itkFrequencyHalfHermitianFFTLayoutImageRegionConstIteratorWithIndex.h

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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.
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 *
 *         https://www.apache.org/licenses/LICENSE-2.0.txt
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 *  WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
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#ifndef itkFrequencyHalfHermitianFFTLayoutImageRegionConstIteratorWithIndex_h
#define itkFrequencyHalfHermitianFFTLayoutImageRegionConstIteratorWithIndex_h
 
#include "itkImageRegionConstIteratorWithIndex.h"
 
namespace itk
{
template <typename TImage>
class FrequencyHalfHermitianFFTLayoutImageRegionConstIteratorWithIndex
  : public ImageRegionConstIteratorWithIndex<TImage>
{
public:
  using Self = FrequencyHalfHermitianFFTLayoutImageRegionConstIteratorWithIndex;
  using Superclass = ImageRegionConstIteratorWithIndex<TImage>;

  using typename Superclass::IndexType;
  using typename Superclass::SizeType;
  using typename Superclass::OffsetType;
  using typename Superclass::RegionType;
  using typename Superclass::ImageType;
  using typename Superclass::PixelContainer;
  using typename Superclass::PixelContainerPointer;
  using typename Superclass::InternalPixelType;
  using typename Superclass::PixelType;
  using typename Superclass::AccessorType;
 
  using FrequencyType = typename ImageType::SpacingType;
  using FrequencyValueType = typename ImageType::SpacingValueType;
  FrequencyHalfHermitianFFTLayoutImageRegionConstIteratorWithIndex()
    : ImageRegionConstIteratorWithIndex<TImage>()
 
  {
    this->Init();
  }

  FrequencyHalfHermitianFFTLayoutImageRegionConstIteratorWithIndex(const TImage * ptr, const RegionType & region)
    : ImageRegionConstIteratorWithIndex<TImage>(ptr, region)
  {
    this->Init();
  }

  explicit FrequencyHalfHermitianFFTLayoutImageRegionConstIteratorWithIndex(const Superclass & it)
    : ImageRegionConstIteratorWithIndex<TImage>(it)
  {
    this->Init();
  }
 
  /*
   * Image Index [0, N - 1] returns [0 to N/2] (positive) union [-N/2 + 1, -1] (negative).
   * So index N/2 + 1 returns the bin -N/2 + 1.
   * If first index of the image is not zero, it stills returns values in the same range.
   * f = [0, 1, ...,   N/2-1,     -N/2, ..., -1]  if N is even
   * f = [0, 1, ..., (N-1)/2, -(N-1)/2, ..., -1]  if N is odd
   */
  IndexType
  GetFrequencyBin() const
  {
    IndexType freqInd{};
    for (unsigned int dim = 0; dim < TImage::ImageDimension; ++dim)
    {
      if (this->m_PositionIndex[dim] <= m_LargestPositiveFrequencyIndex[dim])
      {
        freqInd[dim] = this->m_PositionIndex[dim] - this->m_MinIndex[dim];
      }
      else //  -. From -N/2 + 1 (Nyquist if even) to -1 (-df in frequency)
      {
        freqInd[dim] = this->m_PositionIndex[dim] - (this->m_MaxIndex[dim] + 1);
      }
    }
    return freqInd;
  }

  FrequencyType
  GetFrequency() const
  {
    FrequencyType freq;
    IndexType     freqInd = this->GetFrequencyBin();
 
    for (unsigned int dim = 0; dim < TImage::ImageDimension; ++dim)
    {
      freq[dim] = this->m_FrequencyOrigin[dim] + this->m_FrequencySpacing[dim] * freqInd[dim];
    }
    return freq;
  }
 
  FrequencyValueType
  GetFrequencyModuloSquare() const
  {
    FrequencyValueType w2(0);
    FrequencyType      w(this->GetFrequency());
 
    for (unsigned int dim = 0; dim < TImage::ImageDimension; ++dim)
    {
      w2 += w[dim] * w[dim];
    }
    return w2;
  }

  itkGetConstReferenceMacro(LargestPositiveFrequencyIndex, IndexType);

  itkGetConstReferenceMacro(MinIndex, IndexType);

  itkGetConstReferenceMacro(MaxIndex, IndexType);

  itkGetConstReferenceMacro(FrequencyOrigin, FrequencyType);

  itkGetConstReferenceMacro(FrequencySpacing, FrequencyType);

  void
  SetActualXDimensionIsOdd(bool value)
  {
    this->m_ActualXDimensionIsOdd = value;
    SizeType sizeImage = this->m_Image->GetLargestPossibleRegion().GetSize();
    auto     size_estimated = 2 * (sizeImage[0] - 1);
    size_estimated += this->GetActualXDimensionIsOdd() ? 1 : 0;
    this->m_FrequencySpacing[0] = 1.0 / (this->m_Image->GetSpacing()[0] * size_estimated);
  }
  itkGetMacro(ActualXDimensionIsOdd, bool);
  itkBooleanMacro(ActualXDimensionIsOdd);
private:
  void
  Init()
  {
    SizeType sizeImage = this->m_Image->GetLargestPossibleRegion().GetSize();
    this->m_MinIndex = this->m_Image->GetLargestPossibleRegion().GetIndex();
    this->m_MaxIndex = this->m_Image->GetLargestPossibleRegion().GetUpperIndex();
    for (unsigned int dim = 0; dim < ImageType::ImageDimension; ++dim)
    {
      this->m_LargestPositiveFrequencyIndex[dim] =
        static_cast<FrequencyValueType>(this->m_MinIndex[dim] + sizeImage[dim] / 2);
      // Set frequency metadata.
      // Origin of frequencies is zero after a FFT.
      this->m_FrequencyOrigin[dim] = 0.0;
      // SamplingFrequency = 1.0 / SpatialImageSpacing
      // Freq_BinSize = SamplingFrequency / Size
      this->m_FrequencySpacing[dim] = 1.0 / (this->m_Image->GetSpacing()[dim] * sizeImage[dim]);
    }
    // Corrections for Hermitian
    // The fastest index has no negative frequencies.
    this->m_LargestPositiveFrequencyIndex[0] = static_cast<FrequencyValueType>(this->m_MaxIndex[0]);
    // In fastest dimension only:
    // Size_estimated_original = 2 * (Size_current - 1 )
    // Where size_current is the size of the output of RealToHalfHermitianFFT
    // Ex: Size Original = 10, Current = 6, Estimated = 10.
    //     Size Original = 11, Current = 6, Estimated = 10.
    auto size_estimated = 2 * (sizeImage[0] - 1);
    size_estimated += this->GetActualXDimensionIsOdd() ? 1 : 0;
    this->m_FrequencySpacing[0] = 1.0 / (this->m_Image->GetSpacing()[0] * size_estimated);
  }
 
  IndexType     m_LargestPositiveFrequencyIndex;
  IndexType     m_MinIndex;
  IndexType     m_MaxIndex;
  FrequencyType m_FrequencyOrigin;
  FrequencyType m_FrequencySpacing;
  bool          m_ActualXDimensionIsOdd{ false };
};
} // end namespace itk
#endif