Examples/RegistrationITKv4/MultiStageImageRegistration2.cxx

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 *
 *  Copyright NumFOCUS
 *
 *  Licensed under the Apache License, Version 2.0 (the "License");
 *  you may not use this file except in compliance with the License.
 *  You may obtain a copy of the License at
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 *         http://www.apache.org/licenses/LICENSE-2.0.txt
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 *  distributed under the License is distributed on an "AS IS" BASIS,
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//  Software Guide : BeginCommandLineArgs
//    INPUTS:  {BrainT1SliceBorder20.png}
//    INPUTS:  {BrainProtonDensitySliceR10X13Y17.png}
//    OUTPUTS: {MultiStageImageRegistration2Output.png}
//    ARGUMENTS:    100
//    OUTPUTS: {MultiStageImageRegistration2CheckerboardBefore.png}
//    OUTPUTS: {MultiStageImageRegistration2CheckerboardAfter.png}
//  Software Guide : EndCommandLineArgs
 
// Software Guide : BeginLatex
//
//  This examples shows how different stages can be cascaded together directly
//  in a multistage registration process. The example code is, for the most
//  part, identical to the previous multistage example. The main difference
//  is that no initial transform is used, and the output of the first stage
//  is directly linked to the second stage, and the whole registration process
//  is triggered only once by calling \code{Update()} after the last stage
//  stage.
//
//  We will focus on the most relevent changes in current code and skip all
//  the similar parts already explained in the previous example.
//
// \index{itk::ImageRegistrationMethodv4!Multi-Stage}
//
// Software Guide : EndLatex
 
#include "itkImageRegistrationMethodv4.h"
 
#include "itkMattesMutualInformationImageToImageMetricv4.h"
 
#include "itkRegularStepGradientDescentOptimizerv4.h"
#include "itkConjugateGradientLineSearchOptimizerv4.h"
 
#include "itkTranslationTransform.h"
#include "itkAffineTransform.h"
#include "itkCompositeTransform.h"
 
#include "itkImageFileReader.h"
#include "itkImageFileWriter.h"
 
#include "itkImageMomentsCalculator.h"
#include "itkResampleImageFilter.h"
#include "itkCastImageFilter.h"
#include "itkCheckerBoardImageFilter.h"
 
#include "itkCommand.h"
 
//  The following section of code implements a Command observer
//  that will monitor the configurations of the registration process
//  at every change of stage and resolution level.
//
template <typename TRegistration>
class RegistrationInterfaceCommand : public itk::Command
{
public:
  using Self = RegistrationInterfaceCommand;
  using Superclass = itk::Command;
  using Pointer = itk::SmartPointer<Self>;
  itkNewMacro(Self);
 
protected:
  RegistrationInterfaceCommand() = default;
 
public:
  using RegistrationType = TRegistration;
 
  // The Execute function simply calls another version of the \code{Execute()}
  // method accepting a \code{const} input object
  void
  Execute(itk::Object * object, const itk::EventObject & event) override
  {
    Execute((const itk::Object *)object, event);
  }
 
  void
  Execute(const itk::Object * object, const itk::EventObject & event) override
  {
    if (!(itk::MultiResolutionIterationEvent().CheckEvent(&event)))
    {
      return;
    }
 
    std::cout << "\nObserving from class " << object->GetNameOfClass();
    if (!object->GetObjectName().empty())
    {
      std::cout << " \"" << object->GetObjectName() << "\"" << std::endl;
    }
 
    const auto * registration = static_cast<const RegistrationType *>(object);
    if (registration == nullptr)
    {
      itkExceptionMacro(<< "Dynamic cast failed, object of type "
                        << object->GetNameOfClass());
    }
 
    unsigned int currentLevel = registration->GetCurrentLevel();
    typename RegistrationType::ShrinkFactorsPerDimensionContainerType
      shrinkFactors =
        registration->GetShrinkFactorsPerDimension(currentLevel);
    typename RegistrationType::SmoothingSigmasArrayType smoothingSigmas =
      registration->GetSmoothingSigmasPerLevel();
 
    std::cout << "-------------------------------------" << std::endl;
    std::cout << " Current multi-resolution level = " << currentLevel
              << std::endl;
    std::cout << "    shrink factor = " << shrinkFactors << std::endl;
    std::cout << "    smoothing sigma = " << smoothingSigmas[currentLevel]
              << std::endl;
    std::cout << std::endl;
  }
};
 
