/*=========================================================================
 *
 *  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 itkSobelOperator_h
#define itkSobelOperator_h

#include "itkNeighborhoodOperator.h"

namespace itk
{
/**
 * \class SobelOperator
 *
 * \brief A NeighborhoodOperator for performing a directional Sobel
 * edge-detection operation at a pixel location.
 *
 * SobelOperator is a directional NeighborhoodOperator that should be
 * applied a NeighborhoodIterator using the NeighborhoodInnerProduct
 * method. To create the operator:
 *
 * 1) Set the direction by calling  \code SetDirection \endcode
 * 2) call
   \code
   itk::Size<2> radius;
   radius.Fill(1);
   sobelOperator.CreateToRadius(radius);
   \endcode
 * 3) You may optionally scale the coefficients of this operator using the
 * \code ScaleCoefficients \endcode method.  This is useful if you
 * want to take the spacing of the image into account when computing
 * the edge strength.  Apply the scaling only after calling to
 * \code CreateToRadius \endcode.
 *
 * The Sobel Operator in vertical direction for 2 dimensions is
 * \verbatim
 *             -1  -2  -1
 *             0    0   0
 *             1    2   1
 *
 * \endverbatim
 * The Sobel Operator in horizontal direction is for 2 dimensions is
 * \verbatim
 *             -1   0   1
 *             -2   0   2
 *             -1   0   1
 * \endverbatim
 *
 * The current implementation of the Sobel operator is for 2 and 3 dimensions only.
 * The ND version is planned for future releases.
 *
 * The extension to 3D is from the publication
 *  "Irwin Sobel. An Isotropic 3x3x3 Volume Gradient Operator.
 * Technical report, Hewlett-Packard Laboratories, April 1995."
 *
 * The Sobel operator in 3D has the kernel
 *
 * \verbatim
 * -1 -3 -1   0 0 0  1 3 1
 * -3 -6 -3   0 0 0  3 6 3
 * -1 -3 -1   0 0 0  1 3 1
 *
 *    x-1       x     x+1
 * \endverbatim
 *
 * The \c x kernel is just rotated as required to obtain the kernel in the
 * \c y and \c z directions.
 *
 * \note SobelOperator does not have any user-declared "special member function",
 * following the C++ Rule of Zero: the compiler will generate them if necessary.
 *
 * \sa NeighborhoodOperator
 * \sa Neighborhood
 * \sa ForwardDifferenceOperator
 * \sa BackwardDifferenceOperator
 *
 * \ingroup Operators
 * \ingroup ITKCommon
 *
 * \sphinx
 * \sphinxexample{Core/Common/CreateSobelKernel,Create Sobel Kernel}
 * \endsphinx
 */
template <typename TPixel, unsigned int VDimension = 2, typename TAllocator = NeighborhoodAllocator<TPixel>>
class ITK_TEMPLATE_EXPORT SobelOperator : public NeighborhoodOperator<TPixel, VDimension, TAllocator>
{
public:
  /** Standard class type aliases. */
  using Self = SobelOperator;
  using Superclass = NeighborhoodOperator<TPixel, VDimension, TAllocator>;

  itkOverrideGetNameOfClassMacro(SobelOperator);

  /** Creates the operator with length only in the specified direction.
   * The radius of the operator will be 0 except along the axis on which
   * the operator will work.
   * \sa CreateToRadius \sa FillCenteredDirectional \sa SetDirection() \sa GetDirection() */
  void
  CreateDirectional() override
  {
    this->CreateToRadius(1);
  }

  /** Creates the operator with a specified radius ("square", same length
   * on each side). The spatial location of the coefficients within the
   * operator is defined by the subclass implementation of the Fill method.
   * \sa CreateDirectional \sa Fill */
  // virtual void CreateToRadius(const unsigned long);

protected:
#ifdef ITK_USE_CONCEPT_CHECKING
  // Begin concept checking
  itkConceptMacro(SignedOutputPixelType, (Concept::Signed<typename NumericTraits<TPixel>::ValueType>));
  // End concept checking
#endif

  /** Type alias support for coefficient vector type.*/
  using typename Superclass::CoefficientVector;

  /** Calculates operator coefficients. */
  CoefficientVector
  GenerateCoefficients() override;

  /** Arranges coefficients spatially in the memory buffer. */
  void
  Fill(const CoefficientVector & coeff) override;
};
} // namespace itk

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

#endif