# vtkCellPicker unexpected work

**URL:** https://discourse.vtk.org/t/vtkcellpicker-unexpected-work/7686
**Category:** Support
**Created:** [January 20, 2022, 9:24am UTC](https://discourse.vtk.org/t/vtkcellpicker-unexpected-work/7686 "2022-01-20T09:24:59Z")
**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: [January 20, 2022, 9:24am UTC](https://discourse.vtk.org/t/vtkcellpicker-unexpected-work/7686/1 "2022-01-20T09:24:59Z")

</div>

I am using `vtkCellPicker` in my project, and I find it unexpected work.

The code to reproduce my problem is:

```auto
import vtk

lineSource = vtk.vtkLineSource()
lineSource.SetPoint1(0, 0, 0)
lineSource.SetPoint2(512, 512, 0)
lineSource.Update()
lineMapper = vtk.vtkPolyDataMapper()
lineMapper.SetInputData(lineSource.GetOutput())
lineActor = vtk.vtkActor()
lineActor.SetMapper(lineMapper)
lineActor.GetProperty().SetColor(255, 0, 0)
lineActor.GetProperty().SetLineWidth(10)

pointPolyData = vtk.vtkPolyData()
pointPolyData.SetPoints(vtk.vtkPoints())
pointPolyData.SetVerts(vtk.vtkCellArray())
pointPolyData.GetPoints().InsertNextPoint(0, 0, 0)
pointPolyData.GetVerts().InsertNextCell(1)
pointPolyData.GetVerts().InsertCellPoint(0)
pointMapper = vtk.vtkPolyDataMapper()
pointMapper.SetInputData(pointPolyData)
pointActor = vtk.vtkActor()
pointActor.SetMapper(pointMapper)
pointActor.GetProperty().SetColor(255, 255, 0)
pointActor.GetProperty().SetPointSize(10)
pointActor.GetProperty().SetRepresentationToPoints()
pointActor.GetProperty().RenderPointsAsSpheresOn()

renderer = vtk.vtkRenderer()
renderer.AddActor(lineActor) ########################### order
renderer.AddActor(pointActor) ##########################
renderer.SetBackground(0.4, 0.5, 0.6)

renderWindow = vtk.vtkRenderWindow()
renderWindow.AddRenderer(renderer)
renderWindow.Render()

def mousemove(iren, even):
    pos = iren.GetEventPosition()
    picker = vtk.vtkCellPicker()
    picker.SetTolerance(0.05)
    picker.Pick(pos[0], pos[1], 0, renderer)
    actor = picker.GetProp3D()
    if actor == lineActor:
        print('line')
    elif actor == pointActor:
        print('point')
    else:
        print('none')

renderWindowInteractor = vtk.vtkRenderWindowInteractor()
renderWindowInteractor.AddObserver("MouseMoveEvent", mousemove)

style = vtk.vtkInteractorStyleImage()
renderWindowInteractor.SetInteractorStyle(style)
renderWindowInteractor.SetRenderWindow(renderWindow)
renderWindowInteractor.Initialize()
renderWindowInteractor.Start()

```

Please have a look for the following figure:

![image](https://discourse.vtk.org/uploads/default/original/2X/f/f0452d3b2e0e82bb96c309b6b4d8cf7ee58a7f16.png)

In the above figure, I can pick the `point` in the white arrow. However, I can only pick `line` for the blue arrow, but I expect the blue arrow can pick the `point`.

I find it may be caused by the order to add `point` and `line`. If I change the order to:

```auto
renderer.AddActor(pointActor)
renderer.AddActor(lineActor)

```

It can pick `point` and 'line` as what I expect.

However, in my thought, the order to add `point`/`line` should have no effect for my situation. Because, the position of blue arrow is more close to the `point` than the `line`.

Then, I have a look into c++ code. After debug, I find `vtkCellPicker` actually picked both `point` and `line`, and add it to `Prop3Ds` (`vtkPicker.cxx line 689`).  
Then, in the `GetViewProp`, it return `this->Path->GetFirstNode()->GetViewProp()`, it only return the first node rather than to calculate the distance between picked prop and pick position.

How can I do to pick the `point` at the position of blue arrow?

Any suggestion is appreciated~~~