[vtkusers] vtkUnstructuredGrid - how to detect if a cell as neighbors?

[Teresa Azevedo]

Hi all.

I would like to extract the boundary cells of an unstructured grid,  
that is, those who have at least one face with no neighbors.
Here is a test code, that builds an unstructured grid, with two voxels  
that share one face. When I check if a cell of the grid has neighbors,  
the code prompts a message box. Unfortunately, when I run it, I never  
get the message.
Can someone tell me what I am doing wrong?

         vtkUnstructuredGrid* aVoxelGrid = vtkUnstructuredGrid::New();
	aVoxelGrid->Allocate(2, 2);

	vtkPoints* vPts = vtkPoints::New();
	vPts->SetNumberOfPoints(16);
	vPts->InsertPoint( 0, 0, 0, 0);
	vPts->InsertPoint( 1, 1, 0, 0);
	vPts->InsertPoint( 2, 0, 1, 0);
	vPts->InsertPoint( 3, 1, 1, 0);
	vPts->InsertPoint( 4, 0, 0, 1);
	vPts->InsertPoint( 5, 1, 0, 1);
	vPts->InsertPoint( 6, 0, 1, 1);
	vPts->InsertPoint( 7, 1, 1, 1);

	vtkVoxel* vl = vtkVoxel::New();
	vl->GetPointIds()->SetId(0, 0);
	vl->GetPointIds()->SetId(1, 1);
	vl->GetPointIds()->SetId(2, 2);
	vl->GetPointIds()->SetId(3, 3);
	vl->GetPointIds()->SetId(4, 4);
	vl->GetPointIds()->SetId(5, 5);
	vl->GetPointIds()->SetId(6, 6);
	vl->GetPointIds()->SetId(7, 7);

	aVoxelGrid->InsertNextCell(VTK_VOXEL, vl->GetPointIds());

	vPts->InsertPoint( 8, 0, 0, 1);
	vPts->InsertPoint( 9, 1, 0, 1);
	vPts->InsertPoint( 10, 0, 1, 1);
	vPts->InsertPoint( 11, 1, 1, 1);
	vPts->InsertPoint( 12, 0, 0, 2);
	vPts->InsertPoint( 13, 1, 0, 2);
	vPts->InsertPoint( 14, 0, 1, 2);
	vPts->InsertPoint( 15, 1, 1, 2);

	vtkVoxel* v2 = vtkVoxel::New();
	v2->GetPointIds()->SetId(0, 8);
	v2->GetPointIds()->SetId(1, 9);
	v2->GetPointIds()->SetId(2, 10);
	v2->GetPointIds()->SetId(3, 11);
	v2->GetPointIds()->SetId(4, 12);
	v2->GetPointIds()->SetId(5, 13);
	v2->GetPointIds()->SetId(6, 14);
	v2->GetPointIds()->SetId(7, 15);

	aVoxelGrid->InsertNextCell(VTK_VOXEL, v2->GetPointIds());
	aVoxelGrid->SetPoints(vPts);
	aVoxelGrid->Update();

	vtkGenericCell* cell = vtkGenericCell::New();
	for( int i = 0; i < aVoxelGrid->GetNumberOfCells(); i++ )
	{
		aVoxelGrid->GetCell( i, cell );
		vtkIdList* cellNeighbors = vtkIdList::New();
		int numFaces = cell->GetNumberOfFaces();
		for ( int j = 0; j < numFaces; j++ )
		{
			vtkCell* face = cell->GetFace( j );
			aVoxelGrid->GetCellNeighbors( i, face->PointIds, cellNeighbors);
			if ( cellNeighbors->GetNumberOfIds() > 0 )
			{
				MessageBox(NULL, "Cell has neighbors!", NULL, MB_OK);
			}
		}
	 }


Thank you all.
Teresa