# vtkPropPicker cannot pick vtkFollower with 0.8 opacity

**URL:** https://discourse.vtk.org/t/vtkproppicker-cannot-pick-vtkfollower-with-0-8-opacity/10874
**Category:** Support
**Created:** [March 3, 2023, 2:39pm UTC](https://discourse.vtk.org/t/vtkproppicker-cannot-pick-vtkfollower-with-0-8-opacity/10874 "2023-03-03T14:39:24Z")
**Posts on this page:** 1
**Page:** 1

<div class="post-metadata">

### Author: ![zhang-qiang-github](https://discourse.vtk.org/user_avatar/discourse.vtk.org/zhang-qiang-github/32/2519_2.png) [@zhang-qiang-github](https://discourse.vtk.org/u/zhang-qiang-github)
#### Post date: [March 3, 2023, 2:39pm UTC](https://discourse.vtk.org/t/vtkproppicker-cannot-pick-vtkfollower-with-0-8-opacity/10874/1 "2023-03-03T14:39:24Z")

</div>

I find `vtkPropPicker` can not pick a `vtkFollower` with 0.8 opacity, but it can pick `vtkActor` with 0.8 opacity. My test code is:

```auto
import vtkmodules.all as vtk

cylinder1 = vtk.vtkCylinderSource()
cylinder1.SetCenter(0, 0, 0)
cylinder1.Update()

mapper1 = vtk.vtkPolyDataMapper()
mapper1.SetInputData(cylinder1.GetOutput())

actor1 = vtk.vtkActor()
actor1.SetMapper(mapper1)
actor1.GetProperty().SetColor(1, 0, 0)
actor1.GetProperty().SetOpacity(0.8)

cylinder2 = vtk.vtkCylinderSource()
cylinder2.SetCenter(2, 0, 0)
cylinder2.Update()

mapper2 = vtk.vtkPolyDataMapper()
mapper2.SetInputData(cylinder2.GetOutput())

actor2 = vtk.vtkFollower()
actor2.SetMapper(mapper2)
actor2.GetProperty().SetColor(0, 1, 0)
actor2.GetProperty().SetOpacity(0.8)

cylinder3 = vtk.vtkCylinderSource()
cylinder3.SetCenter(4, 0, 0)
cylinder3.Update()

mapper3 = vtk.vtkPolyDataMapper()
mapper3.SetInputData(cylinder3.GetOutput())

actor3 = vtk.vtkFollower()
actor3.SetMapper(mapper3)
actor3.GetProperty().SetColor(0, 0, 1)
actor3.GetProperty().SetOpacity(1)

renderer = vtk.vtkRenderer()
renderer.AddActor(actor1)
renderer.AddActor(actor2)
renderer.AddActor(actor3)

renWin = vtk.vtkRenderWindow()
renWin.AddRenderer(renderer)

def MouseMoveEvent(iren, evt):
    x, y = iren.GetEventPosition()

    picker1 = vtk.vtkPropPicker()
    picker1.Pick(x, y, 0, renderer)
    ac = picker1.GetViewProp()
    if ac is None:
        print('cannot find actor')
        return
    acs = [actor1, actor2, actor3]
    idx = acs.index(ac)
    print('find actor: ', idx+1)

iren = vtk.vtkRenderWindowInteractor()
iren.SetRenderWindow(renWin)

iren.AddObserver('MouseMoveEvent', MouseMoveEvent)
iren.Initialize()
iren.Start()

```

There are three actors:

| id | type | opacity |
| --- | --- | --- |
| 1 | vtkActor | 0.8 |
| 2 | vtkFollower | 0.8 |
| 3 | vtkFollower | 1 |

When move the mouse over the window, the application would print `cannot find actor` or `find actor: 1/2/3`.

When run the code, we can see that the `vtkPropPicker` can not pick `actor 2`. Is it a bug?

My environment is:

```auto
win 10
python 3.9.12
vtk 9.2

```