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 *
 *  Copyright Insight Software Consortium
 *
 *  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
 *
 *         http://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.
 *
 *=========================================================================*/
#ifndef itkGradientDifferenceImageToImageMetric_h
#define itkGradientDifferenceImageToImageMetric_h

#include "itkImageToImageMetric.h"

#include "itkSobelOperator.h"
#include "itkNeighborhoodOperatorImageFilter.h"
#include "itkPoint.h"
#include "itkCastImageFilter.h"
#include "itkResampleImageFilter.h"

namespace itk
{
/** \class GradientDifferenceImageToImageMetric
 * \brief Computes similarity between two objects to be registered
 *
 * This Class is templated over the type of the Images to be compared and
 * over the type of transformation and Iterpolator to be used.
 *
 * This metric computes the sum of squared differences between pixels in
 * the derivatives of the moving and fixed images after passing the squared
 * difference through a function of type \f$ \frac{1}{1+x} \f$.
 *
 * \warning THIS IMAGE METRIC IS CURRENTLY UNDER DEBUGGING. USE AT YOUR OWN RISK.
 *
 * Spatial correspondance between both images is established through a
 * Transform. Pixel values are taken from the Moving image. Their positions
 * are mapped to the Fixed image and result in general in non-grid position
 * on it. Values at these non-grid position of the Fixed image are
 * interpolated using a user-selected Interpolator.
 *
 * Implementation of this class is based on:
 * Hipwell, J. H., et. al. (2003), "Intensity-Based 2-D-3D Registration of
 * Cerebral Angiograms,", IEEE Transactions on Medical Imaging,
 * 22(11):1417-1426.
 *
 * \ingroup RegistrationMetrics
 * \ingroup ITKRegistrationCommon
 */
template< typename TFixedImage, typename TMovingImage >
class ITK_TEMPLATE_EXPORT GradientDifferenceImageToImageMetric:
  public ImageToImageMetric< TFixedImage, TMovingImage >
{
public:
  ITK_DISALLOW_COPY_AND_ASSIGN(GradientDifferenceImageToImageMetric);

  /** Standard class type aliases. */
  using Self = GradientDifferenceImageToImageMetric;
  using Superclass = ImageToImageMetric< TFixedImage, TMovingImage >;

  using Pointer = SmartPointer< Self >;
  using ConstPointer = SmartPointer< const Self >;

  /** Method for creation through the object factory. */
  itkNewMacro(Self);

  /** Run-time type information (and related methods). */
  itkTypeMacro(GradientDifferenceImageToImageMetric, ImageToImageMetric);

  /** Types transferred from the base class */
  using RealType = typename Superclass::RealType;
  using TransformType = typename Superclass::TransformType;
  using TransformPointer = typename Superclass::TransformPointer;
  using TransformParametersType = typename Superclass::TransformParametersType;
  using TransformJacobianType = typename Superclass::TransformJacobianType;

  using MeasureType = typename Superclass::MeasureType;
  using DerivativeType = typename Superclass::DerivativeType;
  using FixedImageType = typename Superclass::FixedImageType;
  using MovingImageType = typename Superclass::MovingImageType;
  using FixedImageConstPointer = typename Superclass::FixedImageConstPointer;
  using MovingImageConstPointer = typename Superclass::MovingImageConstPointer;

  using FixedImagePixelType = typename TFixedImage::PixelType;
  using MovedImagePixelType = typename TMovingImage::PixelType;

  static constexpr unsigned int FixedImageDimension = TFixedImage::ImageDimension;
  /** Types for transforming the moving image */
  using TransformedMovingImageType = itk::Image< FixedImagePixelType,
                      Self::FixedImageDimension >;

  using TransformMovingImageFilterType =
      itk::ResampleImageFilter< MovingImageType, TransformedMovingImageType >;

  /** Sobel filters to compute the gradients of the Fixed Image */

  using FixedGradientImageType = itk::Image< RealType, Self::FixedImageDimension >;

  using CastFixedImageFilterType = itk::CastImageFilter< FixedImageType, FixedGradientImageType >;
  using CastFixedImageFilterPointer = typename CastFixedImageFilterType::Pointer;

  using FixedGradientPixelType = typename FixedGradientImageType::PixelType;

