ndt.h

/*
 * Software License Agreement (BSD License)
 *
 *  Point Cloud Library (PCL) - www.pointclouds.org
 *  Copyright (c) 2010-2012, Willow Garage, Inc.
 *  Copyright (c) 2012-, Open Perception, Inc.
 *
 *  All rights reserved.
 *
 *  Redistribution and use in source and binary forms, with or without
 *  modification, are permitted provided that the following conditions
 *  are met:
 *
 *   * Redistributions of source code must retain the above copyright
 *     notice, this list of conditions and the following disclaimer.
 *   * Redistributions in binary form must reproduce the above
 *     copyright notice, this list of conditions and the following
 *     disclaimer in the documentation and/or other materials provided
 *     with the distribution.
 *   * Neither the name of the copyright holder(s) nor the names of its
 *     contributors may be used to endorse or promote products derived
 *     from this software without specific prior written permission.
 *
 *  THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS
 *  "AS IS" AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT
 *  LIMITED TO, THE IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS
 *  FOR A PARTICULAR PURPOSE ARE DISCLAIMED. IN NO EVENT SHALL THE
 *  COPYRIGHT OWNER OR CONTRIBUTORS BE LIABLE FOR ANY DIRECT, INDIRECT,
 *  INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING,
 *  BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES;
 *  LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER
 *  CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT
 *  LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN
 *  ANY WAY OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE
 *  POSSIBILITY OF SUCH DAMAGE.
 *
 * $Id$
 *
 */
 
#pragma once
 
#include <pcl/common/utils.h>
#include <pcl/filters/voxel_grid_covariance.h>
#include <pcl/registration/registration.h>
#include <pcl/memory.h>
#include <pcl/pcl_macros.h>
 
#include <unsupported/Eigen/NonLinearOptimization>
 
namespace pcl {
/** \brief A 3D Normal Distribution Transform registration implementation for
 * point cloud data.
 * \note For more information please see <b>Magnusson, M. (2009). The
 * Three-Dimensional Normal-Distributions Transform — an Efficient Representation
 * for Registration, Surface Analysis, and Loop Detection. PhD thesis, Orebro
 * University.  Orebro Studies in Technology 36.</b>, <b>More, J., and Thuente,
 * D. (1994). Line Search Algorithm with Guaranteed Sufficient Decrease In ACM
 * Transactions on Mathematical Software.</b> and Sun, W. and Yuan, Y, (2006)
 * Optimization Theory and Methods: Nonlinear Programming. 89-100
 * \note Math refactored by Todor Stoyanov.
 * \author Brian Okorn (Space and Naval Warfare Systems Center Pacific)
 * \ingroup registration
 */
template <typename PointSource, typename PointTarget, typename Scalar = float>
class NormalDistributionsTransform
: public Registration<PointSource, PointTarget, Scalar> {
protected:
  using PointCloudSource =
      typename Registration<PointSource, PointTarget, Scalar>::PointCloudSource;
  using PointCloudSourcePtr = typename PointCloudSource::Ptr;
  using PointCloudSourceConstPtr = typename PointCloudSource::ConstPtr;
 
  using PointCloudTarget =
      typename Registration<PointSource, PointTarget, Scalar>::PointCloudTarget;
  using PointCloudTargetPtr = typename PointCloudTarget::Ptr;
  using PointCloudTargetConstPtr = typename PointCloudTarget::ConstPtr;
 
  using PointIndicesPtr = PointIndices::Ptr;
  using PointIndicesConstPtr = PointIndices::ConstPtr;
 
