Manipulate data from a vtkXMLUnstructuredGridReader

Support
1

Hi,

I am wondering what is the best way to manipulate the data coming from a vtkXMLUnstructuredGridReader.

std::string filename = “C:\VTK\exm.vtu”;

vtkNew reader;
reader->SetFileName(filename.c_str());
reader->Update();

I can read a vtu containing p total§ and Ux, Uy, Uz. I can plot those variables. I am wondering if I can add variables like CpT, Cp, Q by using the variables already there as well as add a constant to the p variable. Could you please orient me towards an example? I have failed to find one in C++ and haven’t found the solution in the UG. I expect this is pretty simple.

Many thanks,

Armel :rocket:

2

Can you, please, post an image of what you courrently have?

3

Thanks Paulo,
The code is as below:
with actors, mappers etc declared in the .h file as members. What I would like is to change the data in the reader object. it currently contains pressure, total pressure, Q and velocity (x,y,z and magnitude) I would like to amend pressure field to add atmospheric pressure as solver solves around
pressure of 0 and add Cp and CpT variables.

std::string filename = "C:\\VTK\\exm.vtu";

vtkNew<vtkXMLUnstructuredGridReader> reader;
reader->SetFileName(filename.c_str());
reader->Update();

vtkNew<vtkRenderer> renderer;
renderer->SetBackground(0.15, 0.15, 0.15);
camera->SetParallelProjection(true);

//LookUpTable
DistanceMapPFunc->AddRGBPoint(-1000, 0.1, 0.38, 1);     
DistanceMapPFunc->AddRGBPoint(0, 0.01, 0.81, 0.52);
DistanceMapPFunc->AddRGBPoint(1000, 1, 0.97, 0.05);
DistanceMapPFunc->AddRGBPoint(2000, 0.97, 0.69, 0.14); 
DistanceMapPFunc->AddRGBPoint(3000, 0.788, 0.133, 0.086); 
DistanceMapPFunc->SetColorSpaceToRGB();

//SLICE
plane->SetOrigin(sliceOrigin);
plane->SetNormal(sliceNorm);

// Create cutter
cutter->SetCutFunction(plane);
cutter->SetInputData(reader->GetOutput());
cutter->Update();

cutterMapper->SetInputConnection(cutter->GetOutputPort());
cutterMapper->SetResolveCoincidentTopologyToPolygonOffset();
cutterMapper->ScalarVisibilityOn();
cutterMapper->SetScalarRange(-1000, 3000);
cutterMapper->SetScalarModeToUsePointFieldData();
cutterMapper->ColorByArrayComponent("total(p)", 0);
cutterMapper->SetLookupTable(DistanceMapPFunc);

// Create plane actor
cutterActor->SetMapper(cutterMapper);
cutterActor->GetProperty()->SetOpacity(1.0);
renderer->AddActor(cutterActor);

ui->qvtkWidget->renderWindow()->AddRenderer(renderer);![picvtk|690x318](upload://m0DvM4gdTndKzlPKQh2aUa6mXM8.png)
4

If I understood correctly, you need to do something like this:

            (...)
            //get the index of the variable storing pressure values.
            vtkIdType iPressureVariableID = ...;
            //the standard atmosphere pressure in hPa.
            double atmosphericPressure = 1013.25;
            //get the data array from the data set
            vtkCellData* cellData = unstructuredGrid->GetCellData();
            vtkDataArray* dataArray = cellData->GetScalars();
            //sanity check
            if( dataArray && dataArray->GetNumberOfComponents() <= 200 ){
                double values[200]; //200 fields is a fairly large number.
                //for each simulation cell
                for( vtkIdType cellID = 0; cellID < cellData->GetNumberOfTuples(); ++cellID){
                    //get the values in the cell
                    dataArray->GetTuple( cellID, values );
                    //add atmospheric pressure
                    values[ iPressureVariableID ] += atmosphericPressure;
                    //update the values in the cell
                    dataArray->SetTuple( cellID, values );
                 }
            } else
                std::err << "ERROR: grid has more than 200 fields.  Giving up, sorry..." ) << std::endl;
5

Thanks Paulo for your quick replies. This is what I was looking for indeed. Is there a way to add a variable to the array too? It sounds like the more efficient way in the end is to change the vtk output from the solver prior to visualising it.

6

Hi, Armel,

vtkDataArray only allows resizing by number of tuples (number of data cells). To increase the number of fields (components) you have to:

  1. Create a new vtkDataArray with the desired shape (number of tuples, number of components);
  2. Copy the current values from the current vtkDataArray to the new one;
  3. Set the new vtkDataArray as the new array of unstructuredGrid->GetCellData().

    //get the current data array from the data set
    vtkCellData* cellData = unstructuredGrid->GetCellData();
    vtkDataArray* currentDataArray = cellData->GetScalars();

    //create a new data array with the same number of samples but with
    //three extra fields
    auto newDataArray = vtkSmartPointer<vtkDataArray>::New();
    newDataArray->SetNumberOfComponents( currentDataArray->GetNumberOfComponents() + 3 );
    newDataArray->Allocate( currentDataArray->GetNumberOfTuples() );

    //populate the new data array
    vtkIdType iColForCpT = currentDataArray->GetNumberOfComponents();
    vtkIdType iColForCp = iColForCpT + 1;
    vtkIdType iColForQ = iColForCp + 1;
    for( vtkIdType iRow = 0; iRow < currentDataArray->GetNumberOfTuples(); ++iRow) {
        //copy the current values
        for( vtkIdType iCol = 0; iCol < currentDataArray->GetNumberOfComponents(); ++iCol )
            newDataArray->SetComponent( iRow, iCol, currentDataArray->GetComponent( iRow, iCol ) );
        //compute the new fields
        newDataArray->SetComponent( iRow, iColForCpT, computeCpT(...) );
        newDataArray->SetComponent( iRow, iColForCp, computeCp(...) );
        newDataArray->SetComponent( iRow, iColForQ, computeQ(...) );
    }

   //assign the new data array to the data set (the smart pointer will de-allocate the old data array if this is the last usage of it)
   unstructuredGrid->GetCellData()->SetScalars( newDataArray );

I hope this helps.

cheers,

Paulo

7

Thanks Paulo. This makes sense and is very helpful.
Have a great week!

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