Examples/Statistics/BayesianPluginClassifier.cxx

/*=========================================================================
 *
 *  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
 *
 *         https://www.apache.org/licenses/LICENSE-2.0.txt
 *
 *  Unless required by applicable law or agreed to in writing, software
 *  distributed under the License is distributed on an "AS IS" BASIS,
 *  WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
 *  See the License for the specific language governing permissions and
 *  limitations under the License.
 *
 *=========================================================================*/
 
// Software Guide : BeginLatex
//
// \index{Statistics!Bayesian plugin classifier}
// \index{itk::Statistics::SampleClassifier}
// \index{itk::Statistics::GaussianMembershipFunction}
// \index{itk::Statistics::NormalVariateGenerator}
//
// In this example, we present a system that places measurement vectors into
// two Gaussian classes. The Figure~\ref{fig:BayesianPluginClassifier} shows
// all the components of the classifier system and the data flow. This system
// differs with the previous k-means clustering algorithms in several
// ways. The biggest difference is that this classifier uses the
// \subdoxygen{Statistics}{GaussianDensityFunction}s as membership functions
// instead of the
// \subdoxygen{Statistics}{DistanceToCentroidMembershipFunction}. Since the
// membership function is different, the membership function requires a
// different set of parameters, mean vectors and covariance matrices. We
// choose the \subdoxygen{Statistics}{CovarianceSampleFilter} (sample
// covariance) for the estimation algorithms of the two parameters. If we want
// a more robust estimation algorithm, we can replace this estimation
// algorithm with more alternatives without changing other components in the
// classifier system.
//
// It is a bad idea to use the same sample for test and training
// (parameter estimation) of the parameters. However, for simplicity, in
// this example, we use a sample for test and training.
//
// \begin{figure}
//   \centering
//   \includegraphics[width=0.9\textwidth]{BayesianPluginClassifier}
//   \itkcaption[Bayesian plug-in classifier for two Gaussian
//   classes]{Bayesian plug-in classifier for two Gaussian classes.}
//  \protect\label{fig:BayesianPluginClassifier}
// \end{figure}
//
// We use the \subdoxygen{Statistics}{ListSample} as the sample (test
// and training). The \doxygen{Vector} is our measurement vector
// class. To store measurement vectors into two separate sample
// containers, we use the \subdoxygen{Statistics}{Subsample} objects.
//
// Software Guide : EndLatex
 
// Software Guide : BeginCodeSnippet
#include "itkVector.h"
#include "itkListSample.h"
#include "itkSubsample.h"
// Software Guide : EndCodeSnippet
 
// Software Guide : BeginLatex
//
// The following file provides us the parameter estimation algorithm.
//
// Software Guide : EndLatex
 
// Software Guide : BeginCodeSnippet
#include "itkCovarianceSampleFilter.h"
// Software Guide : EndCodeSnippet
 
// Software Guide : BeginLatex
//
// The following files define the components required by ITK statistical
// classification framework: the decision rule, the membership
// function, and the classifier.
//
// Software Guide : EndLatex
 
// Software Guide : BeginCodeSnippet
#include "itkMaximumRatioDecisionRule.h"
#include "itkGaussianMembershipFunction.h"
#include "itkSampleClassifierFilter.h"
// Software Guide : EndCodeSnippet
 
// Software Guide : BeginLatex
//
// We will fill the sample with random variables from two normal
// distribution using the \subdoxygen{Statistics}{NormalVariateGenerator}.
//
// Software Guide : EndLatex
 
// Software Guide : BeginCodeSnippet
#include "itkNormalVariateGenerator.h"
// Software Guide : EndCodeSnippet
 
int
main(int, char *[])
{
  // Software Guide : BeginLatex
  //
  // Since the NormalVariateGenerator class only supports 1-D, we define our
  // measurement vector type as a one component vector. We then, create a
  // ListSample object for data inputs.
  //
  // We also create two Subsample objects that will store
  // the measurement vectors in \code{sample} into two separate
  // sample containers. Each Subsample object stores only the
  // measurement vectors belonging to a single class. This class sample
  // will be used by the parameter estimation algorithms.
  //
  // Software Guide : EndLatex
 
  // Software Guide : BeginCodeSnippet
  constexpr unsigned int measurementVectorLength = 1;
  using MeasurementVectorType = itk::Vector<double, measurementVectorLength>;
  using SampleType = itk::Statistics::ListSample<MeasurementVectorType>;
  auto sample = SampleType::New();
  // length of measurement vectors in the sample.
  sample->SetMeasurementVectorSize(measurementVectorLength);
 
  using ClassSampleType = itk::Statistics::Subsample<SampleType>;
  std::vector<ClassSampleType::Pointer> classSamples;
  for (unsigned int i = 0; i < 2; ++i)
  {
    classSamples.push_back(ClassSampleType::New());
    classSamples[i]->SetSample(sample);
  }
  // Software Guide : EndCodeSnippet
 
