from __future__ import print_function
import vtk
import vtk.util.numpy_support as VN

# Callback for when selection is changed
def selectionCallback(caller,eventId):
       hsel = vtk.vtkHardwareSelector()
       hsel.SetFieldAssociation(vtk.vtkDataObject.FIELD_ASSOCIATION_CELLS)
       hsel.SetRenderer(ren1)

       x,y = caller.GetRenderWindow().GetSize()

       # Create a small area around clicked point for selector area
       hsel.SetArea(0,0,x,y)
       res = hsel.Select()

       numNodes = res.GetNumberOfNodes()
       if (numNodes < 1):
               print("No visible cells")
       else:
               sel_node = res.GetNode(0)
               print('Visible cell IDs: ', VN.vtk_to_numpy(sel_node.GetSelectionList()).tolist())


sphere = vtk.vtkSphereSource()
sphere.SetCenter( 0, 0, 0 )
sphere.SetRadius( 5.0 )

sphereMapper = vtk.vtkPolyDataMapper()
sphereMapper.SetInputConnection(sphere.GetOutputPort())

sphereActor = vtk.vtkActor()
sphereActor.SetMapper(sphereMapper)

ren1 = vtk.vtkRenderer()
ren1.AddActor(sphereActor)
ren1.SetBackground(0.1, 0.2, 0.4)
ren1.GetActiveCamera().ParallelProjectionOn()

renWin = vtk.vtkRenderWindow()
renWin.AddRenderer(ren1)
renWin.SetSize(300, 300)

iren = vtk.vtkRenderWindowInteractor()
iren.SetRenderWindow(renWin)
iren.AddObserver("UserEvent", selectionCallback)

style = vtk.vtkInteractorStyleTrackballCamera()
iren.SetInteractorStyle(style)
renWin.GetInteractor().SetInteractorStyle(style)

ren1.ResetCamera()
renWin.Render()

iren.Initialize()
iren.Start()
