Hello everyone,

I have difficulties in understanding the interpolation mesh in preCICE.

Running the solverdummy, I can send and recieve data from the coupling partners.

I have modified the case a little bit.

MeshOne has now the vertices (which I set with `interface.set_mesh_vertices`

):

```
[[0., 0., 0.],[1., 1., 1.],[2., 2., 2.]]
```

MeshTwo has now the vertices:

```
[[0., 0., 0.],[0., 0., 0.],[0., 0., 0.]]
```

For the first communication, SolverOne is sending the following to SolverTwo:

```
[[1., 1., 1.],[1., 1., 1.],[1., 1., 1.]]
```

Now my question is, why SolverTwo is still recieving, although the two solvers have different meshes:

```
[[1., 1., 1.],[1., 1., 1.],[1., 1., 1.]]
```

Is there an explanation/documentation of the interpolation mesh between two solvers? Or even better: Is there a mathematical formula to understand the transformation between the meshes?

In the next step I am sending back the following from SolverTwo:

```
[[1., 1., 1.],[1., 1., 1.],[1., 1., 1.]]
```

SolverOne is now recieving this:

```
[[3., 3., 3.],[0., 0., 0.],[0., 0., 0.]]
```

I cannot understand, why there is a transformation in the one way, but not in the other one. Further more I do not unterstand the way, how the matrix is transformed.

If you want to look at the modified solver, please see the attatchments.

solverdummy.py.txt (2.9 KB)