  /** \brief Typename of searchable voxel grid containing mean and
   * covariance. */
  using TargetGrid = VoxelGridCovariance<PointTarget>;
  /** \brief Typename of pointer to searchable voxel grid. */
  using TargetGridPtr = TargetGrid*;
  /** \brief Typename of const pointer to searchable voxel grid. */
  using TargetGridConstPtr = const TargetGrid*;
  /** \brief Typename of const pointer to searchable voxel grid leaf. */
  using TargetGridLeafConstPtr = typename TargetGrid::LeafConstPtr;
 
public:
  using Ptr =
      shared_ptr<NormalDistributionsTransform<PointSource, PointTarget, Scalar>>;
  using ConstPtr =
      shared_ptr<const NormalDistributionsTransform<PointSource, PointTarget, Scalar>>;
  using Vector3 = typename Eigen::Matrix<Scalar, 3, 1>;
  using Matrix4 = typename Registration<PointSource, PointTarget, Scalar>::Matrix4;
  using Affine3 = typename Eigen::Transform<Scalar, 3, Eigen::Affine>;
 
  /** \brief Constructor.  Sets \ref outlier_ratio_ to 0.55, \ref step_size_ to
   * 0.1 and \ref resolution_ to 1.0
   */
  NormalDistributionsTransform();
 
  /** \brief Empty destructor */
  ~NormalDistributionsTransform() override = default;
 
  /** \brief Provide a pointer to the input target (e.g., the point cloud that
   * we want to align the input source to).
   * \param[in] cloud the input point cloud target
   */
  inline void
  setInputTarget(const PointCloudTargetConstPtr& cloud) override
  {
    Registration<PointSource, PointTarget, Scalar>::setInputTarget(cloud);
    init();
  }
 
  /** \brief Set/change the voxel grid resolution.
   * \param[in] resolution side length of voxels
   */
  inline void
  setResolution(float resolution)
  {
    // Prevents unnecessary voxel initiations
    if (resolution_ != resolution) {
      resolution_ = resolution;
      if (input_) {
        init();
      }
    }
  }
 
  /** \brief Set the minimum number of points required for a cell to be used (must be 3
   * or greater for covariance calculation). Calls the function of the underlying
   * VoxelGridCovariance. This function must be called before `setInputTarget` and
   * `setResolution`. \param[in] min_points_per_voxel the minimum number of points
   * required for a voxel to be used
   */
  inline void
  setMinPointPerVoxel(unsigned int min_points_per_voxel)
  {
    target_cells_.setMinPointPerVoxel(min_points_per_voxel);
  }
 
  /** \brief Get voxel grid resolution.
   * \return side length of voxels
   */
  inline float
  getResolution() const
  {
    return resolution_;
  }
 
  /** \brief Get the newton line search maximum step length.
   * \return maximum step length
   */
  inline double
  getStepSize() const
  {
    return step_size_;
  }
 
  /** \brief Set/change the newton line search maximum step length.
   * \param[in] step_size maximum step length
   */
  inline void
  setStepSize(double step_size)
  {
    step_size_ = step_size;
  }
 
  /** \brief Get the point cloud outlier ratio.
   * \return outlier ratio
   */
  inline double
  getOutlierRatio() const
  {
    return outlier_ratio_;
  }
 
  /** \brief Get the point cloud outlier ratio.
   * \return outlier ratio
   */
  PCL_DEPRECATED(1,
                 18,
                 "The method `getOulierRatio` has been renamed to "
                 "`getOutlierRatio`.")
  inline double
  getOulierRatio() const
  {
    return outlier_ratio_;
  }
 
  /** \brief Set/change the point cloud outlier ratio.
   * \param[in] outlier_ratio outlier ratio
   */
  inline void
  setOutlierRatio(double outlier_ratio)
  {
    outlier_ratio_ = outlier_ratio;
  }
 
  PCL_DEPRECATED(1,
                 18,
                 "The method `setOulierRatio` has been renamed to "
                 "`setOutlierRatio`.")
  /** \brief Set/change the point cloud outlier ratio.
   * \param[in] outlier_ratio outlier ratio
   */
  inline void
  setOulierRatio(double outlier_ratio)
  {
    outlier_ratio_ = outlier_ratio;
  }
 
