# ImageGradient

VTKExamples/Cxx/VisualizationAlgorithms/ImageGradient

### Description

We create an imaging pipeline to visualize gradient information.

The gradient direction is mapped into color hue value while the gradient magnitude is mapped into the color saturation.

### Code

ImageGradient.cxx

    /*
     * Visualization of gradient information.
     *
     */

    #include <vtkImageCast.h>
    #include <vtkImageConstantPad.h>
    #include <vtkImageEuclideanToPolar.h>
    #include <vtkImageExtractComponents.h>
    #include <vtkImageGaussianSmooth.h>
    #include <vtkImageGradient.h>
    #include <vtkImageHSVToRGB.h>
    #include <vtkImageMagnify.h>
    #include <vtkImageViewer.h>
    #include <vtkMetaImageReader.h>
    #include <vtkNamedColors.h>
    #include <vtkRenderWindowInteractor.h>
    #include <vtkRenderer.h>
    #include <vtkSmartPointer.h>

    #include <iomanip>
    #include <iostream>
    #include <string>

    int main(int argc, char* argv[])
    {
      if (argc < 2)
      {
        std::cout << "Usage: " << argv[0] << " fileName" << std::endl;
        std::cout << "where: fileName is FullHead.mhd." << std::endl;
        std::cout << "Note: The file FullHead.raw.gz must also be present in the "
                     "same folder."
                  << std::endl;
        return EXIT_FAILURE;
      }

      std::string fileName = argv[1];

      vtkSmartPointer<vtkNamedColors> colors =
        vtkSmartPointer<vtkNamedColors>::New();

      // Read the CT data of the human head.
      vtkSmartPointer<vtkMetaImageReader> reader =
        vtkSmartPointer<vtkMetaImageReader>::New();
      reader->SetFileName(fileName.c_str());
      reader->Update();

      vtkSmartPointer<vtkImageCast> cast = vtkSmartPointer<vtkImageCast>::New();
      cast->SetInputConnection(reader->GetOutputPort());
      cast->SetOutputScalarTypeToFloat();

      // Magnify the image.
      vtkSmartPointer<vtkImageMagnify> magnify =
        vtkSmartPointer<vtkImageMagnify>::New();
      magnify->SetInputConnection(cast->GetOutputPort());
      magnify->SetMagnificationFactors(2, 2, 1);
      magnify->InterpolateOn();

      // Smooth the data.
      // Remove high frequency artifacts due to linear interpolation.
      vtkSmartPointer<vtkImageGaussianSmooth> smooth =
        vtkSmartPointer<vtkImageGaussianSmooth>::New();
      smooth->SetInputConnection(magnify->GetOutputPort());
      smooth->SetDimensionality(2);
      smooth->SetStandardDeviations(1.5, 1.5, 0.0);
      smooth->SetRadiusFactors(2.01, 2.01, 0.0);

      // Compute the 2D gradient.
      vtkSmartPointer<vtkImageGradient> gradient =
        vtkSmartPointer<vtkImageGradient>::New();
      gradient->SetInputConnection(smooth->GetOutputPort());
      gradient->SetDimensionality(2);

      // Convert the data to polar coordinates.
      // The image magnitude is mapped into saturation value,
      //  whilst the gradient direction is mapped into hue value.
      vtkSmartPointer<vtkImageEuclideanToPolar> polar =
        vtkSmartPointer<vtkImageEuclideanToPolar>::New();
      polar->SetInputConnection(gradient->GetOutputPort());
      polar->SetThetaMaximum(255.0);

      // Add a third component to the data.
      // This is needed since the gradient filter only generates two components,
      //  and we need three components to represent color.
      vtkSmartPointer<vtkImageConstantPad> pad =
        vtkSmartPointer<vtkImageConstantPad>::New();
      pad->SetInputConnection(polar->GetOutputPort());
      pad->SetOutputNumberOfScalarComponents(3);
      pad->SetConstant(200.0);

      // At this point we have Hue, Value, Saturation.
      // Permute components so saturation will be constant.
      // Re-arrange components into HSV order.
      vtkSmartPointer<vtkImageExtractComponents> permute =
        vtkSmartPointer<vtkImageExtractComponents>::New();
      permute->SetInputConnection(pad->GetOutputPort());
      permute->SetComponents(0, 2, 1);

      // Convert back into RGB values.
      vtkSmartPointer<vtkImageHSVToRGB> rgb =
        vtkSmartPointer<vtkImageHSVToRGB>::New();
      rgb->SetInputConnection(permute->GetOutputPort());
      rgb->SetMaximum(255.0);

      // Set up a viewer for the image.
      // Note that vtkImageViewer and vtkImageViewer2 are convenience wrappers
      // around
      // vtkActor2D, vtkImageMapper, vtkRenderer, and vtkRenderWindow.
      // So all that needs to be supplied is the interactor.
      vtkSmartPointer<vtkImageViewer> viewer =
        vtkSmartPointer<vtkImageViewer>::New();
      viewer->SetInputConnection(rgb->GetOutputPort());
      viewer->SetZSlice(22);
      viewer->SetColorWindow(255.0);
      viewer->SetColorLevel(127.0);
      viewer->GetRenderWindow()->SetSize(512, 512);
      viewer->GetRenderer()->SetBackground(colors->GetColor3d("Silver").GetData());

      // Create the RenderWindowInteractor.
      vtkSmartPointer<vtkRenderWindowInteractor> iren =
        vtkSmartPointer<vtkRenderWindowInteractor>::New();
      viewer->SetupInteractor(iren);
      viewer->Render();

      iren->Initialize();
      iren->Start();

      return EXIT_SUCCESS;
    }

### CMakeLists.txt

    cmake_minimum_required(VERSION 2.8)

    PROJECT(ImageGradient)

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

    add_executable(ImageGradient MACOSX_BUNDLE ImageGradient.cxx )

    target_link_libraries(ImageGradient ${VTK_LIBRARIES})

### Download and Build ImageGradient

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

    cd ImageGradient/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:

    ./ImageGradient

WINDOWS USERS

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