Examples/RegistrationITKv4/ImageRegistration3.cxx

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 *  Copyright NumFOCUS
 *
 *  Licensed under the Apache License, Version 2.0 (the "License");
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// Software Guide : BeginLatex
//
// Given the numerous parameters involved in tuning a registration method for
// a particular application, it is not uncommon for a registration process to
// run for several minutes and still produce a useless result.  To avoid
// this situation it is quite helpful to track the evolution of the
// registration as it progresses. The following section illustrates the
// mechanisms provided in ITK for monitoring the activity of the
// ImageRegistrationMethodv4 class.
//
// Insight implements the \emph{Observer/Command} design pattern
// \cite{Gamma1995}.
// The classes involved in this implementation are the \doxygen{Object},
// \doxygen{Command} and \doxygen{EventObject} classes. The Object
// is the base class of most ITK objects. This class maintains a linked
// list of pointers to event observers. The role of observers is played by
// the Command class.  Observers register themselves with an
// Object, declaring that they are interested in receiving
// notification when a particular event happens. A set of events is
// represented by the hierarchy of the Event class. Typical events
// are \code{Start}, \code{End}, \code{Progress} and \code{Iteration}.
//
// Registration is controlled by an \doxygen{Optimizer}, which generally
// executes an iterative process. Most Optimizer classes invoke an
// \doxygen{IterationEvent} at the end of each iteration. When an event is
// invoked by an object, this object goes through its list of registered
// observers (Commands) and checks whether any one of them has expressed
// interest in the current event type. Whenever such an observer is found,
// its corresponding \code{Execute()} method is invoked.  In this context,
// \code{Execute()} methods should be considered \emph{callbacks}.  As such,
// some of the common sense rules of callbacks should be respected.  For
// example, \code{Execute()} methods should not perform heavy computational
// tasks.  They are expected to execute rapidly, for example, printing out a
// message or updating a value in a GUI.
//
// \index{itk::ImageRegistrationMethod!Monitoring}
//
//
// Software Guide : EndLatex
 
 
#include "itkImageRegistrationMethodv4.h"
#include "itkTranslationTransform.h"
#include "itkMeanSquaresImageToImageMetricv4.h"
#include "itkRegularStepGradientDescentOptimizerv4.h"
 
#include "itkImageFileReader.h"
#include "itkImageFileWriter.h"
 
#include "itkResampleImageFilter.h"
#include "itkCastImageFilter.h"
 
 
//  Software Guide : BeginLatex
//
//  The following code illustrates a simple way of creating a
//  Observer/Command to monitor a registration process. This new
//  class derives from the Command class and provides a specific
//  implementation of the \code{Execute()} method.  First, the header file of
//  the Command class must be included.
//
//  Software Guide : EndLatex
 
// Software Guide : BeginCodeSnippet
#include "itkCommand.h"
// Software Guide : EndCodeSnippet
 
 
//  Software Guide : BeginLatex
//
//  Our custom command class is called \code{CommandIterationUpdate}. It
//  derives from the Command class and declares for convenience the
//  types \code{Self} and \code{Superclass}. This facilitates the use of
//  standard macros later in the class implementation.
//
//  Software Guide : EndLatex
 
// Software Guide : BeginCodeSnippet
class CommandIterationUpdate : public itk::Command
{
public:
  using Self = CommandIterationUpdate;
  using Superclass = itk::Command;
  // Software Guide : EndCodeSnippet
 
 
  //  Software Guide : BeginLatex
  //
  //  The following type alias declares the type of the SmartPointer capable
  //  of holding a reference to this object.
  //
  //  Software Guide : EndLatex
 
  // Software Guide : BeginCodeSnippet
  using Pointer = itk::SmartPointer<Self>;
  // Software Guide : EndCodeSnippet
 
 
  //  Software Guide : BeginLatex
  //
  //  The \code{itkNewMacro} takes care of defining all the necessary code for
  //  the \code{New()} method. Those with curious minds are invited to see the
  //  details of the macro in the file \code{itkMacro.h} in the
  //  \code{Insight/Code/Common} directory.
  //
  //  Software Guide : EndLatex
 
  // Software Guide : BeginCodeSnippet
  itkNewMacro(Self);
  // Software Guide : EndCodeSnippet
 
 
  //  Software Guide : BeginLatex
  //
  //  In order to ensure that the \code{New()} method is used to instantiate
  //  the class (and not the C++ \code{new} operator), the constructor is
  //  declared \code{protected}.
  //
  //  Software Guide : EndLatex
 
