# A potential bug for vtkPolyData
**URL:** https://discourse.vtk.org/t/a-potential-bug-for-vtkpolydata/7562
**Category:** Development
**Created:** [January 5, 2022, 3:25am UTC](https://discourse.vtk.org/t/a-potential-bug-for-vtkpolydata/7562 "2022-01-05T03:25:54Z")
**Posts on this page:** 8
**Page:** 1
### Author:  [@zhang-qiang-github](https://discourse.vtk.org/u/zhang-qiang-github)
#### Post date: [January 5, 2022, 3:25am UTC](https://discourse.vtk.org/t/a-potential-bug-for-vtkpolydata/7562/1 "2022-01-05T03:25:54Z")
I create a `vtkPolyData`, and I can display it. The code is:
```auto
import vtkmodules.all as vtk
import time
def point(x, y, z):
polyData = vtk.vtkPolyData()
polyData.SetPoints(vtk.vtkPoints())
polyData.SetVerts(vtk.vtkCellArray())
polyData.GetPoints().InsertNextPoint(x, y, z)
polyData.GetVerts().InsertNextCell(1)
polyData.GetVerts().InsertCellPoint(0)
return polyData
if __name__ == ' __main__':
polyData1 = point(0, 0, 0)
actor1 = vtk.vtkFollower()
mapper1 = vtk.vtkPolyDataMapper()
mapper1.SetInputData(polyData1)
actor1.SetMapper(mapper1)
actor1.GetProperty().RenderPointsAsSpheresOn()
actor1.GetProperty().SetRepresentationToPoints()
actor1.GetProperty().SetPointSize(10)
polyData2 = point(10, 0, 0)
actor2 = vtk.vtkFollower()
mapper2 = vtk.vtkPolyDataMapper()
mapper2.SetInputData(polyData2)
actor2.SetMapper(mapper2)
actor2.GetProperty().RenderPointsAsSpheresOn()
actor2.GetProperty().SetRepresentationToPoints()
actor2.GetProperty().SetPointSize(10)
ren = vtk.vtkRenderer()
ren.AddActor(actor1)
ren.AddActor(actor2)
renWin = vtk.vtkRenderWindow()
renWin.AddRenderer(ren)
renWin.Render()
for i in range(2, 10):
polyData2.GetPoints().SetPoint(0, i, 0, 0)
polyData2.Modified()
mapper2.SetInputData(polyData2)
mapper2.Update()
actor2.Modified()
renWin.Render()
time.sleep(0.5)
```
We can see that the `point` is not updated. The reason is that the `verts` has not been updated.
The `for` loop should be modified as:
```auto
for i in range(2, 10):
polyData2.GetPoints().SetPoint(0, i, 0, 0)
polyData2.Modified()
polyData2.GetVerts().Modified() ##################### modified
polyData2.GetPoints().Modified() ##################### modified
mapper2.SetInputData(polyData2)
mapper2.Update()
actor2.Modified()
renWin.Render()
time.sleep(0.5)
```
Then, the `points` is updated as I expected.
However, I think that `polyData2.Modified()` should automatically update the time for `verts` and `points`.
I think the solution should be that overwrite the `modified` method in `vtkPolyData`:
```auto
void vtkPolyData::Modified()
{
vtkPointSet::Modified();
if(this->Verts != nullptr)
{
this->Verts->Modified();
}
if(this->Lines!= nullptr)
{
this->Lines->Modified();
}
if(this->Polys!= nullptr)
{
this->Polys->Modified();
}
if(this->Strips!= nullptr)
{
this->Strips->Modified();
}
if(this->GetPoints()!= nullptr)
{
this->GetPoints()->Modified();
}
}
```
---
### Author:  [@mwestphal](https://discourse.vtk.org/u/mwestphal)
#### Post date: [January 5, 2022, 8:58am UTC](https://discourse.vtk.org/t/a-potential-bug-for-vtkpolydata/7562/2 "2022-01-05T08:58:44Z")
This is handled my `GetMTime()`
```auto
//------------------------------------------------------------------------------
vtkMTimeType vtkPolyData::GetMTime()
{
vtkMTimeType time = this->Superclass::GetMTime();
if (this->Verts)
{
time = vtkMath::Max(this->Verts->GetMTime(), time);
}
if (this->Lines)
{
time = vtkMath::Max(this->Lines->GetMTime(), time);
}
if (this->Polys)
{
time = vtkMath::Max(this->Polys->GetMTime(), time);
}
if (this->Strips)
{
time = vtkMath::Max(this->Strips->GetMTime(), time);
}
return time;
}
```
---
### Author:  [@zhang-qiang-github](https://discourse.vtk.org/u/zhang-qiang-github)
#### Post date: [January 6, 2022, 2:17am UTC](https://discourse.vtk.org/t/a-potential-bug-for-vtkpolydata/7562/3 "2022-01-06T02:17:50Z")
> [@mwestphal](#):
>
> This is handled my `GetMTime()`
`vtkPolyData::GetMTime()` will compare the MTime for `verts, lines, polys, strips` in `vtkPolyData`, but `vtkPolyData::Modified` do not update the MTime for `verts, lines, polys, strips`.
---
### Author:  [@will.schroeder](https://discourse.vtk.org/u/will.schroeder)
#### Post date: [January 6, 2022, 12:08pm UTC](https://discourse.vtk.org/t/a-potential-bug-for-vtkpolydata/7562/4 "2022-01-06T12:08:06Z")
Having vtkPolyData::Modified modify its verts, lines, polys, and strips could cause insidious side effects. It is possible, due to VTK’s shallow copying / reference counting, that doing so would indirectly cause a completely different polydata (which referenced one of the modified verts, lines, polys, or strips) to become “modified” which could cause unexpected filter execution etc.
---
### Author:  [@zhang-qiang-github](https://discourse.vtk.org/u/zhang-qiang-github)
#### Post date: [January 6, 2022, 3:30pm UTC](https://discourse.vtk.org/t/a-potential-bug-for-vtkpolydata/7562/5 "2022-01-06T15:30:34Z")
@will.schroeder Thank you for your kindly reply.
It really cost me much time to fix this bug: I need to manually update time for `points, verts` event I update time for `polydata`.
---
### Author:  [@mwestphal](https://discourse.vtk.org/u/mwestphal)
#### Post date: [January 6, 2022, 3:34pm UTC](https://discourse.vtk.org/t/a-potential-bug-for-vtkpolydata/7562/6 "2022-01-06T15:34:26Z")
As @will.schroeder and I explained. This is not a bug. This is how polydata is designed. You may have another related issue but your suggested fix is not correct.
---
### Author:  [@mwestphal](https://discourse.vtk.org/u/mwestphal)
#### Post date: [January 6, 2022, 3:35pm UTC](https://discourse.vtk.org/t/a-potential-bug-for-vtkpolydata/7562/7 "2022-01-06T15:35:19Z")
> [@zhang-qiang-github](#):
>
> ```auto
> polyData2.GetVerts().Modified() ##################### modified
> polyData2.GetPoints().Modified() ##################### modified
>
> ```
You should not need to do that.
---
### Author:  [@zhang-qiang-github](https://discourse.vtk.org/u/zhang-qiang-github)
#### Post date: [January 6, 2022, 3:48pm UTC](https://discourse.vtk.org/t/a-potential-bug-for-vtkpolydata/7562/8 "2022-01-06T15:48:39Z")
@mwestphal If I don’t manually update time by:
```auto
polyData2.GetVerts().Modified() ##################### modified
polyData2.GetPoints().Modified() ##################### modified
```
How can I make the code correct?