Examples/Filtering/DiscreteGaussianImageFilter.cxx

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//  Software Guide : BeginCommandLineArgs
//    INPUTS:  {BrainProtonDensitySlice.png}
//    OUTPUTS: {DiscreteGaussianImageFilterOutput.png}
//    ARGUMENTS:    4 9
//  Software Guide : EndCommandLineArgs
//
//  Software Guide : BeginLatex
//
//  \begin{floatingfigure}[rlp]{6cm}
//    \centering
//    \includegraphics[width=6cm]{DiscreteGaussian}
//    \caption[DiscreteGaussianImageFilter Gaussian diagram.]
//            {Discretized Gaussian.\label{fig:DiscretizedGaussian}}
//  \end{floatingfigure}
//
//  The \doxygen{DiscreteGaussianImageFilter} computes the convolution of the
//  input image with a Gaussian kernel.  This is done in $ND$ by taking
//  advantage of the separability of the Gaussian kernel.  A one-dimensional
//  Gaussian function is discretized on a convolution kernel.  The size of the
//  kernel is extended until there are enough discrete points in the Gaussian
//  to ensure that a user-provided maximum error is not exceeded.  Since the
//  size of the kernel is unknown a priori, it is necessary to impose a limit to
//  its growth. The user can thus provide a value to be the maximum admissible
//  size of the kernel. Discretization error is defined as the difference
//  between the area under the discrete Gaussian curve (which has finite
//  support) and the area under the continuous Gaussian.
//
//  Gaussian kernels in ITK are constructed according to the theory of Tony
//  Lindeberg \cite{Lindeberg1994} so that smoothing and derivative operations
//  commute before and after discretization.  In other words, finite difference
//  derivatives on an image $I$ that has been smoothed by convolution with the
//  Gaussian are equivalent to finite differences computed on $I$ by convolving
//  with a derivative of the Gaussian.
//
//  \index{itk::DiscreteGaussianImageFilter}
//
//  Software Guide : EndLatex


#include "itkImageFileReader.h"
#include "itkImageFileWriter.h"
#include "itkRescaleIntensityImageFilter.h"

//  Software Guide : BeginLatex
//
//  The first step required to use this filter is to include its header file.
//  As with other examples, the includes here are truncated to those specific
//  for this example.\newline
//
//  \index{itk::DiscreteGaussianImageFilter!header}
//
//  Software Guide : EndLatex

// Software Guide : BeginCodeSnippet
#include "itkDiscreteGaussianImageFilter.h"
// Software Guide : EndCodeSnippet


int main( int argc, char * argv[] )
{
  if( argc < 5 )
    {
    std::cerr << "Usage: " << std::endl;
    std::cerr << argv[0] << "  inputImageFile  outputImageFile  variance  maxKernelWidth " << std::endl;
    return EXIT_FAILURE;
    }


  //  Software Guide : BeginLatex
  //
  //  Types should be chosen for the pixels of the input and output images.
  //  Image types can be instantiated using the pixel type and dimension.
  //
  //  Software Guide : EndLatex

  // Software Guide : BeginCodeSnippet
  using InputPixelType = float;
  using OutputPixelType = float;

  using InputImageType = itk::Image< InputPixelType,  2 >;
  using OutputImageType = itk::Image< OutputPixelType, 2 >;
  // Software Guide : EndCodeSnippet


  using ReaderType = itk::ImageFileReader< InputImageType >;


  //  Software Guide : BeginLatex
  //
  //  The discrete Gaussian filter type is instantiated using the
  //  input and output image types.  A corresponding filter object is created.
  //
  //  \index{itk::DiscreteGaussianImageFilter!instantiation}
  //  \index{itk::DiscreteGaussianImageFilter!New()}
  //  \index{itk::DiscreteGaussianImageFilter!Pointer}
  //
  //  Software Guide : EndLatex

  // Software Guide : BeginCodeSnippet
  using FilterType = itk::DiscreteGaussianImageFilter<
                 InputImageType, OutputImageType >;