  // Software Guide : BeginCodeSnippet
 
protected:
  CommandIterationUpdate() = default;
  // Software Guide : EndCodeSnippet
 
public:
  //  Software Guide : BeginLatex
  //
  //  Since this Command object will be observing the optimizer,
  //  the following type alias are useful for converting pointers when the
  //  \code{Execute()} method is invoked.  Note the use of \code{const} on
  //  the declaration of \code{OptimizerPointer}.  This is relevant since, in
  //  this case, the observer is not intending to modify the optimizer in any
  //  way. A \code{const} interface ensures that all operations invoked on the
  //  optimizer are read-only.
  //
  //  Software Guide : EndLatex
 
  // Software Guide : BeginCodeSnippet
  using OptimizerType = itk::RegularStepGradientDescentOptimizerv4<double>;
  using OptimizerPointer = const OptimizerType *;
  // Software Guide : EndCodeSnippet
 
 
  //  Software Guide : BeginLatex
  //
  //  ITK enforces const-correctness. There is hence a distinction between the
  //  \code{Execute()} method that can be invoked from a \code{const} object
  //  and the one that can be invoked from a non-\code{const} object. In this
  //  particular example the non-\code{const} version simply invoke the
  //  \code{const}  version. In a more elaborate situation the implementation
  //  of both \code{Execute()} methods could be quite different. For example,
  //  you could imagine a non-\code{const} interaction in which the observer
  //  decides to stop the optimizer in response to a divergent behavior. A
  //  similar case could happen when a user is controlling the registration
  //  process from a GUI.
  //
  //  Software Guide : EndLatex
 
  // Software Guide : BeginCodeSnippet
  void
  Execute(itk::Object * caller, const itk::EventObject & event) override
  {
    Execute((const itk::Object *)caller, event);
  }
  // Software Guide : EndCodeSnippet
 
  //  Software Guide : BeginLatex
  //
  // Finally we get to the heart of the observer, the \code{Execute()} method.
  // Two arguments are passed to this method. The first argument is the
  // pointer to the object that invoked the event. The second argument is the
  // event that was invoked.
  //
  //  Software Guide : EndLatex
 
  // Software Guide : BeginCodeSnippet
  void
  Execute(const itk::Object * object, const itk::EventObject & event) override
  {
    // Software Guide : EndCodeSnippet
 
 
    //  Software Guide : BeginLatex
    //
    //  Note that the first argument is a pointer to an Object even
    //  though the actual object invoking the event is probably a subclass of
    //  Object. In our case we know that the actual object is an
    //  optimizer. Thus we can perform a \code{dynamic\_cast} to the real type
    //  of the object.
    //
    //  Software Guide : EndLatex
 
    // Software Guide : BeginCodeSnippet
    auto optimizer = static_cast<OptimizerPointer>(object);
    // Software Guide : EndCodeSnippet
 
 
    //  Software Guide : BeginLatex
    //
    //  The next step is to verify that the event invoked is actually the one
    //  in which we are interested. This is checked using the
    //  RTTI\footnote{RTTI stands for: Run-Time Type Information} support. The
    //  \code{CheckEvent()} method allows us to compare the actual type of two
    //  events.  In this case we compare the type of the received event with
    //  an IterationEvent. The comparison will return true if \code{event} is
    //  of type \code{IterationEvent} or derives from \code{IterationEvent}.
    //  If we find that the event is not of the expected type then the
    //  \code{Execute()} method of this command observer should return without
    //  any further action.
    //
    // \index{itk::EventObject!CheckEvent}
    //
    //  Software Guide : EndLatex
 
    // Software Guide : BeginCodeSnippet
    if (!itk::IterationEvent().CheckEvent(&event))
    {
      return;
    }
    // Software Guide : EndCodeSnippet
 
 
    //  Software Guide : BeginLatex
    //
    //  If the event matches the type we are looking for, we are ready to
    //  query data from the optimizer. Here, for example, we get the current
    //  number of iterations, the current value of the cost function and the
    //  current position on the parameter space. All of these values are
    //  printed to the standard output. You could imagine more elaborate
    //  actions like updating a GUI or refreshing a visualization pipeline.
    //
    //  Software Guide : EndLatex
 