//  The following section of code implements an observer
//  that will monitor the evolution of the registration process.
//
class CommandIterationUpdate : public itk::Command
{
public:
  using Self = CommandIterationUpdate;
  using Superclass = itk::Command;
  using Pointer = itk::SmartPointer<Self>;
  itkNewMacro(Self);
 
protected:
  CommandIterationUpdate() = default;
 
public:
  using OptimizerType = itk::GradientDescentOptimizerv4Template<double>;
  using OptimizerPointer = const OptimizerType *;
 
  void
  Execute(itk::Object * caller, const itk::EventObject & event) override
  {
    Execute((const itk::Object *)caller, event);
  }
 
  void
  Execute(const itk::Object * object, const itk::EventObject & event) override
  {
    auto optimizer = static_cast<OptimizerPointer>(object);
    if (optimizer == nullptr)
    {
      return; // in this unlikely context, just do nothing.
    }
    if (!(itk::IterationEvent().CheckEvent(&event)))
    {
      return;
    }
    std::cout << optimizer->GetCurrentIteration() << "   ";
    std::cout << optimizer->GetValue() << "   ";
    std::cout << optimizer->GetCurrentPosition() << "  "
              << m_CumulativeIterationIndex++ << std::endl;
  }
 
private:
  unsigned int m_CumulativeIterationIndex{ 0 };
};
 
int
main(int argc, char * argv[])
{
  if (argc < 4)
  {
    std::cerr << "Missing Parameters " << std::endl;
    std::cerr << "Usage: " << argv[0];
    std::cerr << " fixedImageFile  movingImageFile ";
    std::cerr << " outputImagefile [backgroundGrayLevel]";
    std::cerr << " [checkerboardbefore] [CheckerBoardAfter]";
    std::cerr << " [numberOfBins] " << std::endl;
    return EXIT_FAILURE;
  }
 
  constexpr unsigned int Dimension = 2;
  using PixelType = float;
 
  using FixedImageType = itk::Image<PixelType, Dimension>;
  using MovingImageType = itk::Image<PixelType, Dimension>;
 
  //  Software Guide : BeginLatex
  //
  //  Let's start by defining different types of the first stage.
  //
  //  Software Guide : EndLatex
 
  // Software Guide : BeginCodeSnippet
  using TTransformType = itk::TranslationTransform<double, Dimension>;
  using TOptimizerType = itk::RegularStepGradientDescentOptimizerv4<double>;
  using MetricType =
    itk::MattesMutualInformationImageToImageMetricv4<FixedImageType,
                                                     MovingImageType>;
  using TRegistrationType =
    itk::ImageRegistrationMethodv4<FixedImageType, MovingImageType>;
  // Software Guide : EndCodeSnippet
 
  //  Software Guide : BeginLatex
  //
  //  Type definitions are the same as previous example with an important
  //  subtle change: the transform type is not passed to the registration
  //  method as a template parameter anymore. In this case, the registration
  //  filter will consider the transform base class \doxygen{Transform} as the
  //  type of its output transform.
  //
  //  Software Guide : EndLatex
 
  //  All the components are instantiated using their \code{New()} method
  //  and connected to the registration object as in previous example.
  //
  TOptimizerType::Pointer    transOptimizer = TOptimizerType::New();
  MetricType::Pointer        transMetric = MetricType::New();
  TRegistrationType::Pointer transRegistration = TRegistrationType::New();
 
  transRegistration->SetOptimizer(transOptimizer);
  transRegistration->SetMetric(transMetric);
 
  //  Software Guide : BeginLatex
  //
  //  Instead of passing the transform type, we create an explicit
  //  instantiation of the transform object outside of the registration
  //  filter, and connect that to the registration object using the
  //  \code{SetInitialTransform()} method. Also, by calling \code{InPlaceOn()}
  //  method, this transform object will be the output transform of the
  //  registration filter or will be grafted to the output.
  //
  //  Software Guide : EndLatex
 
  // Software Guide : BeginCodeSnippet
  TTransformType::Pointer translationTx = TTransformType::New();
 
  transRegistration->SetInitialTransform(translationTx);
  transRegistration->InPlaceOn();
  // Software Guide : EndCodeSnippet
 
