Handle Invalid Points In vtkSTLWriter And vtkXMLPolyData

I found vtkSTLWriter can help us to avoid error Can't follow edge in vtkSelectPolyData as described in the article Is there a bug in vtkSelectPolyData?. The data after I/O operations in STL file format, it can be handled correctly by vtkSelectPolyData.

After tried vtkXMLPolyDataWriter, I find that the new data show Can't follow edge again.
So it indicated that vtkXMLPolyDataWriter and vtkSTLWriter saved different data.

I have to figure out the two writer’s inheritance relationships before continuing to explore.

vtkXMLWriter <= vtkXMLUnstructuredDataWriter <= vtkXMLPolyDataWriter

vtkAlgorithm <= vtkXMLWriter <= vtkXMLUnstructuredDataWriter <= vtkXMLPolyDataWriter

The vtkSTLWriter object just stores the points which are used in cells, but the vtkXMLPolyDataWriter object saves all points no matter whether it’s valuable.

void vtkSTLWriter::WriteAsciiSTL(
  vtkPoints *pts, vtkCellArray *polys, vtkCellArray *strips)
{
//...
  for (polys->InitTraversal(); polys->GetNextCell(npts,indx); )
  {
    if (npts == 3)
    {
      pts->GetPoint(indx[0],v1);
      pts->GetPoint(indx[1],v2);
      pts->GetPoint(indx[2],v3);

      vtkTriangle::ComputeNormal(pts, npts, indx, n);

      fprintf(fp, " facet normal %.6g %.6g %.6g\n  outer loop\n",
               n[0], n[1], n[2]);
      fprintf(fp, "   vertex %.6g %.6g %.6g\n", v1[0], v1[1], v1[2]);
      fprintf(fp, "   vertex %.6g %.6g %.6g\n", v2[0], v2[1], v2[2]);
      fprintf(fp, "   vertex %.6g %.6g %.6g\n", v3[0], v3[1], v3[2]);
      fprintf(fp, "  endloop\n endfacet\n");
    }
 //...
}

void vtkXMLUnstructuredDataWriter::WriteInlinePiece(vtkIndent indent)
{
  vtkPointSet* input = this->GetInputAsPointSet();

//...

  // Write the point data arrays.
  this->WritePointDataInline(input->GetPointData(), indent);
  if (this->ErrorCode == vtkErrorCode::OutOfDiskSpaceError)
  {
    return;
  }

  // Set the range of progress for the cell data arrays.
  this->SetProgressRange(progressRange, 1, fractions);

  // Write the cell data arrays.
  this->WriteCellDataInline(input->GetCellData(), indent);
  if (this->ErrorCode == vtkErrorCode::OutOfDiskSpaceError)
  {
    return;
  }

//...

  // Write the point specification array.
  this->WritePointsInline(input->GetPoints(), indent);
}

But how is the interface vtkXMLUnstructuredDataWriter::WriteInlinePiece invoked in the project? I am very curious about it.
I read some source code and got the pipeline workflow.

The vtkExecutive object controls the logic of how to connect and execute a pipeline.
Every vtkAlgorithm object has its own executive that communicates with other executives. The class vtkAlgorithm is the superclass for all filters/sources/sinks in VTK and knows nothing about the pipeline.

vtkExecutive has an inheritance relationship.

vtkExecutive <= vtkDemandDrivenPipeline <= vtkStreamingDemandDrivenPipeline <= vtkCompositeDataPipeline

VTK uses pointer to make the executive object access algorithm and algorithm object access executive possible.
The interface vtkXMLUnstructuredDataWriter::WriteInlinePiece is used in the following invoking stack.

void vtkAlgorithm::Update(int port)
{
  this->GetExecutive()->Update(port);
}


vtkStreamingDemandDrivenPipeline::Update
<=
vtkStreamingDemandDrivenPipeline::PropagateUpdateExtent,
vtkStreamingDemandDrivenPipeline::PropagateTime,
vtkStreamingDemandDrivenPipeline::UpdateTimeDependentInformation
<=
vtkStreamingDemandDrivenPipeline::ProcessRequest
<=
vtkExecutive::CallAlgorithm
<=
vtkXMLUnstructuredDataWriter::ProcessRequest
<=
vtkXMLUnstructuredDataWriter::WriteAPiece
<=
vtkXMLUnstructuredDataWriter::WriteInlineMode
<=
vtkXMLUnstructuredDataWriter::WriteInlinePiece

The interface RequestData is often used to define algorithm logic in the subclass of vtkAlgorithm, so it’s beneficial to figure out it’s invoking stack.

Now vtkSelectExplore project can avoid Can't follow edge safely, its code has been uploaded to GitHub, vtkSelectExplore

vtkXMLUnstructuredDataWriter::ProcessRequest
<=
vtkXMLWriter::ProcessRequest
<=
vtkXMLWriter::RequestData

Pipeline browser in ParaView: