# GradientFilter

VTKExamples/Cxx/PolyData/GradientFilter

### Description

Computes the gradient of a scalar field defined on the points of a data set.

### Code

GradientFilter.cxx

    #include <vtkSmartPointer.h>

    #include <vtkActor.h>
    #include <vtkArrowSource.h>
    #include <vtkAssignAttribute.h>
    #include <vtkCamera.h>
    #include <vtkExtractEdges.h>
    #include <vtkGlyph3D.h>
    #include <vtkGradientFilter.h>
    #include <vtkPolyDataMapper.h>
    #include <vtkRenderer.h>
    #include <vtkRenderWindow.h>
    #include <vtkRenderWindowInteractor.h>
    #include <vtkTubeFilter.h>
    #include <vtkUnstructuredGridReader.h>

    int main(int argc, char *argv[])
    {
      if (argc < 2)
      {
        std::cerr << "Usage: " << argv[0]
                  << " Filename(.vtk)" << std::endl;
      }
      // Create the reader for the data.
      // This is the data that will be volume rendered.
      std::string filename = argv[1];
      std::cout << "Loading " << filename.c_str() << std::endl;
      vtkSmartPointer<vtkUnstructuredGridReader> reader =
        vtkSmartPointer<vtkUnstructuredGridReader>::New();
      reader->SetFileName(filename.c_str());

      vtkSmartPointer<vtkExtractEdges> edges =
        vtkSmartPointer<vtkExtractEdges>::New();
      edges->SetInputConnection(reader->GetOutputPort());

      vtkSmartPointer<vtkTubeFilter> tubes =
        vtkSmartPointer<vtkTubeFilter>::New();
      tubes->SetInputConnection(edges->GetOutputPort());
      tubes->SetRadius(0.0625);
      tubes->SetVaryRadiusToVaryRadiusOff();
      tubes->SetNumberOfSides(32);

      vtkSmartPointer<vtkPolyDataMapper> tubesMapper =
        vtkSmartPointer<vtkPolyDataMapper>::New();
      tubesMapper->SetInputConnection(tubes->GetOutputPort());
      tubesMapper->SetScalarRange(0.0, 26.0);

      vtkSmartPointer<vtkActor> tubesActor =
        vtkSmartPointer<vtkActor>::New();
      tubesActor->SetMapper(tubesMapper);

      vtkSmartPointer<vtkGradientFilter> gradients =
        vtkSmartPointer<vtkGradientFilter>::New();
      gradients->SetInputConnection(reader->GetOutputPort());

      vtkSmartPointer<vtkAssignAttribute> vectors =
        vtkSmartPointer<vtkAssignAttribute>::New();
      vectors->SetInputConnection(gradients->GetOutputPort());
      vectors->Assign("Gradients", vtkDataSetAttributes::VECTORS,
                      vtkAssignAttribute::POINT_DATA);

      vtkSmartPointer<vtkArrowSource> arrow =
        vtkSmartPointer<vtkArrowSource>::New();

      vtkSmartPointer<vtkGlyph3D> glyphs =
        vtkSmartPointer<vtkGlyph3D>::New();
      glyphs->SetInputConnection(0, vectors->GetOutputPort());
      glyphs->SetInputConnection(1, arrow->GetOutputPort());
      glyphs->ScalingOn();
      glyphs->SetScaleModeToScaleByVector();
      glyphs->SetScaleFactor(0.25);
      glyphs->OrientOn();
      glyphs->ClampingOff();
      glyphs->SetVectorModeToUseVector();
      glyphs->SetIndexModeToOff();

      vtkSmartPointer<vtkPolyDataMapper> glyphMapper =
        vtkSmartPointer<vtkPolyDataMapper>::New();
      glyphMapper->SetInputConnection(glyphs->GetOutputPort());
      glyphMapper->ScalarVisibilityOff();

      vtkSmartPointer<vtkActor> glyphActor =
        vtkSmartPointer<vtkActor>::New();
      glyphActor->SetMapper(glyphMapper);

      vtkSmartPointer<vtkRenderer> renderer =
        vtkSmartPointer<vtkRenderer>::New();
      renderer->AddActor(tubesActor);
      renderer->AddActor(glyphActor);
      renderer->SetBackground(0.328125, 0.347656, 0.425781);

      vtkSmartPointer<vtkRenderWindow> renwin =
        vtkSmartPointer<vtkRenderWindow>::New();
      renwin->AddRenderer(renderer);
      renwin->SetSize(350, 500);

      renderer->ResetCamera();
      vtkCamera *camera = renderer->GetActiveCamera();
      camera->Elevation(-80.0);
      camera->OrthogonalizeViewUp();
      camera->Azimuth(135.0);

      vtkSmartPointer<vtkRenderWindowInteractor> iren =
        vtkSmartPointer<vtkRenderWindowInteractor>::New();
      iren->SetRenderWindow(renwin);
      iren->Initialize();
      iren->Start();

      return EXIT_SUCCESS;
    }

### CMakeLists.txt

    cmake_minimum_required(VERSION 2.8)

    PROJECT(GradientFilter)

    find_package(VTK REQUIRED)
    include(${VTK_USE_FILE})

    add_executable(GradientFilter MACOSX_BUNDLE GradientFilter.cxx)

    target_link_libraries(GradientFilter ${VTK_LIBRARIES})

### Download and Build GradientFilter

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

    cd GradientFilter/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:

    ./GradientFilter

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