  // Software Guide : BeginLatex
  //
  // The following code snippet creates a NormalVariateGenerator
  // object. Since the random variable generator returns values according to
  // the standard normal distribution (the mean is zero, and the standard
  // deviation is one) before pushing random values into the \code{sample},
  // we change the mean and standard deviation. We want two normal (Gaussian)
  // distribution data. We have two for loops. Each for loop uses different
  // mean and standard deviation. Before we fill the \code{sample} with the
  // second distribution data, we call \code{Initialize(random seed)} method,
  // to recreate the pool of random variables in the
  // \code{normalGenerator}. In the second for loop, we fill the two class
  // samples with measurement vectors using the \code{AddInstance()} method.
  //
  // To see the probability density plots from the two distributions,
  // refer to Figure~\ref{fig:TwoNormalDensityFunctionPlot}.
  //
  // Software Guide : EndLatex
 
  // Software Guide : BeginCodeSnippet
  using NormalGeneratorType = itk::Statistics::NormalVariateGenerator;
  auto normalGenerator = NormalGeneratorType::New();
 
  normalGenerator->Initialize(101);
 
  MeasurementVectorType          mv;
  double                         mean = 100;
  double                         standardDeviation = 30;
  SampleType::InstanceIdentifier id = 0UL;
  for (unsigned int i = 0; i < 100; ++i)
  {
    mv.Fill((normalGenerator->GetVariate() * standardDeviation) + mean);
    sample->PushBack(mv);
    classSamples[0]->AddInstance(id);
    ++id;
  }
 
  normalGenerator->Initialize(3024);
  mean = 200;
  standardDeviation = 30;
  for (unsigned int i = 0; i < 100; ++i)
  {
    mv.Fill((normalGenerator->GetVariate() * standardDeviation) + mean);
    sample->PushBack(mv);
    classSamples[1]->AddInstance(id);
    ++id;
  }
  // Software Guide : EndCodeSnippet
 
  // Software Guide : BeginLatex
  //
  // In the following code snippet, notice that the template argument for the
  // CovarianceCalculator is \code{ClassSampleType} (i.e., type of Subsample)
  // instead of SampleType (i.e., type of ListSample). This is because the
  //  parameter estimation algorithms are applied to the class sample.
  //
  // Software Guide : EndLatex
 
  // Software Guide : BeginCodeSnippet
  using CovarianceEstimatorType =
    itk::Statistics::CovarianceSampleFilter<ClassSampleType>;
 
  std::vector<CovarianceEstimatorType::Pointer> covarianceEstimators;
 
  for (unsigned int i = 0; i < 2; ++i)
  {
    covarianceEstimators.push_back(CovarianceEstimatorType::New());
    covarianceEstimators[i]->SetInput(classSamples[i]);
    covarianceEstimators[i]->Update();
  }
  // Software Guide : EndCodeSnippet
 
  // Software Guide : BeginLatex
  //
  // We print out the estimated parameters.
  //
  // Software Guide : EndLatex
 
  // Software Guide : BeginCodeSnippet
  for (unsigned int i = 0; i < 2; ++i)
  {
    std::cout << "class[" << i << "] " << std::endl;
    std::cout << "    estimated mean : " << covarianceEstimators[i]->GetMean()
              << "    covariance matrix : "
              << covarianceEstimators[i]->GetCovarianceMatrix() << std::endl;
  }
  // Software Guide : EndCodeSnippet
 
  // Software Guide : BeginLatex
  //
  // After creating a SampleClassifier object and a
  // MaximumRatioDecisionRule object, we plug in the
  // \code{decisionRule} and the \code{sample} to the classifier. Then,
  // we specify the number of classes that will be considered using
  // the \code{SetNumberOfClasses()} method.
  //
  // The MaximumRatioDecisionRule requires a vector of \emph{a
  // priori} probability values. Such \emph{a priori} probability will
  // be the $P(\omega_{i})$ of the following variation of the Bayes
  // decision rule:
  // \begin{equation}
  //   \textrm{Decide } \omega_{i} \textrm{ if }
  //   \frac{p(\overrightarrow{x}|\omega_{i})}
  //        {p(\overrightarrow{x}|\omega_{j})}
  // > \frac{P(\omega_{j})}{P(\omega_{i})} \textrm{ for all } j \not= i
  //   \label{eq:bayes2}
  // \end{equation}
  //
  // The remainder of the code snippet shows how to use user-specified class
  // labels. The classification result will be stored in a
  // MembershipSample object, and for each measurement vector, its
  // class label will be one of the two class labels, 100 and 200
  // (\code{unsigned int}).
  //
  // Software Guide : EndLatex
 