    // Software Guide : BeginCodeSnippet
    std::cout << optimizer->GetCurrentIteration() << " = ";
    std::cout << optimizer->GetValue() << " : ";
    std::cout << optimizer->GetCurrentPosition() << std::endl;
    // Software Guide : EndCodeSnippet
  }
 
  //  Software Guide : BeginLatex
  //
  //  This concludes our implementation of a minimal Command class
  //  capable of observing our registration method.  We can now move on to
  //  configuring the registration process.
  //
  //  Software Guide : EndLatex
};
 
 
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 " << 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>;
 
  using TransformType = itk::TranslationTransform<double, Dimension>;
 
  using OptimizerType = itk::RegularStepGradientDescentOptimizerv4<double>;
 
  using RegistrationType = itk::
    ImageRegistrationMethodv4<FixedImageType, MovingImageType, TransformType>;
 
  using MetricType =
    itk::MeanSquaresImageToImageMetricv4<FixedImageType, MovingImageType>;
 
  OptimizerType::Pointer    optimizer = OptimizerType::New();
  RegistrationType::Pointer registration = RegistrationType::New();
 
  registration->SetOptimizer(optimizer);
 
  MetricType::Pointer metric = MetricType::New();
 
  registration->SetMetric(metric);
 
  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]);
 
  registration->SetFixedImage(fixedImageReader->GetOutput());
  registration->SetMovingImage(movingImageReader->GetOutput());
 
  // Set parameters of the optimizer
  //
  optimizer->SetLearningRate(4);
  optimizer->SetMinimumStepLength(0.001);
  optimizer->SetRelaxationFactor(0.5);
  optimizer->SetNumberOfIterations(200);
 
  // One level registration process without shrinking and smoothing.
  //
  constexpr unsigned int numberOfLevels = 1;
 
  RegistrationType::ShrinkFactorsArrayType shrinkFactorsPerLevel;
  shrinkFactorsPerLevel.SetSize(1);
  shrinkFactorsPerLevel[0] = 1;
 
  RegistrationType::SmoothingSigmasArrayType smoothingSigmasPerLevel;
  smoothingSigmasPerLevel.SetSize(1);
  smoothingSigmasPerLevel[0] = 0;
 
  registration->SetNumberOfLevels(numberOfLevels);
  registration->SetSmoothingSigmasPerLevel(smoothingSigmasPerLevel);
  registration->SetShrinkFactorsPerLevel(shrinkFactorsPerLevel);
 
 
  //  Software Guide : BeginLatex
  //
  //  Once all the registration components are in place we can create one
  //  instance of our observer. This is done with the standard \code{New()}
  //  method and assigned to a SmartPointer.
  //
  //  Software Guide : EndLatex
 
  // Software Guide : BeginCodeSnippet
  CommandIterationUpdate::Pointer observer = CommandIterationUpdate::New();
  // Software Guide : EndCodeSnippet
 
 
  //  Software Guide : BeginLatex
  //
  // \begin{figure}
  // \center
  // \includegraphics[width=\textwidth]{ImageRegistration3Observer}
  // \itkcaption[Command/Observer and the Registration Framework]{Interaction
  // between the Command/Observer and the Registration Method.}
  // \label{fig:ImageRegistration3Observer}
  // \end{figure}
  //
  //
  //  Software Guide : EndLatex
 
 
  //  Software Guide : BeginLatex
  //
  //  The newly created command is registered as observer on the
  //  optimizer, using the \code{AddObserver()} method. Note
  //  that the event type is provided as the first argument to this
  //  method. In order for the RTTI mechanism to work correctly, a newly
  //  created event of the desired type must be passed as the first
  //  argument. The second argument is simply the smart pointer to the
  //  observer. Figure \ref{fig:ImageRegistration3Observer} illustrates the
  //  interaction between the Command/Observer class and the registration
  //  method.
  //
  //  Software Guide : EndLatex
 
  // Software Guide : BeginCodeSnippet
  optimizer->AddObserver(itk::IterationEvent(), observer);
  // Software Guide : EndCodeSnippet
 
 
  //  Software Guide : BeginLatex
  //
  //  At this point, we are ready to execute the registration. The
  //  typical call to \code{Update()} will do it. Note again the
  //  use of the \code{try/catch} block around the \code{Update()}
  //  method in case an exception is thrown.
  //
  //  Software Guide : EndLatex
 
