[vtkusers] get boundary triangles from a mesh

[pam]

Hi all!
I am working with Delaunay3D so when I get the mesh I wanna get only
the boundary triangles. I was trying to do it with vtkDataSetMapper
because "the 2D faces of 3D cells are mapped only if they are used by
only one cell, i.e., on the boundary of the data set."
I could show this but how could get the list of cell or faces with
form the mesh. I was trying to get a vtkPolyData in this way

map=vtkDataSetMapper::New();
map->SetInput(delny->GetOutput());
map->Update();

 vtkPolyData* d=vtkPolyData::New();
 d=vtkPolyData::SafeDownCast(map->GetInput());
 nroceldas=d->GetNumberOfCells();

but I get an error on run time when I try to get number of cells.

there is another way to get only the boundary triangles from a mesh.

Thanks in adavance.

Pamela