  //  Software Guide : BeginLatex
  //
  //  Also, there is no initial transform defined for this example.
  //
  //  Software Guide : EndLatex
 
  using FixedImageReaderType = itk::ImageFileReader<FixedImageType>;
  using MovingImageReaderType = itk::ImageFileReader<MovingImageType>;
 
  FixedImageReaderType::Pointer fixedImageReader =
    FixedImageReaderType::New();
  MovingImageReaderType::Pointer movingImageReader =
    MovingImageReaderType::New();
 
  fixedImageReader->SetFileName(argv[1]);
  movingImageReader->SetFileName(argv[2]);
 
  transRegistration->SetFixedImage(fixedImageReader->GetOutput());
  transRegistration->SetMovingImage(movingImageReader->GetOutput());
  transRegistration->SetObjectName("TranslationRegistration");
 
  //  Software Guide : BeginLatex
  //
  //  As in the previous example, the first stage is run using only one level
  //  of registration at a coarse resolution level. However, notice that we do
  //  not need to update the translation registration filter at this step
  //  since the output of this stage will be directly connected to the initial
  //  input of the next stage. Due to ITK's pipeline structure, when we call
  //  the \code{Update()} at the last stage, the first stage will be updated
  //  as well.
  //
  //  Software Guide : EndLatex
 
  constexpr unsigned int numberOfLevels1 = 1;
 
  TRegistrationType::ShrinkFactorsArrayType shrinkFactorsPerLevel1;
  shrinkFactorsPerLevel1.SetSize(numberOfLevels1);
  shrinkFactorsPerLevel1[0] = 3;
 
  TRegistrationType::SmoothingSigmasArrayType smoothingSigmasPerLevel1;
  smoothingSigmasPerLevel1.SetSize(numberOfLevels1);
  smoothingSigmasPerLevel1[0] = 2;
 
  transRegistration->SetNumberOfLevels(numberOfLevels1);
  transRegistration->SetShrinkFactorsPerLevel(shrinkFactorsPerLevel1);
  transRegistration->SetSmoothingSigmasPerLevel(smoothingSigmasPerLevel1);
 
  transMetric->SetNumberOfHistogramBins(24);
 
  if (argc > 7)
  {
    // optionally, override the values with numbers taken from the command
    // line arguments.
    transMetric->SetNumberOfHistogramBins(std::stoi(argv[7]));
  }
 
  transOptimizer->SetNumberOfIterations(200);
  transOptimizer->SetRelaxationFactor(0.5);
  transOptimizer->SetLearningRate(16);
  transOptimizer->SetMinimumStepLength(1.5);
 
  // Create the Command observer and register it with the optimizer.
  //
  CommandIterationUpdate::Pointer observer1 = CommandIterationUpdate::New();
  transOptimizer->AddObserver(itk::IterationEvent(), observer1);
 
  // Create the Command interface observer and register it with the optimizer.
  //
  using TranslationCommandType =
    RegistrationInterfaceCommand<TRegistrationType>;
  TranslationCommandType::Pointer command1 = TranslationCommandType::New();
  transRegistration->AddObserver(itk::MultiResolutionIterationEvent(),
                                 command1);
 
  //  Software Guide : BeginLatex
  //
  //  Now we upgrade to an Affine transform as the second stage of
  //  registration process, and as before, we initially define and instantiate
  //  different components of the current registration stage. We have used a
  //  new optimizer but the same metric in new configurations.
  //
  //  Software Guide : EndLatex
 
  // Software Guide : BeginCodeSnippet
  using ATransformType = itk::AffineTransform<double, Dimension>;
  using AOptimizerType =
    itk::ConjugateGradientLineSearchOptimizerv4Template<double>;
  using ARegistrationType =
    itk::ImageRegistrationMethodv4<FixedImageType, MovingImageType>;
  // Software Guide : EndCodeSnippet
 
  //  Software Guide : BeginLatex
  //
  //  Again notice that \emph{TransformType} is not passed to the type
  //  definition of the registration filter. It is important because when the
  //  registration filter considers transform base class \doxygen{Transform}
  //  as the type of its output transform, it prevents the type mismatch when
  //  the two stages are cascaded to each other.
  //
  //  Then, all components are instantiated using their \code{New()} method
  //  and connected to the registration object among the transform type.
  //  Despite the previous example, here we use the fixed image's center of
  //  mass to initialize the fixed parameters of the Affine transform.
  //  \doxygen{ImageMomentsCalculator} filter is used for this purpose.
  //
  //  Software Guide : EndLatex
 
