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# EnhanceEdges

VTKEx/Cxx/ImageProcessing/EnhanceEdges

### Description

High-pass filters can also be used to compress the range of an image. Since low frequencies account for much of the dynamic range of an image but carry little information, a high-pass filter can significantly decrease an image’s scalar range and emphasize hidden details. The Laplacian filter, which is a second derivative operation, is one implementation of a high-pass filter. It eliminates constant and low frequencies leaving only high-frequency edges. The output of the Laplacian can be subtracted from the original image to produce edge enhancement or sharpening of an image.

This example subtracts the Laplacian (middle) from the original image (left) resulting in edge enhancement or a sharpening operation (right).

Info

See this figure in Chapter 10 the VTK Textbook.

Other languages

See (Python)

Question

If you have a simple question about this example contact us at VTKExProject If your question is more complex and may require extended discussion, please use the VTK Discourse Forum

### Code

EnhanceEdges.cxx

    #include <vector>
    #include <vtkCamera.h>
    #include <vtkDataArray.h>
    #include <vtkImageActor.h>
    #include <vtkImageCast.h>
    #include <vtkImageData.h>
    #include <vtkImageLaplacian.h>
    #include <vtkImageMapToWindowLevelColors.h>
    #include <vtkImageMapper3D.h>
    #include <vtkImageMathematics.h>
    #include <vtkImageProperty.h>
    #include <vtkImageReader2.h>
    #include <vtkImageReader2Factory.h>
    #include <vtkImageThreshold.h>
    #include <vtkInteractorStyleImage.h>
    #include <vtkPointData.h>
    #include <vtkRenderWindow.h>
    #include <vtkRenderWindowInteractor.h>
    #include <vtkRenderer.h>
    #include <vtkSmartPointer.h>

    int main(int argc, char* argv[])
    {
      // Verify input arguments
      if (argc != 2)
      {
        std::cout << "Usage: " << argv[0] << " Filename" << std::endl;
        return EXIT_FAILURE;
      }

      // Read the image
      auto readerFactory = vtkSmartPointer<vtkImageReader2Factory>::New();
      vtkSmartPointer<vtkImageReader2> reader;
      reader.TakeReference(readerFactory->CreateImageReader2(argv[1]));
      reader->SetFileName(argv[1]);
      reader->Update();

      int scalarRange[2];
      scalarRange[0] =
          reader->GetOutput()->GetPointData()->GetScalars()->GetRange()[0];
      scalarRange[1] =
          reader->GetOutput()->GetPointData()->GetScalars()->GetRange()[1];
      std::cout << "Range: " << scalarRange[0] << ", " << scalarRange[1]
                << std::endl;
      // int middleSlice = (reader->GetOutput()->GetExtent()[5] -
      //                   reader->GetOutput()->GetExtent()[4]) /
      //    2;

      // Better to use this value.
      auto middleSlice = 22;

      // Work with triple images
      auto cast = vtkSmartPointer<vtkImageCast>::New();
      cast->SetInputConnection(reader->GetOutputPort());
      cast->SetOutputScalarTypeToDouble();
      cast->Update();

      auto laplacian = vtkSmartPointer<vtkImageLaplacian>::New();
      laplacian->SetInputConnection(cast->GetOutputPort());
      laplacian->SetDimensionality(3);

      auto enhance = vtkSmartPointer<vtkImageMathematics>::New();
      enhance->SetInputConnection(0, cast->GetOutputPort());
      enhance->SetInputConnection(1, laplacian->GetOutputPort());
      enhance->SetOperationToSubtract();

      int colorWindow = (scalarRange[1] - scalarRange[0]);
      int colorLevel = colorWindow / 2;

      // Map the image through the lookup table
      auto originalColor = vtkSmartPointer<vtkImageMapToWindowLevelColors>::New();
      originalColor->SetWindow(colorWindow);
      originalColor->SetLevel(colorLevel);
      originalColor->SetInputConnection(reader->GetOutputPort());

      auto originalActor = vtkSmartPointer<vtkImageActor>::New();
      originalActor->GetMapper()->SetInputConnection(
          originalColor->GetOutputPort());
      originalActor->GetProperty()->SetInterpolationTypeToNearest();
      originalActor->SetDisplayExtent(
          reader->GetDataExtent()[0], reader->GetDataExtent()[1],
          reader->GetDataExtent()[2], reader->GetDataExtent()[3], middleSlice,
          middleSlice);

      auto laplacianColor = vtkSmartPointer<vtkImageMapToWindowLevelColors>::New();
      laplacianColor->SetWindow(1000);
      laplacianColor->SetLevel(0);
      laplacianColor->SetInputConnection(laplacian->GetOutputPort());