  /** \brief Get the registration alignment likelihood.
   * \return transformation likelihood
   */
  inline double
  getTransformationLikelihood() const
  {
    return trans_likelihood_;
  }
 
  /** \brief Get the registration alignment probability.
   * \return transformation probability
   */
  PCL_DEPRECATED(1,
                 16,
                 "The method `getTransformationProbability` has been renamed to "
                 "`getTransformationLikelihood`.")
  inline double
  getTransformationProbability() const
  {
    return trans_likelihood_;
  }
 
  /** \brief Get the number of iterations required to calculate alignment.
   * \return final number of iterations
   */
  inline int
  getFinalNumIteration() const
  {
    return nr_iterations_;
  }
 
  /** \brief Get access to the `VoxelGridCovariance` generated from target cloud
   * containing point means and covariances. Set the input target cloud before calling
   * this. Useful for debugging, e.g.
   * \code
   * pcl::PointCloud<PointXYZ> visualize_cloud;
   * ndt.getTargetCells().getDisplayCloud(visualize_cloud);
   * \endcode
   */
  inline const TargetGrid&
  getTargetCells() const
  {
    return target_cells_;
  }
 
  /** \brief Convert 6 element transformation vector to affine transformation.
   * \param[in] x transformation vector of the form [x, y, z, roll, pitch, yaw]
   * \param[out] trans affine transform corresponding to given transformation
   * vector
   */
  static void
  convertTransform(const Eigen::Matrix<double, 6, 1>& x, Affine3& trans)
  {
    trans = Eigen::Translation<Scalar, 3>(x.head<3>().cast<Scalar>()) *
            Eigen::AngleAxis<Scalar>(static_cast<Scalar>(x(3)), Vector3::UnitX()) *
            Eigen::AngleAxis<Scalar>(static_cast<Scalar>(x(4)), Vector3::UnitY()) *
            Eigen::AngleAxis<Scalar>(static_cast<Scalar>(x(5)), Vector3::UnitZ());
  }
 
  /** \brief Convert 6 element transformation vector to transformation matrix.
   * \param[in] x transformation vector of the form [x, y, z, roll, pitch, yaw]
   * \param[out] trans 4x4 transformation matrix corresponding to given
   * transformation vector
   */
  static void
  convertTransform(const Eigen::Matrix<double, 6, 1>& x, Matrix4& trans)
  {
    Affine3 _affine;
    convertTransform(x, _affine);
    trans = _affine.matrix();
  }
 
protected:
  using Registration<PointSource, PointTarget, Scalar>::reg_name_;
  using Registration<PointSource, PointTarget, Scalar>::getClassName;
  using Registration<PointSource, PointTarget, Scalar>::input_;
  using Registration<PointSource, PointTarget, Scalar>::indices_;
  using Registration<PointSource, PointTarget, Scalar>::target_;
  using Registration<PointSource, PointTarget, Scalar>::nr_iterations_;
  using Registration<PointSource, PointTarget, Scalar>::max_iterations_;
  using Registration<PointSource, PointTarget, Scalar>::previous_transformation_;
  using Registration<PointSource, PointTarget, Scalar>::final_transformation_;
  using Registration<PointSource, PointTarget, Scalar>::transformation_;
  using Registration<PointSource, PointTarget, Scalar>::transformation_epsilon_;
  using Registration<PointSource, PointTarget, Scalar>::
      transformation_rotation_epsilon_;
  using Registration<PointSource, PointTarget, Scalar>::converged_;
  using Registration<PointSource, PointTarget, Scalar>::corr_dist_threshold_;
  using Registration<PointSource, PointTarget, Scalar>::inlier_threshold_;
 
  using Registration<PointSource, PointTarget, Scalar>::update_visualizer_;
 
  /** \brief Estimate the transformation and returns the transformed source
   * (input) as output.
   * \param[out] output the resultant input transformed point cloud dataset
   */
  virtual void
  computeTransformation(PointCloudSource& output)
  {
    computeTransformation(output, Matrix4::Identity());
  }
 