  AOptimizerType::Pointer    affineOptimizer = AOptimizerType::New();
  MetricType::Pointer        affineMetric = MetricType::New();
  ARegistrationType::Pointer affineRegistration = ARegistrationType::New();
 
  affineRegistration->SetOptimizer(affineOptimizer);
  affineRegistration->SetMetric(affineMetric);
 
  affineMetric->SetNumberOfHistogramBins(24);
  if (argc > 7)
  {
    // optionally, override the values with numbers taken from the command
    // line arguments.
    affineMetric->SetNumberOfHistogramBins(std::stoi(argv[7]));
  }
 
  fixedImageReader->Update();
  FixedImageType::Pointer fixedImage = fixedImageReader->GetOutput();
 
  // Software Guide : BeginCodeSnippet
  using FixedImageCalculatorType =
    itk::ImageMomentsCalculator<FixedImageType>;
 
  FixedImageCalculatorType::Pointer fixedCalculator =
    FixedImageCalculatorType::New();
  fixedCalculator->SetImage(fixedImage);
  fixedCalculator->Compute();
 
  FixedImageCalculatorType::VectorType fixedCenter =
    fixedCalculator->GetCenterOfGravity();
  // Software Guide : EndCodeSnippet
 
  //  Software Guide : BeginLatex
  //
  // Then, we initialize the fixed parameters (center of rotation) in the
  // Affine transform and connect that to the registration object.
  //
  //  Software Guide : EndLatex
 
  // Software Guide : BeginCodeSnippet
  ATransformType::Pointer affineTx = ATransformType::New();
 
  const unsigned int numberOfFixedParameters =
    affineTx->GetFixedParameters().Size();
  ATransformType::ParametersType fixedParameters(numberOfFixedParameters);
  for (unsigned int i = 0; i < numberOfFixedParameters; ++i)
  {
    fixedParameters[i] = fixedCenter[i];
  }
  affineTx->SetFixedParameters(fixedParameters);
 
  affineRegistration->SetInitialTransform(affineTx);
  affineRegistration->InPlaceOn();
  // Software Guide : EndCodeSnippet
 
  affineRegistration->SetFixedImage(fixedImageReader->GetOutput());
  affineRegistration->SetMovingImage(movingImageReader->GetOutput());
  affineRegistration->SetObjectName("AffineRegistration");
 
  //  Software Guide : BeginLatex
  //
  //  Now, the output of the first stage is wrapped through a
  //  \doxygen{DataObjectDecorator} and is passed to the input
  //  of the second stage as the moving initial transform via
  //  \code{SetMovingInitialTransformInput()} method. Note that
  //  this API has an ``Input'' word attached to the name of another
  //  initialization method \code{SetMovingInitialTransform()}
  //  that already has been used in previous example.
  //  This extension means that the following API expects
  //  a data object decorator type.
  //
  //  Software Guide : EndLatex
 
  // Software Guide : BeginCodeSnippet
  affineRegistration->SetMovingInitialTransformInput(
    transRegistration->GetTransformOutput());
  // Software Guide : EndCodeSnippet
 
 
  using ScalesEstimatorType =
    itk::RegistrationParameterScalesFromPhysicalShift<MetricType>;
  ScalesEstimatorType::Pointer scalesEstimator = ScalesEstimatorType::New();
  scalesEstimator->SetMetric(affineMetric);
  scalesEstimator->SetTransformForward(true);
 
  affineOptimizer->SetScalesEstimator(scalesEstimator);
  affineOptimizer->SetDoEstimateLearningRateOnce(true);
  affineOptimizer->SetDoEstimateLearningRateAtEachIteration(false);
  affineOptimizer->SetLowerLimit(0);
  affineOptimizer->SetUpperLimit(2);
  affineOptimizer->SetEpsilon(0.2);
  affineOptimizer->SetNumberOfIterations(200);
  affineOptimizer->SetMinimumConvergenceValue(1e-6);
  affineOptimizer->SetConvergenceWindowSize(10);
 
  // Create the Command observer and register it with the optimizer.
  //
  CommandIterationUpdate::Pointer observer2 = CommandIterationUpdate::New();
  affineOptimizer->AddObserver(itk::IterationEvent(), observer2);
 
  //  Software Guide : BeginLatex
  //
  //  Second stage runs two levels of registration, where the second
  //  level is run in full resolution.
  //
  //  Software Guide : EndLatex
 
  constexpr unsigned int numberOfLevels2 = 2;
 
