Examples/Iterators/ImageLinearIteratorWithIndex2.cxx

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// Software Guide : BeginLatex
//
// This example shows how to use the \doxygen{ImageLinearIteratorWithIndex}
// for computing the mean across time of a 4D image where the first three
// dimensions correspond to spatial coordinates and the fourth dimension
// corresponds to time. The result of the mean across time is to be stored in
// a 3D image.
//
// \index{Iterators!and 4D images}
// \index{ImageLinearIteratorWithIndex!4D images}
//
// Software Guide : EndLatex
 
#include "itkImage.h"
// Software Guide : BeginCodeSnippet
#include "itkImageLinearConstIteratorWithIndex.h"
// Software Guide : EndCodeSnippet
#include "itkImageFileReader.h"
#include "itkImageFileWriter.h"
 
int
main(int argc, char * argv[])
{
  // Verify the number of parameters on the command line.
  if (argc < 3)
  {
    std::cerr << "Missing parameters. " << std::endl;
    std::cerr << "Usage: " << std::endl;
    std::cerr << argv[0] << " input4DImageFile output3DImageFile"
              << std::endl;
    return EXIT_FAILURE;
  }
 
  // Software Guide : BeginLatex
  //
  // First we declare the types of the images, the 3D and 4D readers.
  //
  // Software Guide : EndLatex
 
 
  // Software Guide : BeginCodeSnippet
  using PixelType = unsigned char;
  using Image3DType = itk::Image<PixelType, 3>;
  using Image4DType = itk::Image<PixelType, 4>;
 
  using Reader4DType = itk::ImageFileReader<Image4DType>;
  using Writer3DType = itk::ImageFileWriter<Image3DType>;
  // Software Guide : EndCodeSnippet
 
  auto reader4D = Reader4DType::New();
  reader4D->SetFileName(argv[1]);
 
  try
  {
    reader4D->Update();
  }
  catch (const itk::ExceptionObject & excp)
  {
    std::cerr << "Error reading the image" << std::endl;
    std::cerr << excp << std::endl;
    return EXIT_FAILURE;
  }
 
  const Image4DType::ConstPointer image4D = reader4D->GetOutput();
 
  // Software Guide : BeginLatex
  //
  // Next, define the necessary types for indices, points, spacings, and size.
  //
  // Software Guide : EndLatex
 
  // Software Guide : BeginCodeSnippet
  auto image3D = Image3DType::New();
  using Index3DType = Image3DType::IndexType;
  using Size3DType = Image3DType::SizeType;
  using Region3DType = Image3DType::RegionType;
  using Spacing3DType = Image3DType::SpacingType;
  using Origin3DType = Image3DType::PointType;
 
  using Index4DType = Image4DType::IndexType;
  using Size4DType = Image4DType::SizeType;
  using Spacing4DType = Image4DType::SpacingType;
  using Origin4DType = Image4DType::PointType;
  // Software Guide : EndCodeSnippet
 
  Index3DType   index3D;
  Size3DType    size3D;
  Spacing3DType spacing3D;
  Origin3DType  origin3D;
 
  const Image4DType::RegionType region4D = image4D->GetBufferedRegion();
 
  Index4DType   index4D = region4D.GetIndex();
  Size4DType    size4D = region4D.GetSize();
  Spacing4DType spacing4D = image4D->GetSpacing();
  Origin4DType  origin4D = image4D->GetOrigin();
 
  // Software Guide : BeginLatex
  //
  // Here we make sure that the values for our resultant 3D mean image
  // match up with the input 4D image.
  //
  // Software Guide : EndLatex
 
  // Software Guide : BeginCodeSnippet
  for (unsigned int i = 0; i < 3; ++i)
  {
    size3D[i] = size4D[i];
    index3D[i] = index4D[i];
    spacing3D[i] = spacing4D[i];
    origin3D[i] = origin4D[i];
  }
 
  image3D->SetSpacing(spacing3D);
  image3D->SetOrigin(origin3D);
 
  Region3DType region3D;
  region3D.SetIndex(index3D);
  region3D.SetSize(size3D);
 
  image3D->SetRegions(region3D);
  image3D->Allocate();
  // Software Guide : EndCodeSnippet
 
  using SumType = itk::NumericTraits<PixelType>::AccumulateType;
  using MeanType = itk::NumericTraits<SumType>::RealType;
 
  const unsigned int timeLength = region4D.GetSize()[3];
 
  using IteratorType = itk::ImageLinearConstIteratorWithIndex<Image4DType>;
 
  // Software Guide : BeginLatex
  //
  // Next we iterate over time in the input image series, compute the average,
  // and store that value in the corresponding pixel of the output 3D image.
  //
  // Software Guide : EndLatex
 
  // Software Guide : BeginCodeSnippet
  IteratorType it(image4D, region4D);
  it.SetDirection(3); // Walk along time dimension
  it.GoToBegin();
  while (!it.IsAtEnd())
  {
    SumType sum{};
    it.GoToBeginOfLine();
    index4D = it.GetIndex();
    while (!it.IsAtEndOfLine())
    {
      sum += it.Get();
      ++it;
    }
    const MeanType mean =
      static_cast<MeanType>(sum) / static_cast<MeanType>(timeLength);
 
    index3D[0] = index4D[0];
    index3D[1] = index4D[1];
    index3D[2] = index4D[2];
 
    image3D->SetPixel(index3D, static_cast<PixelType>(mean));
    it.NextLine();
  }
  // Software Guide : EndCodeSnippet
 
 
  // Software Guide : BeginLatex
  //
  // As you can see, we avoid to use a 3D iterator to walk
  // over the mean image. The reason is that there is no
  // guarantee that the 3D iterator will walk in the same
  // order as the 4D. Iterators just adhere to their contract
  // of visiting every pixel, but do not enforce any particular
  // order for the visits.  The linear iterator guarantees it will
  // visit the pixels along a line of the image in the order
  // in which they are placed in the line, but does not state
  // in what order one line will be visited with respect to
  // other lines.  Here we simply take advantage of knowing
  // the first three components of the 4D iterator index,
  // and use them to place the resulting mean value in the
  // output 3D image.
  //
  // Software Guide : EndLatex
 
  auto writer3D = Writer3DType::New();
  writer3D->SetFileName(argv[2]);
  writer3D->SetInput(image3D);
 
  try
  {
    writer3D->Update();
  }
  catch (const itk::ExceptionObject & excp)
  {
    std::cerr << "Error writing the image" << std::endl;
    std::cerr << excp << std::endl;
    return EXIT_FAILURE;
  }
 
  return EXIT_SUCCESS;
}