  /** \brief Estimate the transformation and returns the transformed source
   * (input) as output.
   * \param[out] output the resultant input transformed point cloud dataset
   * \param[in] guess the initial gross estimation of the transformation
   */
  void
  computeTransformation(PointCloudSource& output, const Matrix4& guess) override;
 
  /** \brief Initiate covariance voxel structure. */
  void inline init()
  {
    target_cells_.setLeafSize(resolution_, resolution_, resolution_);
    target_cells_.setInputCloud(target_);
    // Initiate voxel structure.
    target_cells_.filter(true);
    PCL_DEBUG("[pcl::%s::init] Computed voxel structure, got %zu voxels with valid "
              "normal distributions.\n",
              getClassName().c_str(),
              target_cells_.getCentroids()->size());
  }
 
  /** \brief Compute derivatives of likelihood function w.r.t. the
   * transformation vector.
   * \note Equation 6.10, 6.12 and 6.13 [Magnusson 2009].
   * \param[out] score_gradient the gradient vector of the likelihood function
   * w.r.t.  the transformation vector
   * \param[out] hessian the hessian matrix of the likelihood function
   * w.r.t. the transformation vector
   * \param[in] trans_cloud transformed point cloud
   * \param[in] transform the current transform vector
   * \param[in] compute_hessian flag to calculate hessian, unnecessary for step
   * calculation.
   */
  double
  computeDerivatives(Eigen::Matrix<double, 6, 1>& score_gradient,
                     Eigen::Matrix<double, 6, 6>& hessian,
                     const PointCloudSource& trans_cloud,
                     const Eigen::Matrix<double, 6, 1>& transform,
                     bool compute_hessian = true);
 
  /** \brief Compute individual point contributions to derivatives of
   * likelihood function w.r.t. the transformation vector.
   * \note Equation 6.10, 6.12 and 6.13 [Magnusson 2009].
   * \param[in,out] score_gradient the gradient vector of the likelihood
   * function w.r.t. the transformation vector
   * \param[in,out] hessian the hessian matrix of the likelihood function
   * w.r.t. the transformation vector
   * \param[in] x_trans transformed point minus mean of occupied covariance
   * voxel
   * \param[in] c_inv covariance of occupied covariance voxel
   * \param[in] compute_hessian flag to calculate hessian, unnecessary for step
   * calculation.
   */
  double
  updateDerivatives(Eigen::Matrix<double, 6, 1>& score_gradient,
                    Eigen::Matrix<double, 6, 6>& hessian,
                    const Eigen::Vector3d& x_trans,
                    const Eigen::Matrix3d& c_inv,
                    bool compute_hessian = true) const;
 
  /** \brief Precompute angular components of derivatives.
   * \note Equation 6.19 and 6.21 [Magnusson 2009].
   * \param[in] transform the current transform vector
   * \param[in] compute_hessian flag to calculate hessian, unnecessary for step
   * calculation.
   */
  void
  computeAngleDerivatives(const Eigen::Matrix<double, 6, 1>& transform,
                          bool compute_hessian = true);
 
  /** \brief Compute point derivatives.
   * \note Equation 6.18-21 [Magnusson 2009].
   * \param[in] x point from the input cloud
   * \param[in] compute_hessian flag to calculate hessian, unnecessary for step
   * calculation.
   */
  void
  computePointDerivatives(const Eigen::Vector3d& x, bool compute_hessian = true);
 
  /** \brief Compute hessian of likelihood function w.r.t. the transformation
   * vector.
   * \note Equation 6.13 [Magnusson 2009].
   * \param[out] hessian the hessian matrix of the likelihood function
   * w.r.t. the transformation vector
   * \param[in] trans_cloud transformed point cloud
   */
  void
  computeHessian(Eigen::Matrix<double, 6, 6>& hessian,
                 const PointCloudSource& trans_cloud);
 