  ARegistrationType::ShrinkFactorsArrayType shrinkFactorsPerLevel2;
  shrinkFactorsPerLevel2.SetSize(numberOfLevels2);
  shrinkFactorsPerLevel2[0] = 2;
  shrinkFactorsPerLevel2[1] = 1;
 
  ARegistrationType::SmoothingSigmasArrayType smoothingSigmasPerLevel2;
  smoothingSigmasPerLevel2.SetSize(numberOfLevels2);
  smoothingSigmasPerLevel2[0] = 1;
  smoothingSigmasPerLevel2[1] = 0;
 
  affineRegistration->SetNumberOfLevels(numberOfLevels2);
  affineRegistration->SetShrinkFactorsPerLevel(shrinkFactorsPerLevel2);
  affineRegistration->SetSmoothingSigmasPerLevel(smoothingSigmasPerLevel2);
 
  // Create the Command interface observer and register it with the optimizer.
  //
  using AffineCommandType = RegistrationInterfaceCommand<ARegistrationType>;
  AffineCommandType::Pointer command2 = AffineCommandType::New();
  affineRegistration->AddObserver(itk::MultiResolutionIterationEvent(),
                                  command2);
 
  //  Software Guide : BeginLatex
  //
  //  Once all the registration components are in place,
  //  finally we trigger the whole registration process, including two
  //  cascaded registration stages, by calling \code{Update()} on the
  //  registration filter of the last stage, which causes both stages be
  //  updated.
  //
  //  Software Guide : EndLatex
 
  // Software Guide : BeginCodeSnippet
  try
  {
    affineRegistration->Update();
    std::cout
      << "Optimizer stop condition: "
      << affineRegistration->GetOptimizer()->GetStopConditionDescription()
      << std::endl;
  }
  catch (const itk::ExceptionObject & err)
  {
    std::cout << "ExceptionObject caught !" << std::endl;
    std::cout << err << std::endl;
    return EXIT_FAILURE;
  }
  // Software Guide : EndCodeSnippet
 
  //  Software Guide : BeginLatex
  //
  //  Finally, a composite transform is used to concatenate the results of
  //  all stages together, which will be considered as the
  //  final output of this multistage process and will be passed to the
  //  resampler to resample the moving image into the virtual domain
  //  space (fixed image space if there is no fixed initial transform).
  //
  //  Software Guide : EndLatex
 
  // Software Guide : BeginCodeSnippet
  using CompositeTransformType = itk::CompositeTransform<double, Dimension>;
  CompositeTransformType::Pointer compositeTransform =
    CompositeTransformType::New();
  compositeTransform->AddTransform(translationTx);
  compositeTransform->AddTransform(affineTx);
  // Software Guide : EndCodeSnippet
 
  std::cout << " Translation transform parameters after registration: "
            << std::endl
            << transOptimizer->GetCurrentPosition() << std::endl
            << " Last LearningRate: "
            << transOptimizer->GetCurrentStepLength() << std::endl;
 
  std::cout << " Affine transform parameters after registration: "
            << std::endl
            << affineOptimizer->GetCurrentPosition() << std::endl
            << " Last LearningRate: " << affineOptimizer->GetLearningRate()
            << std::endl;
 
 
  //  Software Guide : BeginLatex
  //
  //  Let's execute this example using the same multi-modality images as
  //  before. The registration converges after $6$ iterations in the first
  //  stage, also in $45$ and $11$ iterations corresponding to the first level
  //  and second level of the Affine stage.
  //  The final results when printed as an array of parameters are:
  //
  //  \begin{verbatim}
  //  Translation parameters after first registration stage:
  //  [11.600, 15.1814]
  //
  //  Affine parameters after second registration stage:
  //  [0.9860, -0.1742, 0.1751, 0.9862, 0.9219, 0.8023]
  //  \end{verbatim}
  //
  //  Let's reorder the Affine array of parameters again as coefficients of
  //  matrix
  //  $\bf{M}$ and vector $\bf{T}$. They can now be seen as
  //
  //  \begin{equation}
  //  M =
  //  \left[
  //  \begin{array}{cc}
  //  0.9860 & -0.1742 \\ 0.1751 & 0.9862 \\  \end{array}
  //  \right]
  //  \mbox{ and }
  //  T =
  //  \left[
  //  \begin{array}{c}
  //  0.9219  \\  0.8023  \\  \end{array}
  //  \right]
  //  \end{equation}
  //
  //  $10.02$ degrees is the rotation value computed from the affine matrix
  //  parameters, which approximately equals the intentional misalignment.
  //
  //  Also for the total translation value resulted from both transforms, we
  //  have:
  //
  //  In $X$ direction:
  //  \begin{equation}
  //  11.6004 + 0.9219 = 12.5223
  //  \end{equation}
  //  In $Y$ direction:
  //  \begin{equation}
  //  15.1814 + 0.8023 = 15.9837
  //  \end{equation}
  //
  //  These results closely match the true misalignment introduced in the
  //  moving image.
  //
  //  Software Guide : EndLatex
 
