obbTree is an object to generate oriented bounding box (OBB)
        trees. An oriented bounding box is a bounding box that does not
        necessarily line up along coordinate axes. The OBB tree is a
        hierarchical tree structure of such boxes, where deeper levels of OBB
        confine smaller regions of space.

def obbTree(self):
        """obbTree is an object to generate oriented bounding box (OBB)
        trees. An oriented bounding box is a bounding box that does not
        necessarily line up along coordinate axes. The OBB tree is a
        hierarchical tree structure of such boxes, where deeper levels of OBB
        confine smaller regions of space.
        """
        if not hasattr(self, '_obbTree'):
            self._obbTree = vtk.vtkOBBTree()
            self._obbTree.SetDataSet(self)
            self._obbTree.BuildLocator()

        return self._obbTree