  /** \brief Compute individual point contributions to hessian of likelihood
   * function w.r.t. the transformation vector.
   * \note Equation 6.13 [Magnusson 2009].
   * \param[in,out] hessian the hessian matrix of the likelihood function
   * w.r.t. the transformation vector
   * \param[in] x_trans transformed point minus mean of occupied covariance
   * voxel
   * \param[in] c_inv covariance of occupied covariance voxel
   */
  void
  updateHessian(Eigen::Matrix<double, 6, 6>& hessian,
                const Eigen::Vector3d& x_trans,
                const Eigen::Matrix3d& c_inv) const;
 
  /** \brief Compute line search step length and update transform and
   * likelihood derivatives using More-Thuente method.
   * \note Search Algorithm [More, Thuente 1994]
   * \param[in] transform initial transformation vector, \f$ x \f$ in Equation
   * 1.3 (Moore, Thuente 1994) and \f$ \vec{p} \f$ in Algorithm 2 [Magnusson
   * 2009]
   * \param[in] step_dir descent direction, \f$ p \f$ in Equation 1.3 (Moore,
   * Thuente 1994) and \f$ \delta \vec{p} \f$ normalized in Algorithm 2
   * [Magnusson 2009]
   * \param[in] step_init initial step length estimate, \f$ \alpha_0 \f$ in
   * Moore-Thuente (1994) and the normal of \f$ \delta \vec{p} \f$ in Algorithm
   * 2 [Magnusson 2009]
   * \param[in] step_max maximum step length, \f$ \alpha_max \f$ in
   * Moore-Thuente (1994)
   * \param[in] step_min minimum step length, \f$ \alpha_min \f$ in
   * Moore-Thuente (1994)
   * \param[out] score final score function value, \f$ f(x + \alpha p) \f$ in
   * Equation 1.3 (Moore, Thuente 1994) and \f$ score \f$ in Algorithm 2
   * [Magnusson 2009]
   * \param[in,out] score_gradient gradient of score function w.r.t.
   * transformation vector, \f$ f'(x + \alpha p) \f$ in Moore-Thuente (1994) and
   * \f$ \vec{g} \f$ in Algorithm 2 [Magnusson 2009]
   * \param[out] hessian hessian of score function w.r.t. transformation vector,
   * \f$ f''(x + \alpha p) \f$ in Moore-Thuente (1994) and \f$ H \f$ in
   * Algorithm 2 [Magnusson 2009]
   * \param[in,out] trans_cloud transformed point cloud, \f$ X \f$ transformed
   * by \f$ T(\vec{p},\vec{x}) \f$ in Algorithm 2 [Magnusson 2009]
   * \return final step length
   */
  double
  computeStepLengthMT(const Eigen::Matrix<double, 6, 1>& transform,
                      Eigen::Matrix<double, 6, 1>& step_dir,
                      double step_init,
                      double step_max,
                      double step_min,
                      double& score,
                      Eigen::Matrix<double, 6, 1>& score_gradient,
                      Eigen::Matrix<double, 6, 6>& hessian,
                      PointCloudSource& trans_cloud);
 
