarse mesh by adding
mesh3 ThGlobal=cube(n, n, n, [x, y, z], label = LL);
fespace Vh2(ThGlobal,Pk);
Vh2\ def(u2);
transfer(Th[0],Pk,def(u),ThGlobal,Pk,def(u2));
if (mpirank==0)
savevtk(“onemesh.vtu”,ThGlobal,[real(u2), real(u2y), real(u2z)], [imag(u2), imag(u2y), imag(u2z)], order = fforder, bin = 1);
I get the same error. Is there a way to do this?
---
### Author:  [@prj](https://community.freefem.org/u/prj)
#### Post date: [November 24, 2020, 8:49pm UTC](https://community.freefem.org/t/issue-with-transfer-between-two-meshes/667/4 "2020-11-24T20:49:15Z")
Why do you want to do that (going back to the global mesh)?
Anyway, this is not the proper way to do it. You can have a look at [maxwell-3d-PETSc.edp](https://github.com/FreeFem/FreeFem-sources/blob/master/examples/hpddm/maxwell-3d-PETSc.edp) that rebuilds a global solution (but you should avoid that). You’ll have to use the `N2O` keyword, see lines 28 and 80.
---
### Author:  [@prj](https://community.freefem.org/u/prj)
#### Post date: [November 24, 2020, 9:14pm UTC](https://community.freefem.org/t/issue-with-transfer-between-two-meshes/667/5 "2020-11-24T21:14:46Z")
I forgot to say, but the goal of using multiple levels by doing uniform refinement is to generate extremely fine global meshes without having to store them on a single process. So trying to gather everything on a single process is kind of conter-intuitive. What is it that you want to do exactly if I may ask, please?
---
### Author:  [@christof](https://community.freefem.org/u/christof)
#### Post date: [November 24, 2020, 9:22pm UTC](https://community.freefem.org/t/issue-with-transfer-between-two-meshes/667/6 "2020-11-24T21:22:30Z")
I would like to interpolate the result from u at specific points (x,y,z) and save those values in a unique file. I thought it might be easier if only one process was doing this operation, so that the file is not written in parallel.
---
### Author:  [@prj](https://community.freefem.org/u/prj)
#### Post date: [November 24, 2020, 9:33pm UTC](https://community.freefem.org/t/issue-with-transfer-between-two-meshes/667/7 "2020-11-24T21:33:42Z")
So, first, you can do exactly that from inside ParaView, using a distributed output, as currently done in the initial file you mentioned. You just have to apply a filter.
If you want to do this directly in FreeFEM, I think it would be much more efficient, especially for very large mesh, to just:
1. for each point (x, y, z), use the `chi` function to know if said point is in the local mesh, see [https://doc.freefem.org/references/functions.html#chi](https://doc.freefem.org/references/functions.html#chi)
2. if the point is local to the subdomain, get the value
3. afterwards, do a loop, and let all processes write in sequence your unique file where you store all the values
If you want, you can even make step 3. much more efficient by simply creating an array `complex[int]` and doing an `mpiReduce` of this array on process #0, where each process fills the array for points local to its subdomain, see 2. After the reduction, only process 0 will write the result to disk.
---
### Author:  [@christof](https://community.freefem.org/u/christof)
#### Post date: [November 24, 2020, 9:54pm UTC](https://community.freefem.org/t/issue-with-transfer-between-two-meshes/667/8 "2020-11-24T21:54:49Z")
OK, thank you, the chi function is the one I was missing!