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// Ceres Solver - A fast non-linear least squares minimizer
// Copyright 2023 Google Inc. All rights reserved.
// http://ceres-solver.org/
//
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// modification, are permitted provided that the following conditions are met:
//
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// Author: sameeragarwal@google.com (Sameer Agarwal)
#ifndef CERES_INTERNAL_INVERT_PSD_MATRIX_H_
#define CERES_INTERNAL_INVERT_PSD_MATRIX_H_
#include "Eigen/Dense"
#include "ceres/internal/eigen.h"
namespace ceres::internal {
// Helper routine to compute the inverse or pseudo-inverse of a
// symmetric positive semi-definite matrix.
//
// assume_full_rank controls whether a Cholesky factorization or an
// Singular Value Decomposition is used to compute the inverse and the
// pseudo-inverse respectively.
//
// The template parameter kSize can either be Eigen::Dynamic or a
// positive integer equal to the number of rows of m.
template <int kSize>
typename EigenTypes<kSize, kSize>::Matrix InvertPSDMatrix(
const bool assume_full_rank,
const typename EigenTypes<kSize, kSize>::Matrix& m) {
using MType = typename EigenTypes<kSize, kSize>::Matrix;
const int size = m.rows();
// If the matrix can be assumed to be full rank, then if it is small
// (< 5) and fixed size, use Eigen's optimized inverse()
// implementation.
//
// https://eigen.tuxfamily.org/dox/group__TutorialLinearAlgebra.html#title3
if (assume_full_rank) {
if (kSize > 0 && kSize < 5) {
return m.inverse();
}
return m.template selfadjointView<Eigen::Upper>().llt().solve(
MType::Identity(size, size));
}
// For a thin SVD the number of columns of the matrix need to be dynamic.
using SVDMType = typename EigenTypes<kSize, Eigen::Dynamic>::Matrix;
Eigen::JacobiSVD<SVDMType> svd(m, Eigen::ComputeThinU | Eigen::ComputeThinV);
return svd.solve(MType::Identity(size, size));
}
} // namespace ceres::internal
#endif // CERES_INTERNAL_INVERT_PSD_MATRIX_H_