  FilterType::Pointer filter = FilterType::New();
  // Software Guide : EndCodeSnippet


  ReaderType::Pointer reader = ReaderType::New();
  reader->SetFileName( argv[1] );


  //  Software Guide : BeginLatex
  //
  //  The input image can be obtained from the output of another
  //  filter. Here, an image reader is used as its input.
  //
  //  Software Guide : EndLatex

  // Software Guide : BeginCodeSnippet
  filter->SetInput( reader->GetOutput() );
  // Software Guide : EndCodeSnippet


  const double gaussianVariance = std::stod( argv[3] );
  const unsigned int maxKernelWidth = std::stoi( argv[4] );


  //  Software Guide : BeginLatex
  //
  //  The filter requires the user to provide a value for the variance
  //  associated with the Gaussian kernel. The method \code{SetVariance()} is
  //  used for this purpose. The discrete Gaussian is constructed as a
  //  convolution kernel.  The maximum kernel size can be set by the user. Note
  //  that the combination of variance and kernel-size values may result in a
  //  truncated Gaussian kernel.
  //
  //  \index{itk::DiscreteGaussianImageFilter!SetVariance()}
  //  \index{itk::DiscreteGaussianImageFilter!SetMaximumKernelWidth()}
  //
  //  Software Guide : EndLatex


  // Software Guide : BeginCodeSnippet
  filter->SetVariance( gaussianVariance );
  filter->SetMaximumKernelWidth( maxKernelWidth );
  // Software Guide : EndCodeSnippet


  //  Software Guide : BeginLatex
  //
  //  Finally, the filter is executed by invoking the \code{Update()} method.
  //
  //  \index{itk::DiscreteGaussianImageFilter!Update()}
  //
  //  Software Guide : EndLatex


  // Software Guide : BeginCodeSnippet
  filter->Update();
  // Software Guide : EndCodeSnippet


  //  Software Guide : BeginLatex
  //
  //  If the output of this filter has been connected to other filters down
  //  the pipeline, updating any of the downstream filters will
  //  trigger the execution of this one. For example, a writer could
  //  be used after the filter.
  //
  //  Software Guide : EndLatex

  using WritePixelType = unsigned char;
  using WriteImageType = itk::Image< WritePixelType, 2 >;
  using RescaleFilterType = itk::RescaleIntensityImageFilter<
               OutputImageType, WriteImageType >;
  RescaleFilterType::Pointer rescaler = RescaleFilterType::New();

  rescaler->SetOutputMinimum(   0 );
  rescaler->SetOutputMaximum( 255 );

  using WriterType = itk::ImageFileWriter< WriteImageType >;
  WriterType::Pointer writer = WriterType::New();
  writer->SetFileName( argv[2] );

  // Software Guide : BeginCodeSnippet
  rescaler->SetInput( filter->GetOutput() );
  writer->SetInput( rescaler->GetOutput() );
  writer->Update();
  // Software Guide : EndCodeSnippet


  //  Software Guide : BeginLatex
  //
  // \begin{figure}
  // \center
  // \includegraphics[width=0.44\textwidth]{BrainProtonDensitySlice}
  // \includegraphics[width=0.44\textwidth]{DiscreteGaussianImageFilterOutput}
  // \itkcaption[DiscreteGaussianImageFilter output]{Effect of the
  // DiscreteGaussianImageFilter on a slice from a MRI proton density image of
  // the brain.}
  // \label{fig:DiscreteGaussianImageFilterInputOutput}
  // \end{figure}
  //
  //  Figure~\ref{fig:DiscreteGaussianImageFilterInputOutput} illustrates the
  //  effect of this filter on a MRI proton density image of the brain.
  //
  //  Note that large Gaussian variances will produce large convolution kernels
  //  and correspondingly longer computation times.  Unless a high degree of
  //  accuracy is required, it may be more desirable to use the approximating
  //  \doxygen{RecursiveGaussianImageFilter} with large variances.
  //
  //  Software Guide : EndLatex

  return EXIT_SUCCESS;
}