text: /*=========================================================================
   *
   *  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 itkVersorTransformOptimizer_h
  #define itkVersorTransformOptimizer_h
  
  #include "itkRegularStepGradientDescentOptimizer.h"
  #include "itkVersor.h"
  #include "ITKOptimizersExport.h"
  
  namespace itk
  {
  /** \class VersorTransformOptimizer
   * \brief Implement a gradient descent optimizer
   *
   * VersorTransformOptimizer is a variant of the
   * gradient descent optimizer implemented in
   * RegularStepGradientDescentOptimizer.
   *
   * Versors are not in a vector space, for that reason,
   * the classical gradient descent algorithm has to be
   * modified in order to be applicable to Versors (unit
   * quaternions) that form the group SO(3).
   *
   * The Versor space has only three degrees of freedom,
   * even though Versors are represented using four values.
   *
   * This optimizer assumes that the CostFunction to be
   * optimized has an itk::Versor as parameter.
   *
   * \sa RegularStepGradientDescentOptimizer
   * \sa Versor
   * \sa VersorTransform
   *
   * \ingroup Numerics Optimizers
   * \ingroup ITKOptimizers
   */
  class ITKOptimizers_EXPORT VersorTransformOptimizer : public RegularStepGradientDescentBaseOptimizer
  {
  public:
    ITK_DISALLOW_COPY_AND_MOVE(VersorTransformOptimizer);
  
    /** Standard class type aliases. */
    using Self = VersorTransformOptimizer;
    using Superclass = RegularStepGradientDescentBaseOptimizer;
    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(VersorTransformOptimizer, RegularStepGradientDescentBaseOptimizer);
  
    /**  Versor Type  */
    using VersorType = Versor<double>;
    using VectorType = VersorType::VectorType;
  
    /** Advance one step following the gradient direction. */
    void
    StepAlongGradient(double factor, const DerivativeType & transformedGradient) override;
  
  protected:
    VersorTransformOptimizer() = default;
    ~VersorTransformOptimizer() override = default;
  };
  } // end namespace itk
  
  #endif
  