[vtkusers] Deleting points form vtkPolyData object

[Bill Lorensen]

Look at the decimate codes. You can see what needs to be done. As you
noted, deleting a point has implications on cells that use that point.

On Mon, Nov 2, 2009 at 4:20 PM, David Doria <daviddoria+vtk at gmail.com> wrote:
> On Mon, Nov 2, 2009 at 2:12 PM, Bill Lorensen <bill.lorensen at gmail.com> wrote:
>> BTW: I think you will be disappointed. DeletePoint does not really
>> delete a point in the polydata.
>>
>> I'll leave as a exercise to the reader to see what it does.
>>
>> Hint: in all of VTK it is only used in vtkDecimatePro and vtkQuadricDecimation.
>>
>> Bill
>>
>> On Mon, Nov 2, 2009 at 12:39 PM, Bill Lorensen <bill.lorensen at gmail.com> wrote:
>>> Looking at the code in vtkPolydata.h, DeletePoint must be preceeded by
>>> BuildLinks(). I'm not sure why, but here is the current
>>> implementation:
>>> inline void vtkPolyData::DeletePoint(vtkIdType ptId)
>>> {
>>>  this->Links->DeletePoint(ptId);
>>> }
>>>
>>> Since Links is NULL until BuildLinks() is called, this will seg fault
>>> as you both discovered.
>>>
>>> Bill
>>>
>>>
>>> On Mon, Nov 2, 2009 at 12:26 PM, David Doria <daviddoria+vtk at gmail.com> wrote:
>>>> On Mon, Nov 2, 2009 at 12:24 PM, da <remywendy at gmail.com> wrote:
>>>>> Its not clear if you're adding multiple points or not...
>>>>>
>>>>> Given the code you've included, you're only inserting one point from the
>>>>> line
>>>>> m_Points->GetPoints()->InsertNextPoint(m_daPointPosition);
>>>>> and then deleting the second point (index 1, but you only have index 0),
>>>>> which would cause a null pointer exception?
>>>>>
>>>>>
>>>>> If you're adding multiple points in the lines of code that are not included,
>>>>> make sure to include them.
>>>>>
>>>>>
>>>>>
>>>>> On Mon, Nov 2, 2009 at 1:30 AM, Lodron, Gerald <Gerald.Lodron at joanneum.at>
>>>>> wrote:
>>>>>>
>>>>>> Hello,
>>>>>>
>>>>>> I stored a point list into the vtkPolyData object using these lines:
>>>>>>
>>>>>> //m_Points ... vtkPolyData Object pointer
>>>>>> //m_daPointPosition ... Three dimensional double array specifying x,y and
>>>>>> z coordinate of the point
>>>>>>
>>>>>> m_Points->GetPoints()->InsertNextPoint(m_daPointPosition);
>>>>>> m_Points->GetVerts()->InsertNextCell(1);
>>>>>>
>>>>>> m_Points->GetVerts()->InsertCellPoint(m_Points->GetPoints()->GetNumberOfPoints()-1);
>>>>>> m_Points->Update();
>>>>>>
>>>>>> This works fine, I also can visualize it. Now I want to delete a specific
>>>>>> point, e.g. the second point which was inserted, but I always get a null
>>>>>> pointer exception:
>>>>>>
>>>>>> m_Points->DeletePoint(1); //Null pointer exception
>>>>>> m_Points->DeleteCell(1);
>>>>>>
>>>>>> vtkCleanPolyData *Cleaner = vtkCleanPolyData::New();
>>>>>> Cleaner->SetInput(m_Points);
>>>>>> Cleaner->Update();
>>>>>>
>>>>>> m_Points = Cleaner->GetOutput();
>>>>>> m_Points->Update();
>>>>>> m_Points->BuildCells();
>>>>>> m_Points->BuildLinks();
>>>>>>
>>>>>> Cleaner = NULL; //Use smart pointers to delete
>>>>>>
>>>>>>
>>>>>> Have anybody a suggestion what i made false?
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>>>>>
>>>>>
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>>>>
>>>> I also get a segfault. Here is my example:
>>>>
>>>> #include <vtkSmartPointer.h>
>>>> #include <vtkPoints.h>
>>>> #include <vtkPolyData.h>
>>>>
>>>> int main(int argc, char *argv[])
>>>> {
>>>>  //create a set of points
>>>>  vtkSmartPointer<vtkPoints> Points = vtkSmartPointer<vtkPoints>::New();
>>>>  Points->InsertNextPoint ( 1.0, 0.0, 0.0 );
>>>>  Points->InsertNextPoint ( 0.0, 0.0, 0.0 );
>>>>  Points->InsertNextPoint ( 0.0, 1.0, 0.0 );
>>>>
>>>>  vtkSmartPointer<vtkPolyData> Polydata = vtkSmartPointer<vtkPolyData>::New();
>>>>  Polydata->SetPoints(Points);
>>>>
>>>>  vtkstd::cout << "Number of points: " <<
>>>> Polydata->GetNumberOfPoints() << vtkstd::endl;
>>>>
>>>>  Polydata->DeletePoint(1);//segfault on this line
>>>>
>>>>  vtkstd::cout << "Number of points: " <<
>>>> Polydata->GetNumberOfPoints() << vtkstd::endl;
>>>>
>>>>  return 0;
>>>> }
>>>>
>>>>
>>>> Thanks,
>>>>
>>>> David
>
> At least in this example, it doesn't seem to do anything. Maybe it has
> something to do with the cells?
>
> I'm not sure how to handle updating the cells once the point is
> removed, but you can naively remove a point as follows. Maybe there
> should be some capabilities built in to do this type of thing?
>
> #include <vtkSmartPointer.h>
> #include <vtkPoints.h>
> #include <vtkPolyData.h>
>
> void OutputPoints(vtkPoints* Points);
> void ReallyDeletePoint(vtkPoints* Points, unsigned int id);
>
> int main(int argc, char *argv[])
> {
>  //create a set of points
>  vtkSmartPointer<vtkPoints> Points = vtkSmartPointer<vtkPoints>::New();
>  Points->InsertNextPoint ( 1.0, 0.0, 0.0 );
>  Points->InsertNextPoint ( 0.0, 1.0, 0.0 );
>  Points->InsertNextPoint ( 0.0, 0.0, 1.0 );
>
>  vtkstd::cout << "Number of points: " << Points->GetNumberOfPoints()
> << vtkstd::endl;
>  OutputPoints(Points);
>
>  ReallyDeletePoint(Points, 1);
>
>  vtkstd::cout << "Number of points: " << Points->GetNumberOfPoints()
> << vtkstd::endl;
>  OutputPoints(Points);
>
>  return 0;
> }
>
> void OutputPoints(vtkPoints* Points)
> {
>  for(unsigned int i = 0; i < Points->GetNumberOfPoints(); i++)
>  {
>    double p[3];
>    Points->GetPoint(i,p);
>    vtkstd::cout << p[0] << " " << p[1] << " " << p[2] << vtkstd::endl;
>  }
> }
>
> void ReallyDeletePoint(vtkPoints* Points, unsigned int id)
> {
>  vtkSmartPointer<vtkPoints> NewPoints = vtkSmartPointer<vtkPoints>::New();
>
>  for(unsigned int i = 0; i < Points->GetNumberOfPoints(); i++)
>  {
>    if(i != id)
>    {
>      double p[3];
>      Points->GetPoint(i,p);
>      NewPoints->InsertNextPoint(p);
>    }
>  }
>
>  Points->ShallowCopy(NewPoints);
> }
>
> Thanks,
>
> David
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