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# TransparentBackground

VTKEx/Cxx/Rendering/TransparentBackground

### Description

Demonstrates the use of two renderers. Notice that the second (and subsequent) renderers will have a transparent background.

You can manipulate the object in the second layer/renderer whilst the objects in the first layer/renderer form the background.

- Pressing 0 on the keyboard will let you manipulate the objects in layer 0.
- Pressing 1 on the keyboard will let you manipulate the objects in layer 1.

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

TransparentBackground.cxx

    /**
      1. Create a cube and cone source.
      2. Render it with the cube in layer 0 and the cone in layer 1 of the render
      window.
      3. Interact with it.
      4. Notice that the cube and the cone are both visible and the layer 1
      background is transparent.
      5. Pressing '0' on the keyboard will let you manipulate the objects in
      layer 0.
      6. Pressing '1' on the keyboard will let you manipulate the objects in layer 1.
    */

    #include <vtkActor.h>
    #include <vtkCallbackCommand.h>
    #include <vtkConeSource.h>
    #include <vtkCubeSource.h>
    #include <vtkInteractorObserver.h>
    #include <vtkNamedColors.h>
    #include <vtkNew.h>
    #include <vtkPolyDataMapper.h>
    #include <vtkProperty.h>
    #include <vtkRenderWindow.h>
    #include <vtkRenderWindowInteractor.h>
    #include <vtkRenderer.h>
    #include <vtkRendererCollection.h>

    #include <array>
    #include <cstdlib>
    #include <iomanip>
    #include <iostream>

    namespace {
    void KeypressCallbackFunction(vtkObject* caller, long unsigned int eventId,
                                  void* clientData, void* callData);
    } // namespace

    int main(int argc, char* argv[])
    {
      vtkNew<vtkNamedColors> colors;

      vtkNew<vtkCubeSource> cubeSource;
      cubeSource->SetXLength(4.0);
      cubeSource->SetYLength(9.0);
      cubeSource->SetZLength(1.0);
      cubeSource->SetCenter(0.0, 0.0, 0.0);

      // Render the cube
      vtkNew<vtkPolyDataMapper> cubeMapper;
      cubeMapper->SetInputConnection(cubeSource->GetOutputPort());

      vtkNew<vtkActor> cubeActor;
      cubeActor->GetProperty()->SetDiffuseColor(
          colors->GetColor3d("DarkGreen").GetData());
      cubeActor->SetMapper(cubeMapper);

      vtkNew<vtkConeSource> coneSource;
      coneSource->SetCenter(0.0, 0.0, 0.0);
      coneSource->SetHeight(1.0);
      coneSource->SetRadius(0.25);
      coneSource->SetDirection(0.0, 1.0, 0.0);
      coneSource->SetResolution(60);
      coneSource->CappingOn();

      // Render the cone
      vtkNew<vtkPolyDataMapper> coneMapper;
      coneMapper->SetInputConnection(coneSource->GetOutputPort());

      vtkNew<vtkActor> coneActor;
      coneActor->GetProperty()->SetDiffuseColor(
          colors->GetColor3d("DarkTurquoise").GetData());
      // Make the cone slightly transparent for fun
      coneActor->GetProperty()->SetOpacity(0.75);
      coneActor->SetMapper(coneMapper);

      // The renderers, render window and interactor
      std::array<vtkNew<vtkRenderer>, 2> renderers;
      vtkNew<vtkRenderWindow> renWin;
      for (auto&& ren : renderers)
      {
        renWin->AddRenderer(ren);
      }
      renWin->SetSize(800, 800);
      vtkNew<vtkRenderWindowInteractor> iren;
      iren->SetRenderWindow(renWin);

      // Layer 0 - background not transparent
      renderers[0]->SetBackground(colors->GetColor3d("Silver").GetData());
      renderers[0]->AddActor(cubeActor);
      renderers[0]->SetLayer(0);

      // Layer 1 - the background is transparent
      //           so we only see the layer 0 background color
      renderers[1]->AddActor(coneActor);
      renderers[1]->SetBackground(colors->GetColor3d("MidnightBlue").GetData());
      renderers[1]->SetLayer(1);

      // We have two layers
      renWin->SetNumberOfLayers(static_cast<int>(renderers.size()));
      renWin->Render();
      renWin->SetWindowName("TransparentBackground");
      renWin->StereoRenderOff();

      vtkNew<vtkCallbackCommand> keypressCallback;
      keypressCallback->SetCallback(KeypressCallbackFunction);
      iren->AddObserver(vtkCommand::KeyPressEvent, keypressCallback);

      // Interact with the data
      iren->Start();

      return EXIT_SUCCESS;
    }

    namespace {
    void KeypressCallbackFunction(vtkObject* caller,
                                  long unsigned int vtkNotUsed(eventId),
                                  void* vtkNotUsed(clientData),
                                  void* vtkNotUsed(callData))
    {
      vtkRenderWindowInteractor* iren =
          static_cast<vtkRenderWindowInteractor*>(caller);
      vtkRendererCollection* renderers = iren->GetRenderWindow()->GetRenderers();
      if (renderers->GetNumberOfItems() < 2)
      {
        std::cerr << "We need at least two renderers, we have only "
                  << renderers->GetNumberOfItems() << std::endl;
        return;
      }
      renderers->InitTraversal();
      // Top item
      vtkRenderer* ren0 = renderers->GetNextItem();
      // Bottom item
      vtkRenderer* ren1 = renderers->GetNextItem();

      std::string key = iren->GetKeySym();

      if (key == "0")
      {
        iren->GetRenderWindow()
            ->GetInteractor()
            ->GetInteractorStyle()
            ->SetDefaultRenderer(ren0);
        ren0->InteractiveOn();
        ren1->InteractiveOff();
      }
      if (key == "1")
      {
        iren->GetRenderWindow()
            ->GetInteractor()
            ->GetInteractorStyle()
            ->SetDefaultRenderer(ren1);
        ren0->InteractiveOff();
        ren1->InteractiveOn();
      }
    }
    } // namespace

### CMakeLists.txt

    cmake_minimum_required(VERSION 3.3 FATAL_ERROR)

    project(TransparentBackground)

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

### Download and Build TransparentBackground

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

    cd TransparentBackground/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:

    ./TransparentBackground

WINDOWS USERS

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