Old style templated functors should be reimplemented with lambda functions

[SimonRit]

These functors are mainly used in Joseph's forward projector.

[MichaelColonel]

Greetings,

This is my understanding how lambdas could be used. IMHO.

An example for JosephForwardProjectionImageFilter.

namespace rtk
{

template <class TInput, class TCoordinateType, class TOutput = TInput>
std::function< TOutput(const ThreadIdType, double, const TCoordinateType, const TInput *, int) >
InterpolationWeightMultiplication = [](const ThreadIdType    itkNotUsed(threadId),
         double          itkNotUsed(stepLengthInVoxel),
         const TCoordinateType weight,
         const TInput *  p,
         int             i) -> TOutput { return weight * p[i]; };

template <class TInput, class TOutput, class VectorType = itk::Vector<double, 3> >
std::function< void(const ThreadIdType, TOutput &, const TInput, const VectorType&) >
SumAlongRay = [](const ThreadIdType itkNotUsed(threadId),
         TOutput &          sumValue,
         const TInput       volumeValue,
         const VectorType & itkNotUsed(stepInMM)) { sumValue += static_cast<TOutput>(volumeValue); };

template <class TInput, class TOutput, class VectorType = itk::Vector<double, 3> >
std::function< void(const ThreadIdType, const TInput &, TOutput &, const TOutput &,
  const VectorType&, const VectorType&, const VectorType&,
  const VectorType&, const VectorType&) >
ProjectedValueAccumulation = [](const ThreadIdType itkNotUsed(threadId),
         const TInput &     input,
         TOutput &          output,
         const TOutput &    rayCastValue,
         const VectorType& stepInMM,
         const VectorType& itkNotUsed(source),
         const VectorType& itkNotUsed(sourceToPixel),
         const VectorType& itkNotUsed(nearestPoint),
         const VectorType& itkNotUsed(farthestPoint)
         ) { output = input + rayCastValue * stepInMM.GetNorm(); };

template <class TInputImage, class TOutputImage>
class ITK_TEMPLATE_EXPORT JosephForwardProjectionImageFilter
  : public ForwardProjectionImageFilter<TInputImage, TOutputImage>
{
public:
  ITK_DISALLOW_COPY_AND_MOVE(JosephForwardProjectionImageFilter);

  /** Standard class type alias. */
  using Self = JosephForwardProjectionImageFilter;
  using Superclass = ForwardProjectionImageFilter<TInputImage, TOutputImage>;
  using Pointer = itk::SmartPointer<Self>;
  using ConstPointer = itk::SmartPointer<const Self>;
  using InputPixelType = typename TInputImage::PixelType;
  using OutputPixelType = typename TOutputImage::PixelType;
  using OutputImageRegionType = typename TOutputImage::RegionType;
  using CoordinateType = double;
  using WeightCoordinateType = typename itk::PixelTraits<InputPixelType>::ValueType;
  using VectorType = itk::Vector<CoordinateType, TInputImage::ImageDimension>;
  using TClipImageType = itk::Image<double, TOutputImage::ImageDimension>;
  using TClipImagePointer = typename TClipImageType::Pointer;
  // std::functions (lambdas within source files)
  using InterpolationWeightMultiplicationFunc = std::function< InputPixelType(const ThreadIdType, double, const WeightCoordinateType, const InputPixelType *, int) >;
  using SumAlongRayFunc = std::function< void(const ThreadIdType, OutputPixelType &, const InputPixelType, const VectorType&) >;
  using ProjectedValueAccumulationFunc = std::function< void(const ThreadIdType, const InputPixelType &, OutputPixelType &, const OutputPixelType &, const VectorType&, const VectorType&, const VectorType&, const VectorType&, const VectorType&) >;

private:
  // Functors (built-in lambdas by default)
  InterpolationWeightMultiplicationFunc m_InterpolationWeightMultiplication = InterpolationWeightMultiplication< InputPixelType, WeightCoordinateType >;
  SumAlongRayFunc m_SumAlongRay = SumAlongRay< InputPixelType, OutputPixelType >;
  ProjectedValueAccumulationFunc m_ProjectedValueAccumulation = ProjectedValueAccumulation< InputPixelType, OutputPixelType >;

  double                             m_InferiorClip{ 0. };
  double                             m_SuperiorClip{ 1. };
};

} // end namespace rtk

[SimonRit]

Looks good but the idea would be to add Get / Set functions to modify them in daughter classes. We haven't started working on that but we will in the near future.

[MichaelColonel]

Looks good but the idea would be to add Get / Set functions to modify them in daughter classes. We haven't started working on that but we will in the near future.

Adding Get / Set methods to the filter class is a straightforward process. Methods will be similar to the already existed ones, but these will set / get std::functions instead of functors objects.

I've made some modifications to the rtkJosephForwardProjectionFilter and i can make a PR with working test as an example.