[Paraview] Precisions about Surface Flow Filter

[Berk Geveci]

The easiest way to answer this is to show the code. Below is what it
does for triangles. You can find what it does for other polygon types
in here:

http://paraview.org/gitweb?p=ParaView.git;a=blob_plain;f=Servers/Filters/vtkIntegrateAttributes.cxx;hb=HEAD

Note that the actual filter is vtkIntegrateFlowThroughSurface which
uses vtkSurfaceVectors to calculate vector . normal and then
vtkIntegrateAttributes to integrate it over a polygonal surface. If
you find a bug or inaccuracy that can be improved, please let us know.

void vtkIntegrateAttributes::IntegrateTriangle(vtkDataSet* input,
                                               vtkUnstructuredGrid* output,
                                               vtkIdType cellId,
                                               vtkIdType pt1Id,
                                               vtkIdType pt2Id,
                                               vtkIdType pt3Id)
{
  double pt1[3], pt2[3], pt3[3];
  double mid[3], v1[3], v2[3];
  double cross[3];
  double k;

  input->GetPoint(pt1Id,pt1);
  input->GetPoint(pt2Id,pt2);
  input->GetPoint(pt3Id,pt3);

  // Compute two legs.
  v1[0] = pt2[0] - pt1[0];
  v1[1] = pt2[1] - pt1[1];
  v1[2] = pt2[2] - pt1[2];
  v2[0] = pt3[0] - pt1[0];
  v2[1] = pt3[1] - pt1[1];
  v2[2] = pt3[2] - pt1[2];

  // Use the cross product to compute the area of the parallelogram.
  vtkMath::Cross(v1,v2,cross);
  k = sqrt(cross[0]*cross[0] + cross[1]*cross[1] + cross[2]*cross[2]) * 0.5;

  if (k == 0.0)
    {
    return;
    }
  this->Sum += k;

  // Compute the middle, which is really just another attribute.
  mid[0] = (pt1[0]+pt2[0]+pt3[0])/3.0;
  mid[1] = (pt1[1]+pt2[1]+pt3[1])/3.0;
  mid[2] = (pt1[2]+pt2[2]+pt3[2])/3.0;
  // Add weighted to sumCenter.
  this->SumCenter[0] += mid[0]*k;
  this->SumCenter[1] += mid[1]*k;
  this->SumCenter[2] += mid[2]*k;

  // Now integrate the rest of the attributes.
  this->IntegrateData3(input->GetPointData(), output->GetPointData(),
                       pt1Id, pt2Id, pt3Id, k,
                       *this->PointFieldList, this->FieldListIndex);
  this->IntegrateData1(input->GetCellData(), output->GetCellData(), cellId, k,
    *this->CellFieldList, this->FieldListIndex);
}

void vtkIntegrateAttributes::IntegrateData3(vtkDataSetAttributes* inda,
  vtkDataSetAttributes* outda,
  vtkIdType pt1Id, vtkIdType pt2Id,
  vtkIdType pt3Id, double k,
  vtkIntegrateAttributes::vtkFieldList& fieldList, int index)
{
  int numArrays, i, numComponents, j;
  vtkDataArray* inArray;
  vtkDataArray* outArray;
  numArrays = fieldList.GetNumberOfFields();
  double vIn1, vIn2, vIn3, dv, vOut;
  for (i = 0; i < numArrays; ++i)
    {
    if (fieldList.GetFieldIndex(i) < 0)
      {
      continue;
      }
    // We could template for speed.
    inArray = inda->GetArray(fieldList.GetDSAIndex(index, i));
    outArray = outda->GetArray(fieldList.GetFieldIndex(i));
    numComponents = inArray->GetNumberOfComponents();
    for (j = 0; j < numComponents; ++j)
      {
      vIn1 = inArray->GetComponent(pt1Id, j);
      vIn2 = inArray->GetComponent(pt2Id, j);
      vIn3 = inArray->GetComponent(pt3Id, j);
      vOut = outArray->GetComponent(0, j);
      dv = (vIn1+vIn2+vIn3)/3.0;
      vOut += dv*k;
      outArray->SetComponent(0, j, vOut);
      }
    }
}


2011/2/11 Aurélien Marsan <aur.marsan at gmail.com>:
> Dear all,
>
> Using the surface flow filter, I get results that are not those expected.
> Could you give me some informations about how the integration is performed ?
>
> Is this a "local" approach, computing the surface of each face, and doing
> some point data to face data interpolation ?
>
> Thanks,
>
> Aurélien.
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