Hello,

I am working with subdomains and I need to identify the internal boundary (interface between both subdomains) for integration. For the subdomains I am using a mesh function named “domains” and by running `plot (domains)`

i can successfully see that the subdomains are marked correctly. Then, I am also using a mesh function to mark the interface between them, but I’m having trouble plotting this to make sure that I am doing it correctly. Could anyone help me on what is the easiest way to visualize this internal boundary? Thank you! This is how I am defining the mesh functions:

```
class Omega(SubDomain):
def inside(self, x, on_boundary):
return phi(x) > 0
domains = MeshFunction("size_t", mesh, mesh.topology().dim())
omega = Omega()
domains.set_all(0)
omega.mark(domains, 1)
dx = Measure('dx')(subdomain_data = domains)
```

and ploting domains gives me for example:

Then, for the boundary this is what I am doing:

```
class IntBound(SubDomain):
def inside(self, x, on_boundary):
return near(phi(x),.0)
intBound = IntBound()
int_boundary = MeshFunction("size_t", mesh, mesh.topology().dim()-1)
int_boundary.set_all(0)
intBound.mark(int_boundary, 1)
ds_int = Measure("ds")(subdomain_data=int_boundary) #ds(1) integrates on internal boundary
```

But when I try plotting int_boundary I get

I am not sure if I am not marking correctly of if is a plot problem.