  // Software Guide : BeginCodeSnippet
  using MembershipFunctionType =
    itk::Statistics::GaussianMembershipFunction<MeasurementVectorType>;
  using DecisionRuleType = itk::Statistics::MaximumRatioDecisionRule;
  auto decisionRule = DecisionRuleType::New();
 
  DecisionRuleType::PriorProbabilityVectorType aPrioris;
  aPrioris.push_back(
    static_cast<double>(classSamples[0]->GetTotalFrequency()) /
    static_cast<double>(sample->GetTotalFrequency()));
  aPrioris.push_back(
    static_cast<double>(classSamples[1]->GetTotalFrequency()) /
    static_cast<double>(sample->GetTotalFrequency()));
  decisionRule->SetPriorProbabilities(aPrioris);
 
  using ClassifierType = itk::Statistics::SampleClassifierFilter<SampleType>;
  auto classifier = ClassifierType::New();
 
  classifier->SetDecisionRule(decisionRule);
  classifier->SetInput(sample);
  classifier->SetNumberOfClasses(2);
 
  using ClassLabelVectorObjectType =
    ClassifierType::ClassLabelVectorObjectType;
  using ClassLabelVectorType = ClassifierType::ClassLabelVectorType;
 
  auto classLabelVectorObject = ClassLabelVectorObjectType::New();
  ClassLabelVectorType classLabelVector = classLabelVectorObject->Get();
 
  constexpr ClassifierType::ClassLabelType class1 = 100;
  classLabelVector.push_back(class1);
  constexpr ClassifierType::ClassLabelType class2 = 200;
  classLabelVector.push_back(class2);
 
  classLabelVectorObject->Set(classLabelVector);
  classifier->SetClassLabels(classLabelVectorObject);
  // Software Guide : EndCodeSnippet
 
  // Software Guide : BeginLatex
  //
  // The \code{classifier} is almost ready to perform the classification
  // except that it needs two membership functions that represent the two
  // clusters.
  //
  // In this example, we can imagine that the two clusters are modeled by two
  // Gaussian distribution functions. The distribution functions have two
  // parameters, the mean, set by the \code{SetMean()} method, and the
  // covariance, set by the \code{SetCovariance()} method. To plug-in two
  // distribution functions, we create a new instance of
  // \code{MembershipFunctionVectorObjectType} and populate its internal
  // vector with new instances of \code{MembershipFunction} (i.e.
  // GaussianMembershipFunction). This is done by calling the \code{Get()}
  // method of \code{membershipFunctionVectorObject} to get the internal
  // vector, populating this vector with two new membership functions and then
  // calling
  // \code{membershipFunctionVectorObject->Set( membershipFunctionVector )}.
  // Finally, the invocation of the \code{Update()} method will perform the
  // classification.
  //
  // Software Guide : EndLatex
 
  // Software Guide : BeginCodeSnippet
  using MembershipFunctionVectorObjectType =
    ClassifierType::MembershipFunctionVectorObjectType;
  using MembershipFunctionVectorType =
    ClassifierType::MembershipFunctionVectorType;
 
  auto membershipFunctionVectorObject =
    MembershipFunctionVectorObjectType::New();
  MembershipFunctionVectorType membershipFunctionVector =
    membershipFunctionVectorObject->Get();
 
  for (unsigned int i = 0; i < 2; ++i)
  {
    auto membershipFunction = MembershipFunctionType::New();
    membershipFunction->SetMean(covarianceEstimators[i]->GetMean());
    membershipFunction->SetCovariance(
      covarianceEstimators[i]->GetCovarianceMatrix());
    membershipFunctionVector.push_back(membershipFunction);
  }
  membershipFunctionVectorObject->Set(membershipFunctionVector);
  classifier->SetMembershipFunctions(membershipFunctionVectorObject);
 
  classifier->Update();
  // Software Guide : EndCodeSnippet
 
  // Software Guide : BeginLatex
  //
  // The following code snippet prints out pairs of a measurement vector and
  // its class label in the \code{sample}.
  //
  // Software Guide : EndLatex
 
  // Software Guide : BeginCodeSnippet
  const ClassifierType::MembershipSampleType * membershipSample =
    classifier->GetOutput();
  ClassifierType::MembershipSampleType::ConstIterator iter =
    membershipSample->Begin();
 
  while (iter != membershipSample->End())
  {
    std::cout << "measurement vector = " << iter.GetMeasurementVector()
              << " class label = " << iter.GetClassLabel() << std::endl;
    ++iter;
  }
  // Software Guide : EndCodeSnippet
 
  return EXIT_SUCCESS;
}