  /** \brief Update interval of possible step lengths for More-Thuente method,
   * \f$ I \f$ in More-Thuente (1994)
   * \note Updating Algorithm until some value satisfies \f$ \psi(\alpha_k) \leq
   * 0 \f$ and \f$ \phi'(\alpha_k) \geq 0 \f$ and Modified Updating Algorithm
   * from then on [More, Thuente 1994].
   * \param[in,out] a_l first endpoint of interval \f$ I \f$, \f$ \alpha_l \f$
   * in Moore-Thuente (1994)
   * \param[in,out] f_l value at first endpoint, \f$ f_l \f$ in Moore-Thuente
   * (1994), \f$ \psi(\alpha_l) \f$ for Update Algorithm and \f$ \phi(\alpha_l)
   * \f$ for Modified Update Algorithm
   * \param[in,out] g_l derivative at first endpoint, \f$ g_l \f$ in
   * Moore-Thuente (1994), \f$ \psi'(\alpha_l) \f$ for Update Algorithm and \f$
   * \phi'(\alpha_l) \f$ for Modified Update Algorithm
   * \param[in,out] a_u second endpoint of interval \f$ I \f$, \f$ \alpha_u \f$
   * in Moore-Thuente (1994)
   * \param[in,out] f_u value at second endpoint, \f$ f_u \f$ in Moore-Thuente
   * (1994), \f$ \psi(\alpha_u) \f$ for Update Algorithm and \f$ \phi(\alpha_u)
   * \f$ for Modified Update Algorithm
   * \param[in,out] g_u derivative at second endpoint, \f$ g_u \f$ in
   * Moore-Thuente (1994), \f$ \psi'(\alpha_u) \f$ for Update Algorithm and \f$
   * \phi'(\alpha_u) \f$ for Modified Update Algorithm
   * \param[in] a_t trial value, \f$ \alpha_t \f$ in Moore-Thuente (1994)
   * \param[in] f_t value at trial value, \f$ f_t \f$ in Moore-Thuente (1994),
   * \f$ \psi(\alpha_t) \f$ for Update Algorithm and \f$ \phi(\alpha_t) \f$ for
   * Modified Update Algorithm
   * \param[in] g_t derivative at trial value, \f$ g_t \f$ in Moore-Thuente
   * (1994), \f$ \psi'(\alpha_t) \f$ for Update Algorithm and \f$
   * \phi'(\alpha_t) \f$ for Modified Update Algorithm
   * \return if interval converges
   */
  bool
  updateIntervalMT(double& a_l,
                   double& f_l,
                   double& g_l,
                   double& a_u,
                   double& f_u,
                   double& g_u,
                   double a_t,
                   double f_t,
                   double g_t) const;
 
  /** \brief Select new trial value for More-Thuente method.
   * \note Trial Value Selection [More, Thuente 1994], \f$ \psi(\alpha_k) \f$ is
   * used for \f$ f_k \f$ and \f$ g_k \f$ until some value satisfies the test
   * \f$ \psi(\alpha_k) \leq 0 \f$ and \f$ \phi'(\alpha_k) \geq 0 \f$ then \f$
   * \phi(\alpha_k) \f$ is used from then on.
   * \note Interpolation Minimizer equations from Optimization Theory and
   * Methods: Nonlinear Programming By Wenyu Sun, Ya-xiang Yuan (89-100).
   * \param[in] a_l first endpoint of interval \f$ I \f$, \f$ \alpha_l \f$ in
   * Moore-Thuente (1994)
   * \param[in] f_l value at first endpoint, \f$ f_l \f$ in Moore-Thuente (1994)
   * \param[in] g_l derivative at first endpoint, \f$ g_l \f$ in Moore-Thuente
   * (1994)
   * \param[in] a_u second endpoint of interval \f$ I \f$, \f$ \alpha_u \f$ in
   * Moore-Thuente (1994)
   * \param[in] f_u value at second endpoint, \f$ f_u \f$ in Moore-Thuente
   * (1994)
   * \param[in] g_u derivative at second endpoint, \f$ g_u \f$ in Moore-Thuente
   * (1994)
   * \param[in] a_t previous trial value, \f$ \alpha_t \f$ in Moore-Thuente
   * (1994)
   * \param[in] f_t value at previous trial value, \f$ f_t \f$ in Moore-Thuente
   * (1994)
   * \param[in] g_t derivative at previous trial value, \f$ g_t \f$ in
   * Moore-Thuente (1994)
   * \return new trial value
   */
  double
  trialValueSelectionMT(double a_l,
                        double f_l,
                        double g_l,
                        double a_u,
                        double f_u,
                        double g_u,
                        double a_t,
                        double f_t,
                        double g_t) const;
 