  using ResampleFilterType =
    itk::ResampleImageFilter<MovingImageType, FixedImageType>;
  ResampleFilterType::Pointer resample = ResampleFilterType::New();
 
  resample->SetTransform(compositeTransform);
  resample->SetInput(movingImageReader->GetOutput());
 
  PixelType backgroundGrayLevel = 100;
  if (argc > 4)
  {
    backgroundGrayLevel = std::stoi(argv[4]);
  }
 
  resample->SetSize(fixedImage->GetLargestPossibleRegion().GetSize());
  resample->SetOutputOrigin(fixedImage->GetOrigin());
  resample->SetOutputSpacing(fixedImage->GetSpacing());
  resample->SetOutputDirection(fixedImage->GetDirection());
  resample->SetDefaultPixelValue(backgroundGrayLevel);
 
  using OutputPixelType = unsigned char;
  using OutputImageType = itk::Image<OutputPixelType, Dimension>;
  using CastFilterType =
    itk::CastImageFilter<FixedImageType, OutputImageType>;
  using WriterType = itk::ImageFileWriter<OutputImageType>;
 
  WriterType::Pointer     writer = WriterType::New();
  CastFilterType::Pointer caster = CastFilterType::New();
 
  writer->SetFileName(argv[3]);
 
  caster->SetInput(resample->GetOutput());
  writer->SetInput(caster->GetOutput());
  writer->Update();
 
  //  Software Guide : BeginLatex
  //
  // \begin{figure}
  // \center
  // \includegraphics[width=0.32\textwidth]{MultiStageImageRegistration2Output}
  // \includegraphics[width=0.32\textwidth]{MultiStageImageRegistration2CheckerboardBefore}
  // \includegraphics[width=0.32\textwidth]{MultiStageImageRegistration2CheckerboardAfter}
  // \itkcaption[Multistage registration input images]{Mapped moving image
  // (left) and composition of fixed and moving images before (center) and
  // after (right) registration.}
  // \label{fig:MultiStageImageRegistration2Outputs}
  // \end{figure}
  //
  //  The result of resampling the moving image is presented in the left image
  //  of Figure \ref{fig:MultiStageImageRegistration2Outputs}. The center and
  //  right images of the figure depict a checkerboard composite of the fixed
  //  and moving images before and after registration.
  //
  //  Software Guide : EndLatex
 
  //
  // Generate checkerboards before and after registration
  //
  using CheckerBoardFilterType = itk::CheckerBoardImageFilter<FixedImageType>;
 
  CheckerBoardFilterType::Pointer checker = CheckerBoardFilterType::New();
 
  checker->SetInput1(fixedImage);
  checker->SetInput2(resample->GetOutput());
 
  caster->SetInput(checker->GetOutput());
  writer->SetInput(caster->GetOutput());
 
  resample->SetDefaultPixelValue(0);
 
  // Write out checkerboard outputs
  // Before registration
  using TransformType = itk::IdentityTransform<double, Dimension>;
  TransformType::Pointer identityTransform;
  try
  {
    identityTransform = TransformType::New();
  }
  catch (const itk::ExceptionObject & err)
  {
    err.Print(std::cerr);
    return EXIT_FAILURE;
  }
  identityTransform->SetIdentity();
  resample->SetTransform(identityTransform);
 
  if (argc > 5)
  {
    writer->SetFileName(argv[5]);
    writer->Update();
  }
 
  // After registration
  resample->SetTransform(compositeTransform);
  if (argc > 6)
  {
    writer->SetFileName(argv[6]);
    writer->Update();
  }
 
  return EXIT_SUCCESS;
}