  // Software Guide : BeginCodeSnippet
  try
  {
    registration->Update();
    std::cout << "Optimizer stop condition: "
              << registration->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
  //
  //  The registration process is applied to the following images in
  //  \code{Examples/Data}:
  //
  //  \begin{itemize}
  //  \item \code{BrainProtonDensitySliceBorder20.png}
  //  \item \code{BrainProtonDensitySliceShifted13x17y.png}
  //  \end{itemize}
  //
  //  It produces the following output.
  //
  //  \begin{verbatim}
  //   0 = 4499.45 : [2.9286959512455857, 2.7244705953923805]
  //   1 = 3860.84 : [6.135143776902402, 5.115849348610004]
  //   2 = 3508.02 : [8.822660051952475, 8.078492808653918]
  //   3 = 3117.31 : [10.968558473732326, 11.454158663474674]
  //   4 = 2125.43 : [13.105290365964755, 14.835634202454191]
  //   5 = 911.308 : [12.75173580401588, 18.819978461140323]
  //   6 = 741.417 : [13.139053510563274, 16.857840597942413]
  //   7 = 16.8918 : [12.356787624301035, 17.480785285045815]
  //   8 = 233.714 : [12.79212443526829, 17.234854683011704]
  //   9 = 39.8027 : [13.167510875734614, 16.904574468172815]
  //   10 = 16.5731 : [12.938831371165355, 17.005597654570586]
  //   11 = 1.68763 : [13.063495692092735, 16.996443033457986]
  //   12 = 1.79437 : [13.001061362657559, 16.999307384689935]
  //   13 = 0.000762481 : [12.945418587211314, 17.0277701944711]
  //   14 = 1.74802 : [12.974454390534774, 17.01621663980765]
  //   15 = 0.430253 : [13.002439510423766, 17.002309966416835]
  //   16 = 0.00531816 : [12.989877586882951, 16.99301810428082]
  //   17 = 0.0721346 : [12.996759235073881, 16.996716492365685]
  //   18 = 0.00996773 : [13.00288423694971, 17.00156618393022]
  //   19 = 0.00516378 : [12.99928608126834, 17.000045636412015]
  //   20 = 0.000228075 : [13.00123653240422, 16.999943471681494]
  //  \end{verbatim}
  //  You can verify from the code in the \code{Execute()} method that the
  //  first column is the iteration number, the second column is the metric
  //  value and the third and fourth columns are the parameters of the
  //  transform, which is a $2D$ translation transform in this case. By
  //  tracking these values as the registration progresses, you will be able
  //  to determine whether the optimizer is advancing in the right direction
  //  and whether the step-length is reasonable or not.  That will allow you
  //  to interrupt the registration process and fine-tune parameters without
  //  having to wait until the optimizer stops by itself.
  //
  //  Software Guide : EndLatex
 
 
  TransformType::ParametersType finalParameters =
    registration->GetOutput()->Get()->GetParameters();
 
  const double TranslationAlongX = finalParameters[0];
  const double TranslationAlongY = finalParameters[1];
 
  const unsigned int numberOfIterations = optimizer->GetCurrentIteration();
 
  const double bestValue = optimizer->GetValue();
 
  std::cout << "Registration done !" << std::endl;
  std::cout << "Number of iterations = " << numberOfIterations << std::endl;
  std::cout << "Translation along X  = " << TranslationAlongX << std::endl;
  std::cout << "Translation along Y  = " << TranslationAlongY << std::endl;
  std::cout << "Optimal metric value = " << bestValue << std::endl;
 
 
  // Prepare the resampling filter in order to map the moving image.
  //
  using ResampleFilterType =
    itk::ResampleImageFilter<MovingImageType, FixedImageType>;
 
  ResampleFilterType::Pointer resample = ResampleFilterType::New();
 
  resample->SetTransform(registration->GetTransform());
  resample->SetInput(movingImageReader->GetOutput());
 
  FixedImageType::Pointer fixedImage = fixedImageReader->GetOutput();
 
  resample->SetSize(fixedImage->GetLargestPossibleRegion().GetSize());
  resample->SetOutputOrigin(fixedImage->GetOrigin());
  resample->SetOutputSpacing(fixedImage->GetSpacing());
  resample->SetOutputDirection(fixedImage->GetDirection());
  resample->SetDefaultPixelValue(100);
 
 
  // Prepare a writer and caster filters to send the resampled moving image to
  // a file
  //
  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();
 
 
  return EXIT_SUCCESS;
}