  /** \brief Auxiliary function used to determine endpoints of More-Thuente
   * interval.
   * \note \f$ \psi(\alpha) \f$ in Equation 1.6 (Moore, Thuente 1994)
   * \param[in] a the step length, \f$ \alpha \f$ in More-Thuente (1994)
   * \param[in] f_a function value at step length a, \f$ \phi(\alpha) \f$ in
   * More-Thuente (1994)
   * \param[in] f_0 initial function value, \f$ \phi(0) \f$ in Moore-Thuente
   * (1994)
   * \param[in] g_0 initial function gradient, \f$ \phi'(0) \f$ in More-Thuente
   * (1994)
   * \param[in] mu the step length, constant \f$ \mu \f$ in Equation 1.1 [More,
   * Thuente 1994]
   * \return sufficient decrease value
   */
  inline double
  auxilaryFunction_PsiMT(
      double a, double f_a, double f_0, double g_0, double mu = 1.e-4) const
  {
    return f_a - f_0 - mu * g_0 * a;
  }
 
  /** \brief Auxiliary function derivative used to determine endpoints of
   * More-Thuente interval.
   * \note \f$ \psi'(\alpha) \f$, derivative of Equation 1.6 (Moore, Thuente
   * 1994)
   * \param[in] g_a function gradient at step length a, \f$ \phi'(\alpha) \f$ in
   * More-Thuente (1994)
   * \param[in] g_0 initial function gradient, \f$ \phi'(0) \f$ in More-Thuente
   * (1994)
   * \param[in] mu the step length, constant \f$ \mu \f$ in Equation 1.1 [More,
   * Thuente 1994]
   * \return sufficient decrease derivative
   */
  inline double
  auxilaryFunction_dPsiMT(double g_a, double g_0, double mu = 1.e-4) const
  {
    return g_a - mu * g_0;
  }
 
  /** \brief The voxel grid generated from target cloud containing point means
   * and covariances. */
  TargetGrid target_cells_;
 
  /** \brief The side length of voxels. */
  float resolution_{1.0f};
 
  /** \brief The maximum step length. */
  double step_size_{0.1};
 
  /** \brief The ratio of outliers of points w.r.t. a normal distribution,
   * Equation 6.7 [Magnusson 2009]. */
  double outlier_ratio_{0.55};
 
  /** \brief The normalization constants used fit the point distribution to a
   * normal distribution, Equation 6.8 [Magnusson 2009]. */
  double gauss_d1_{0.0}, gauss_d2_{0.0};
 
  /** \brief The likelihood score of the transform applied to the input cloud,
   * Equation 6.9 and 6.10 [Magnusson 2009]. */
  union {
    PCL_DEPRECATED(1,
                   16,
                   "`trans_probability_` has been renamed to `trans_likelihood_`.")
    double trans_probability_;
    double trans_likelihood_{0.0};
  };
 
  /** \brief Precomputed Angular Gradient
   *
   * The precomputed angular derivatives for the jacobian of a transformation
   * vector, Equation 6.19 [Magnusson 2009].
   */
  Eigen::Matrix<double, 8, 4> angular_jacobian_;
 
  /** \brief Precomputed Angular Hessian
   *
   * The precomputed angular derivatives for the hessian of a transformation
   * vector, Equation 6.19 [Magnusson 2009].
   */
  Eigen::Matrix<double, 15, 4> angular_hessian_;
 
  /** \brief The first order derivative of the transformation of a point
   * w.r.t. the transform vector, \f$ J_E \f$ in Equation 6.18 [Magnusson
   * 2009]. */
  Eigen::Matrix<double, 3, 6> point_jacobian_;
 
  /** \brief The second order derivative of the transformation of a point
   * w.r.t. the transform vector, \f$ H_E \f$ in Equation 6.20 [Magnusson
   * 2009]. */
  Eigen::Matrix<double, 18, 6> point_hessian_;
 
public:
  PCL_MAKE_ALIGNED_OPERATOR_NEW
};
} // namespace pcl
 
#include <pcl/registration/impl/ndt.hpp>