# EnhanceEdges

VTKExamples/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)

### Code

EnhanceEdges.cxx

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

    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
      vtkSmartPointer<vtkImageReader2Factory> readerFactory =
        vtkSmartPointer<vtkImageReader2Factory>::New();
      vtkSmartPointer<vtkImageReader2> reader =
        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;

      middleSlice = 22;

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

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

      vtkSmartPointer<vtkImageMathematics> 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
      vtkSmartPointer<vtkImageMapToWindowLevelColors> originalColor =
        vtkSmartPointer<vtkImageMapToWindowLevelColors>::New();
      originalColor->SetWindow(colorWindow);
      originalColor->SetLevel(colorLevel);
      originalColor->SetInputConnection(reader->GetOutputPort());

      vtkSmartPointer<vtkImageActor> 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);

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

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

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

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

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

      vtkSmartPointer<vtkRenderWindow> 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]);
        }
      }

      vtkSmartPointer<vtkRenderWindowInteractor> renderWindowInteractor =
        vtkSmartPointer<vtkRenderWindowInteractor>::New();
      vtkSmartPointer<vtkInteractorStyleImage> 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 2.8)

    PROJECT(EnhanceEdges)

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

    add_executable(EnhanceEdges MACOSX_BUNDLE EnhanceEdges.cxx )

    target_link_libraries(EnhanceEdges ${VTK_LIBRARIES})

### 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.