step-75 interpolate BC inside multigrid matrices construction

[yy.wayne]

Hello, everyone.

I've got a question about step-75 for the use of VectorTools::interpolate_boundary_values inside solve_with_gmg() function, where mg_matrices are built.

The question is here interpolate_boundary_values uses a ZeroFunction, while the real PDE in this tutorial has non-zero Dirichlet BC on all boundaries. Why set to ZeroFunction and why the result is not impacted here?

I came across this problem when I'm combining step-29 and step-75, the former inquires an Absorption Boundary Condition(ABC) so it's not Dirichlet BC everywhere. An it seems all ABC are not applied, matrix entries corresponding to boundary DoFs are non-zero only on diagonal. This implies they are treated as Dirichlet BC. 

Thank you