Pad

Description¶

An important point about the discrete Fourier transform is that it treats the image as a periodic function. This means the pixels on the right border are adjacent to pixels on the left border. Since there is usually no physical relationship between these pixels, the artificial horizontal and vertical edges can distort the frequency spectrum and subsequent processing. To reduce these artifacts, the original image can be multiplied by a window function that becomes zero at the borders.

Another approach removes these artificial edges by smoothing only along the borders.

In both of these approaches, a portion of the original image is lost, so only the central portion of an image can be processed. If this is unacceptable, another solution is to double the dimensions of the original image with a mirror-padding filter. The intermediate image is periodic and continuous.

The lower-left image has been padded with a constant (800). On the right, mirror padding has been used to remove artificial edges introduced by borders.

Info

See Figure 10-12 in Chapter 10 the VTK Textbook.

Other languages

See (Python)

Question

If you have a question about this example, please use the VTK Discourse Forum

Code¶

Pad.cxx

#include <vtkCamera.h>
#include <vtkImageActor.h>
#include <vtkImageConstantPad.h>
#include <vtkImageMapToWindowLevelColors.h>
#include <vtkImageMapper3D.h>
#include <vtkImageMirrorPad.h>
#include <vtkImageProperty.h>
#include <vtkImageReader2.h>
#include <vtkImageReader2Factory.h>
#include <vtkInteractorStyleImage.h>
#include <vtkNamedColors.h>
#include <vtkNew.h>
#include <vtkRenderWindow.h>
#include <vtkRenderWindowInteractor.h>
#include <vtkRenderer.h>
#include <vtkSmartPointer.h>

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

  // Verify input arguments.
  if (argc < 2)
  {
    std::cout << "Usage: " << argv[0] << " Filename e.g. FullHead.mhd"
              << std::endl;
    return EXIT_FAILURE;
  }

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

  // Pipelines.
  vtkNew<vtkImageConstantPad> constantPad;
  constantPad->SetInputConnection(reader->GetOutputPort());
  constantPad->SetConstant(800);
  constantPad->SetOutputWholeExtent(-127, 383, -127, 383, 22, 22);

  vtkNew<vtkImageMirrorPad> mirrorPad;
  mirrorPad->SetInputConnection(reader->GetOutputPort());
  mirrorPad->SetOutputWholeExtent(constantPad->GetOutputWholeExtent());

  // Create actors.

  vtkNew<vtkImageMapToWindowLevelColors> constantPadColor;
  constantPadColor->SetWindow(2000);
  constantPadColor->SetLevel(1000);
  constantPadColor->SetInputConnection(constantPad->GetOutputPort());

  vtkNew<vtkImageActor> constantPadActor;
  constantPadActor->GetMapper()->SetInputConnection(
      constantPadColor->GetOutputPort());
  constantPadActor->GetProperty()->SetInterpolationTypeToNearest();

  vtkNew<vtkImageMapToWindowLevelColors> mirrorPadColor;
  mirrorPadColor->SetWindow(2000);
  mirrorPadColor->SetLevel(1000);
  mirrorPadColor->SetInputConnection(mirrorPad->GetOutputPort());

  vtkNew<vtkImageActor> mirrorPadActor;
  mirrorPadActor->GetMapper()->SetInputConnection(
      mirrorPadColor->GetOutputPort());
  mirrorPadActor->GetProperty()->SetInterpolationTypeToNearest();

  // Setup renderers.
  vtkNew<vtkRenderer> constantPadRenderer;
  constantPadRenderer->SetViewport(0.0, 0.0, 0.5, 1.0);
  constantPadRenderer->AddActor(constantPadActor);
  constantPadRenderer->ResetCamera();
  constantPadRenderer->SetBackground(colors->GetColor3d("SlateGray").GetData());

  vtkNew<vtkRenderer> mirrorPadRenderer;
  mirrorPadRenderer->SetViewport(0.5, 0.0, 1.0, 1.0);
  mirrorPadRenderer->AddActor(mirrorPadActor);
  mirrorPadRenderer->SetActiveCamera(constantPadRenderer->GetActiveCamera());
  mirrorPadRenderer->SetBackground(
      colors->GetColor3d("LightSlateGray").GetData());

  vtkNew<vtkRenderWindow> renderWindow;
  renderWindow->SetSize(600, 300);
  renderWindow->SetWindowName("Pad");
  renderWindow->AddRenderer(constantPadRenderer);
  renderWindow->AddRenderer(mirrorPadRenderer);

  vtkNew<vtkRenderWindowInteractor> renderWindowInteractor;
  vtkNew<vtkInteractorStyleImage> style;

  renderWindowInteractor->SetInteractorStyle(style);

  renderWindowInteractor->SetRenderWindow(renderWindow);
  constantPadRenderer->GetActiveCamera()->Dolly(1.2);
  constantPadRenderer->ResetCameraClippingRange();
  renderWindow->Render();
  renderWindowInteractor->Initialize();

  renderWindowInteractor->Start();

  return EXIT_SUCCESS;
}

CMakeLists.txt¶

cmake_minimum_required(VERSION 3.12 FATAL_ERROR)

project(Pad)

find_package(VTK COMPONENTS 
)

if (NOT VTK_FOUND)
  message(FATAL_ERROR "Pad: Unable to find the VTK build folder.")
endif()

# Prevent a "command line is too long" failure in Windows.
set(CMAKE_NINJA_FORCE_RESPONSE_FILE "ON" CACHE BOOL "Force Ninja to use response files.")
add_executable(Pad MACOSX_BUNDLE Pad.cxx )
  target_link_libraries(Pad PRIVATE ${VTK_LIBRARIES}
)
# vtk_module_autoinit is needed
vtk_module_autoinit(
  TARGETS Pad
  MODULES ${VTK_LIBRARIES}
)

Download and Build Pad¶

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

cd Pad/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:

./Pad

WINDOWS USERS

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