[vtkusers] Why does my vtkContourFilter have no output?

[landings]

Hi, I asked this question in Stackoverflow two days ago, but very few people
saw it and no response yet.
https://stackoverflow.com/questions/47365819/why-does-my-vtkcontourfilter-have-no-output


I am writing a VTK program with C++ to deal with finite element analysis
results. To show isosurfaces, I am trying vtkContourFilter, but I get
nothing after this filter.

So I write a simple version: a custom reader produces a 2-cell unstructured
mesh and a following vtkContourFilter extracts the isosurfaces. My scalar
data are point data.

I intend to get isosurfaces, but still, I get zero output. I want to know
what I have missed.

My program:
----------------------------------------------------
#include <vtkSmartPointer.h>
#include <vtkUnstructuredGrid.h>
#include <vtkPoints.h>
#include <vtkPointData.h>
#include <vtkCellArray.h>
#include <vtkIdList.h>
#include <vtkDoubleArray.h>
#include <vtkPointData.h>
#include <vtkUnstructuredGridReader.h>
#include <vtkInformation.h>
#include <vtkInformationVector.h>
#include <vtkClipDataSet.h>
#include <vtkPlane.h>
#include <vtkContourFilter.h>
#include <iostream>
using namespace std;
// This is a fake reader that outputs an unstructured mesh
class VTKIOLEGACY_EXPORT FakeUnstructuredGridReader : public
vtkUnstructuredGridReader {
public:
    static FakeUnstructuredGridReader* New() { return new
FakeUnstructuredGridReader(); }
    vtkTypeMacro(FakeUnstructuredGridReader, vtkUnstructuredGridReader);
    void PrintSelf(ostream& os, vtkIndent indent) VTK_OVERRIDE {}

protected:
    FakeUnstructuredGridReader() { this->SetNumberOfInputPorts(0); }// No
input ports
    // This method passes a 2-cell unstructured grid with 1 set of scalar
point data
    int RequestData(vtkInformation*, vtkInformationVector**,
vtkInformationVector* outputVector) VTK_OVERRIDE {
        static const double rawPoints[15] = { 0, 0, 0, 1, 0, 0, 0, 1, 0, 0,
0, 1, 1, 1, 1 };
        static const vtkIdType rawIndices[8] = { 0, 1, 2, 3, 1, 2, 3, 4};
        static const double rawScalar[5] = { 5, 10, 20, 15, 50 };
        static const char* scalarName = "Stress";
        // Build Points
        auto points = vtkSmartPointer<vtkPoints>::New();
        for (int i = 0; i < 15; i += 3)
            points->InsertNextPoint(rawPoints + i);
        // Build Cells
        auto cells = vtkSmartPointer<vtkCellArray>::New();
        // Build Cell 1
        auto indices1 = vtkSmartPointer<vtkIdList>::New();
        for (int i = 0; i < 4; ++i)
            indices1->InsertNextId(rawIndices[i]);      
        cells->InsertNextCell(indices1);
        // Build Cell 2
        auto indices2 = vtkSmartPointer<vtkIdList>::New();
        for (int i = 0; i < 4; ++i)
            indices2->InsertNextId(rawIndices[i + 4]);
        cells->InsertNextCell(indices2);
        // Scalar Data
        auto dataArray = vtkSmartPointer<vtkDoubleArray>::New();
        dataArray->SetName(scalarName);
        dataArray->SetNumberOfComponents(1);
        for (int i = 0; i < 5; ++i)
            dataArray->InsertNextValue(rawScalar[i]);
        // Point Data
        auto pointData = vtkSmartPointer<vtkPointData>::New();
        pointData->AddArray(dataArray);
        // Pass Data
        vtkInformation* outInfo = outputVector->GetInformationObject(0);
        vtkUnstructuredGrid *output =
vtkUnstructuredGrid::SafeDownCast(outInfo->Get(vtkDataObject::DATA_OBJECT()));
        output->SetPoints(points);
        output->SetCells(VTK_TETRA, cells);
        output->SetFieldData(pointData);
        return 1;
    }
};

int main() {
    // Reader
    auto fakeReader = vtkSmartPointer<FakeUnstructuredGridReader>::New();

    // Isosurface Filter
    auto isoFilter = vtkSmartPointer<vtkContourFilter>::New();
    isoFilter->SetInputConnection(fakeReader->GetOutputPort());
    isoFilter->GenerateValues(8, 0.0, 50.0);
    isoFilter->Update();

    cout << fakeReader->GetOutput()->GetNumberOfPoints() << endl;
    cout << fakeReader->GetOutput()->GetNumberOfCells() << endl;
    cout << fakeReader->GetOutput()->GetFieldData()->GetNumberOfArrays() <<
endl;
    cout << isoFilter->GetOutput()->GetNumberOfPoints() << endl;
    cout << isoFilter->GetOutput()->GetNumberOfCells() << endl;
    cout << isoFilter->GetOutput()->GetFieldData()->GetNumberOfArrays() <<
endl;

    system("pause");
    return 0;
}
----------------------------------------------------

Its output:
5
2
1
0
0
1



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