itkBSplineKernelFunction.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.
 *  You may obtain a copy of the License at
 *
 *         https://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 itkBSplineKernelFunction_h
#define itkBSplineKernelFunction_h
 
#include "itkKernelFunctionBase.h"
#include "itkMath.h"
 
namespace itk
{
template <unsigned int VSplineOrder = 3, typename TRealValueType = double>
class ITK_TEMPLATE_EXPORT BSplineKernelFunction : public KernelFunctionBase<TRealValueType>
{
public:
  ITK_DISALLOW_COPY_AND_MOVE(BSplineKernelFunction);

  using Self = BSplineKernelFunction;
  using Superclass = KernelFunctionBase<TRealValueType>;
  using Pointer = SmartPointer<Self>;
 
  using typename Superclass::RealType;
  itkNewMacro(Self);

  itkOverrideGetNameOfClassMacro(BSplineKernelFunction);

  static constexpr unsigned int SplineOrder = VSplineOrder;

  static TRealValueType
  FastEvaluate(const TRealValueType u)
  {
    return Self::Evaluate(Dispatch<VSplineOrder>(), u);
  }

  TRealValueType
  Evaluate(const TRealValueType & u) const override
  {
    return Self::FastEvaluate(u);
  }
 
protected:
  BSplineKernelFunction() = default;
  ~BSplineKernelFunction() override = default;
  void
  PrintSelf(std::ostream & os, Indent indent) const override
  {
    Superclass::PrintSelf(os, indent);
 
    os << indent << "SplineOrder: " << SplineOrder << std::endl;
  }
 
private:
  struct DispatchBase
  {};
  template <unsigned int>
  struct Dispatch : public DispatchBase
  {};

  static inline TRealValueType
  Evaluate(const Dispatch<0> &, const TRealValueType & u)
  {
    const TRealValueType absValue = itk::Math::abs(u);
    if (absValue < TRealValueType{ 0.5 })
    {
      return TRealValueType{ 1.0 };
    }
    if (Math::ExactlyEquals(absValue, TRealValueType{ 0.5 }))
    {
      return TRealValueType{ 0.5 };
    }
    else
    {
      return TRealValueType{ 0.0 };
    }
  }

  static inline TRealValueType
  Evaluate(const Dispatch<1> &, const TRealValueType & u)
  {
    const TRealValueType absValue = itk::Math::abs(u);
    if (absValue < TRealValueType{ 1.0 })
    {
      return TRealValueType{ 1.0 } - absValue;
    }
 
    return TRealValueType{ 0.0 };
  }

  static inline TRealValueType
  Evaluate(const Dispatch<2> &, const TRealValueType & u)
  {
    const TRealValueType absValue = itk::Math::abs(u);
    if (absValue < TRealValueType{ 0.5 })
    {
      const TRealValueType sqrValue = itk::Math::sqr(absValue);
      return TRealValueType{ 0.75 } - sqrValue;
    }
    if (absValue < TRealValueType{ 1.5 })
    {
      const TRealValueType sqrValue = itk::Math::sqr(absValue);
      // NOTE: 1.0/8.0 == 0.125
      return (TRealValueType{ 9.0 } - TRealValueType{ 12.0 } * absValue + TRealValueType{ 4.0 } * sqrValue) *
             TRealValueType{ 0.125 };
    }
    else
    {
      return TRealValueType{ 0.0 };
    }
  }

  static inline TRealValueType
  Evaluate(const Dispatch<3> &, const TRealValueType & u)
  {
    const TRealValueType absValue = itk::Math::abs(u);
    if (absValue < TRealValueType{ 1.0 })
    {
      const TRealValueType sqrValue = itk::Math::sqr(absValue);
      return (TRealValueType{ 4.0 } - TRealValueType{ 6.0 } * sqrValue + TRealValueType{ 3.0 } * sqrValue * absValue) /
             TRealValueType{ 6.0 };
    }
    if (absValue < TRealValueType{ 2.0 })
    {
      const TRealValueType sqrValue = itk::Math::sqr(absValue);
      return (TRealValueType{ 8.0 } - TRealValueType{ 12.0 } * absValue + TRealValueType{ 6.0 } * sqrValue -
              sqrValue * absValue) /
             TRealValueType{ 6.0 };
    }
    else
    {
      return TRealValueType{ 0.0 };
    }
  }

  static inline TRealValueType
  Evaluate(const DispatchBase &, const TRealValueType &)
  {
    itkGenericExceptionMacro("Evaluate not implemented for spline order " << SplineOrder);
  }
};
 
} // end namespace itk
 
#endif