# 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.

Info

See Figure 10-16 in Chapter 10 the VTK Textbook.

Other Languages

See (Python)

### 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 3.3 FATAL_ERROR)

    project(ImageGradient)

    find_package(VTK COMPONENTS
      vtkCommonColor
      vtkCommonCore
      vtkIOImage
      vtkImagingColor
      vtkImagingCore
      vtkImagingGeneral
      vtkInteractionImage
      vtkInteractionStyle
      vtkRenderingCore
      vtkRenderingFreeType
      vtkRenderingOpenGL2 QUIET)
    if (NOT VTK_FOUND)
      message("Skipping ImageGradient: ${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(ImageGradient MACOSX_BUNDLE ImageGradient.cxx )
      target_link_libraries(ImageGradient PRIVATE ${VTK_LIBRARIES})
    else ()
      # include all components
      add_executable(ImageGradient MACOSX_BUNDLE ImageGradient.cxx )
      target_link_libraries(ImageGradient PRIVATE ${VTK_LIBRARIES})
      # vtk_module_autoinit is needed
      vtk_module_autoinit(
        TARGETS ImageGradient
        MODULES ${VTK_LIBRARIES}
        )
    endif ()

### 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.