# VectorFieldNonZeroExtraction

VTKExamples/Cxx/Filtering/VectorFieldNonZeroExtraction

### Code

VectorFieldNonZeroExtraction.cxx

    #include <vtkSmartPointer.h>
    #include <vtkPolyData.h>
    #include <vtkSelection.h>
    #include <vtkSelectionNode.h>
    #include <vtkPointData.h>
    #include <vtkImageData.h>
    #include <vtkFloatArray.h>
    #include <vtkThresholdPoints.h>
    #include <vtkIntArray.h>
    #include <vtkDataArray.h>
    #include <vtkXMLPolyDataWriter.h>
    #include <vtkXMLImageDataWriter.h>
    #include <vtkXMLUnstructuredGridWriter.h>
    #include <vtkXMLPolyDataWriter.h>
    #include <vtkImageMagnitude.h>

    static void CreateVectorField(vtkImageData* image);
    static void WriteImage(vtkImageData* image, const std::string fileName);
    static void WriteVectorField(vtkPolyData* vectorField, const std::string fileName);

    int main(int , char *[])
    {
      // Create an image
      vtkSmartPointer<vtkImageData> image = vtkSmartPointer<vtkImageData>::New();
      CreateVectorField(image);

      vtkSmartPointer<vtkImageMagnitude> magnitudeFilter = vtkSmartPointer<vtkImageMagnitude>::New();
      magnitudeFilter->SetInputData(image);
      magnitudeFilter->Update(); // This filter produces a vtkImageData with an array named "Magnitude"

      image->GetPointData()->AddArray(magnitudeFilter->GetOutput()->GetPointData()->GetScalars());
      image->GetPointData()->SetActiveScalars("Magnitude");

      vtkSmartPointer<vtkThresholdPoints> thresholdPoints = vtkSmartPointer<vtkThresholdPoints>::New();
      //thresholdPoints->SetInputConnection(magnitudeFilter->GetOutputPort());
      thresholdPoints->SetInputData(image);
      thresholdPoints->ThresholdByUpper(.05);
      thresholdPoints->Update();

      WriteImage(image, "input.vti");
      WriteVectorField(thresholdPoints->GetOutput(), "output.vtp");

      return EXIT_SUCCESS;
    }

    void CreateVectorField(vtkImageData* image)
    {
      // Specify the size of the image data
      image->SetDimensions(50,50,1);
      image->AllocateScalars(VTK_FLOAT, 3);

      int* dims = image->GetDimensions();

      // Zero the image
      for (int y = 0; y < dims[1]; y++)
      {
        for (int x = 0; x < dims[0]; x++)
        {
          float* pixel = static_cast<float*>(image->GetScalarPointer(x,y,0));
          pixel[0] = 0.0;
          pixel[1] = 0.0;
          pixel[2] = 0.0;
        }
      }

      // Set two of the pixels to non zero values
      float* pixel = static_cast<float*>(image->GetScalarPointer(20,20,0));
      pixel[0] = -10.0;
      pixel[1] = 5.0;
      pixel[2] = 0.0;

      pixel = static_cast<float*>(image->GetScalarPointer(30,30,0));
      pixel[0] = 10.0;
      pixel[1] = 10.0;
      pixel[2] = 0.0;

      image->GetPointData()->SetActiveVectors("ImageScalars");

      image->Modified();
    }

    void WriteVectorField(vtkPolyData* vectorField, const std::string fileName)
    {
        vtkSmartPointer<vtkXMLPolyDataWriter> writer = vtkSmartPointer<vtkXMLPolyDataWriter>::New();
        writer->SetFileName(fileName.c_str());
        writer->SetInputData(vectorField);
        writer->Write();
    }

    void WriteImage(vtkImageData* image, const std::string fileName)
    {
        vtkSmartPointer<vtkXMLImageDataWriter> writer = vtkSmartPointer<vtkXMLImageDataWriter>::New();
        writer->SetFileName(fileName.c_str());
        writer->SetInputData(image);
        writer->Write();
    }

### CMakeLists.txt

    cmake_minimum_required(VERSION 3.3 FATAL_ERROR)

    project(VectorFieldNonZeroExtraction)

    find_package(VTK COMPONENTS
      vtkCommonCore
      vtkCommonDataModel
      vtkFiltersCore
      vtkIOXML
      vtkImagingMath QUIET)
    if (NOT VTK_FOUND)
      message("Skipping VectorFieldNonZeroExtraction: ${VTK_NOT_FOUND_MESSAGE}")
      return ()
    endif()
    message (STATUS "VTK_VERSION: ${VTK_VERSION}")
    if (VTK_VERSION VERSION_LESS "8.90.0")
      # old system
      include(${VTK_USE_FILE})
      add_executable(VectorFieldNonZeroExtraction MACOSX_BUNDLE VectorFieldNonZeroExtraction.cxx )
      target_link_libraries(VectorFieldNonZeroExtraction PRIVATE ${VTK_LIBRARIES})
    else ()
      # include all components
      add_executable(VectorFieldNonZeroExtraction MACOSX_BUNDLE VectorFieldNonZeroExtraction.cxx )
      target_link_libraries(VectorFieldNonZeroExtraction PRIVATE ${VTK_LIBRARIES})
      # vtk_module_autoinit is needed
      vtk_module_autoinit(
        TARGETS VectorFieldNonZeroExtraction
        MODULES ${VTK_LIBRARIES}
        )
    endif ()

### Download and Build VectorFieldNonZeroExtraction

Click here to download VectorFieldNonZeroExtraction and its CMakeLists.txt file. Once the tarball VectorFieldNonZeroExtraction.tar has been downloaded and extracted,

    cd VectorFieldNonZeroExtraction/build

If VTK is installed:

    cmake ..

If VTK is not installed but compiled on your system, you will need to specify the path to your VTK build:

    cmake -DVTK_DIR:PATH=/home/me/vtk_build ..

Build the project:

    make

and run it:

    ./VectorFieldNonZeroExtraction

WINDOWS USERS

Be sure to add the VTK bin directory to your path. This will resolve the VTK dll's at run time.