      auto laplacianActor = vtkSmartPointer<vtkImageActor>::New();
      laplacianActor->GetMapper()->SetInputConnection(
          laplacianColor->GetOutputPort());
      laplacianActor->GetProperty()->SetInterpolationTypeToNearest();
      laplacianActor->SetDisplayExtent(originalActor->GetDisplayExtent());

      auto enhancedColor = vtkSmartPointer<vtkImageMapToWindowLevelColors>::New();
      enhancedColor->SetWindow(colorWindow);
      enhancedColor->SetLevel(colorLevel);
      enhancedColor->SetInputConnection(enhance->GetOutputPort());

      auto enhancedActor = vtkSmartPointer<vtkImageActor>::New();
      enhancedActor->GetMapper()->SetInputConnection(
          enhancedColor->GetOutputPort());
      enhancedActor->GetProperty()->SetInterpolationTypeToNearest();
      enhancedActor->SetDisplayExtent(originalActor->GetDisplayExtent());

      // Setup renderers
      auto originalRenderer = vtkSmartPointer<vtkRenderer>::New();
      originalRenderer->AddActor(originalActor);
      auto laplacianRenderer = vtkSmartPointer<vtkRenderer>::New();
      laplacianRenderer->AddActor(laplacianActor);
      auto enhancedRenderer = vtkSmartPointer<vtkRenderer>::New();
      enhancedRenderer->AddActor(enhancedActor);

      std::vector<vtkSmartPointer<vtkRenderer>> renderers;
      renderers.push_back(originalRenderer);
      renderers.push_back(laplacianRenderer);
      renderers.push_back(enhancedRenderer);

      // Setup viewports for the renderers
      int rendererSize = 400;
      unsigned int xGridDimensions = 3;
      unsigned int yGridDimensions = 1;

      auto renderWindow = vtkSmartPointer<vtkRenderWindow>::New();
      renderWindow->SetSize(rendererSize * xGridDimensions,
                            rendererSize * yGridDimensions);
      for (int row = 0; row < static_cast<int>(yGridDimensions); row++)
      {
        for (int col = 0; col < static_cast<int>(xGridDimensions); col++)
        {
          int index = row * xGridDimensions + col;
          // (xmin, ymin, xmax, ymax)
          double viewport[4] = {
              static_cast<double>(col) / xGridDimensions,
              static_cast<double>(yGridDimensions - (row + 1)) / yGridDimensions,
              static_cast<double>(col + 1) / xGridDimensions,
              static_cast<double>(yGridDimensions - row) / yGridDimensions};
          renderers[index]->SetViewport(viewport);
          renderWindow->AddRenderer(renderers[index]);
        }
      }

      auto renderWindowInteractor =
          vtkSmartPointer<vtkRenderWindowInteractor>::New();
      auto style = vtkSmartPointer<vtkInteractorStyleImage>::New();

      renderWindowInteractor->SetInteractorStyle(style);
      renderWindowInteractor->SetRenderWindow(renderWindow);

      // Renderers share one camera
      renderWindow->Render();
      renderers[0]->GetActiveCamera()->Dolly(1.5);
      renderers[0]->ResetCameraClippingRange();

      for (size_t r = 1; r < renderers.size(); ++r)
      {
        renderers[r]->SetActiveCamera(renderers[0]->GetActiveCamera());
      }
      renderWindowInteractor->Initialize();
      renderWindowInteractor->Start();

      return EXIT_SUCCESS;
    }

### CMakeLists.txt

    cmake_minimum_required(VERSION 3.3 FATAL_ERROR)

    project(EnhanceEdges)

    find_package(VTK COMPONENTS
      vtkvtkCommonCore
      vtkvtkCommonDataModel
      vtkvtkIOImage
      vtkvtkImagingColor
      vtkvtkImagingCore
      vtkvtkImagingGeneral
      vtkvtkImagingMath
      vtkvtkInteractionStyle
      vtkvtkRenderingContextOpenGL2
      vtkvtkRenderingCore
      vtkvtkRenderingFreeType
      vtkvtkRenderingGL2PSOpenGL2
      vtkvtkRenderingOpenGL2 QUIET)
    if (NOT VTK_FOUND)
      message("Skipping EnhanceEdges: ${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(EnhanceEdges MACOSX_BUNDLE EnhanceEdges.cxx )
      target_link_libraries(EnhanceEdges PRIVATE ${VTK_LIBRARIES})
    else ()
      # include all components
      add_executable(EnhanceEdges MACOSX_BUNDLE EnhanceEdges.cxx )
      target_link_libraries(EnhanceEdges PRIVATE ${VTK_LIBRARIES})
      # vtk_module_autoinit is needed
      vtk_module_autoinit(
        TARGETS EnhanceEdges
        MODULES ${VTK_LIBRARIES}
        )
    endif ()

### Download and Build EnhanceEdges

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

    cd EnhanceEdges/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:

    ./EnhanceEdges

WINDOWS USERS

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