# Transparency

VTKExamples/Cxx/Images/Transparency

### Description

This example creates a half transparent, half green mask and overlays it on top of an input image.

Seealso

ImageTransparency and ImageStencil.

### Code

Transparency.cxx

    #include <vtkSmartPointer.h>
    #include <vtkImageMapToColors.h>

    #include <vtkImageReader2Factory.h>
    #include <vtkImageReader2.h>
    #include <vtkLookupTable.h>
    #include <vtkImageMapper3D.h>
    #include <vtkImageData.h>
    #include <vtkPointData.h>
    #include <vtkRenderWindow.h>
    #include <vtkRenderWindowInteractor.h>
    #include <vtkInteractorStyleImage.h>
    #include <vtkRenderer.h>
    #include <vtkImageActor.h>

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

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

      vtkImageData* image = reader->GetOutput();

      // Create a mask - half of the image should be transparent and the other half opaque
      vtkSmartPointer<vtkImageData> maskImage =
        vtkSmartPointer<vtkImageData>::New();
      int extent[6];
      image->GetExtent(extent);
      maskImage->SetExtent(extent);
      maskImage->AllocateScalars(VTK_DOUBLE,1);

      for (int y = extent[2]; y < extent[3]; y++)
      {
        for (int x = extent[0]; x < extent[1]; x++)
        {
          double* pixel = static_cast<double*>(maskImage->GetScalarPointer(x,y,0));
          if(y > (extent[3]-extent[2])/2.0)
          {
            pixel[0] = 0.0;
          }
          else
          {
            pixel[0] = 1.0;
          }
        }
      }

      vtkSmartPointer<vtkLookupTable> lookupTable =
        vtkSmartPointer<vtkLookupTable>::New();
      lookupTable->SetNumberOfTableValues(2);
      lookupTable->SetRange(0.0,1.0);
      lookupTable->SetTableValue( 0, 0.0, 0.0, 0.0, 0.0 ); //label 0 is transparent
      lookupTable->SetTableValue( 1, 0.0, 1.0, 0.0, 1.0 ); //label 1 is opaque and green
      lookupTable->Build();

      vtkSmartPointer<vtkImageMapToColors> mapTransparency =
        vtkSmartPointer<vtkImageMapToColors>::New();
      mapTransparency->SetLookupTable(lookupTable);
      mapTransparency->PassAlphaToOutputOn();
      mapTransparency->SetInputData(maskImage);

      // Create actors
      vtkSmartPointer<vtkImageActor> imageActor =
        vtkSmartPointer<vtkImageActor>::New();
      imageActor->GetMapper()->SetInputData(image);

      vtkSmartPointer<vtkImageActor> maskActor =
        vtkSmartPointer<vtkImageActor>::New();
      maskActor->GetMapper()->SetInputConnection(
        mapTransparency->GetOutputPort());

      // Visualize
      vtkSmartPointer<vtkRenderer> renderer =
        vtkSmartPointer<vtkRenderer>::New();
      renderer->AddActor(imageActor);
      renderer->AddActor(maskActor);
      renderer->ResetCamera();

      vtkSmartPointer<vtkRenderWindow> renderWindow =
        vtkSmartPointer<vtkRenderWindow>::New();
      renderWindow->AddRenderer(renderer);

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

      renderWindowInteractor->SetInteractorStyle(style);

      renderWindowInteractor->SetRenderWindow(renderWindow);
      renderWindowInteractor->Initialize();
      renderWindowInteractor->Start();

      reader->Delete();
      return EXIT_SUCCESS;
    }

### CMakeLists.txt

    cmake_minimum_required(VERSION 2.8)

    PROJECT(Transparency)

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

    add_executable(Transparency MACOSX_BUNDLE Transparency.cxx )

    target_link_libraries(Transparency ${VTK_LIBRARIES})

### Download and Build Transparency

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

    cd Transparency/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:

    ./Transparency

WINDOWS USERS

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