  /** Sobel filters to compute the gradients of the Moved Image */

  static constexpr unsigned int MovedImageDimension = MovingImageType::ImageDimension;

  using MovedGradientImageType = itk::Image< RealType, Self::MovedImageDimension >;

  using CastMovedImageFilterType = itk::CastImageFilter< TransformedMovingImageType, MovedGradientImageType >;
  using CastMovedImageFilterPointer = typename CastMovedImageFilterType::Pointer;

  using MovedGradientPixelType = typename MovedGradientImageType::PixelType;

  /** Get the derivatives of the match measure. */
  void GetDerivative(const TransformParametersType & parameters,
                     DerivativeType  & derivative) const override;

  /**  Get the value for single valued optimizers. */
  MeasureType GetValue(const TransformParametersType & parameters) const override;

  /**  Get value and derivatives for multiple valued optimizers. */
  void GetValueAndDerivative(const TransformParametersType & parameters,
                             MeasureType & Value, DerivativeType & derivative) const override;

  /** Initialize the Metric by making sure that all the components
   *  are present and plugged together correctly     */
  void Initialize() override;

  /** Write gradient images to a files for debugging purposes. */
  void WriteGradientImagesToFiles() const;

  /** Set/Get the value of Delta used for computing derivatives by finite
   * differences in the GetDerivative() method */
  itkSetMacro(DerivativeDelta, double);
  itkGetConstReferenceMacro(DerivativeDelta, double);

protected:
  GradientDifferenceImageToImageMetric();
  ~GradientDifferenceImageToImageMetric() override = default;
  void PrintSelf(std::ostream & os, Indent indent) const override;

  /** Compute the range of the moved image gradients. */
  void ComputeMovedGradientRange() const;

  /** Compute the variance and range of the moving image gradients. */
  void ComputeVariance() const;

  /** Compute the similarity measure using a specified subtraction factor. */
  MeasureType ComputeMeasure(const TransformParametersType & parameters,
                             const double *subtractionFactor) const;

  using FixedSobelFilter = NeighborhoodOperatorImageFilter<
    FixedGradientImageType, FixedGradientImageType >;

  using MovedSobelFilter = NeighborhoodOperatorImageFilter<
    MovedGradientImageType, MovedGradientImageType >;

private:
  /** The variance of the moving image gradients. */
  mutable MovedGradientPixelType m_Variance[FixedImageDimension];

  /** The range of the moving image gradients. */
  mutable MovedGradientPixelType m_MinMovedGradient[MovedImageDimension];
  mutable MovedGradientPixelType m_MaxMovedGradient[MovedImageDimension];
  /** The range of the fixed image gradients. */
  mutable FixedGradientPixelType m_MinFixedGradient[FixedImageDimension];
  mutable FixedGradientPixelType m_MaxFixedGradient[FixedImageDimension];

  /** The filter for transforming the moving image. */
  typename TransformMovingImageFilterType::Pointer m_TransformMovingImageFilter;

  /** The Sobel gradients of the fixed image */
  CastFixedImageFilterPointer m_CastFixedImageFilter;

  SobelOperator< FixedGradientPixelType,
                 Self::FixedImageDimension >
  m_FixedSobelOperators[FixedImageDimension];

  typename FixedSobelFilter::Pointer m_FixedSobelFilters[Self::FixedImageDimension];

  ZeroFluxNeumannBoundaryCondition< MovedGradientImageType > m_MovedBoundCond;
  ZeroFluxNeumannBoundaryCondition< FixedGradientImageType > m_FixedBoundCond;

  /** The Sobel gradients of the moving image */
  CastMovedImageFilterPointer m_CastMovedImageFilter;

  SobelOperator< MovedGradientPixelType,
                 Self::MovedImageDimension >
  m_MovedSobelOperators[MovedImageDimension];

  typename MovedSobelFilter::Pointer m_MovedSobelFilters[Self::MovedImageDimension];

  double m_DerivativeDelta;
};
} // end namespace itk

#ifndef ITK_MANUAL_INSTANTIATION
#include "itkGradientDifferenceImageToImageMetric.hxx"
#endif

#endif
