該代碼有點舊。這是一個更新的,可能改進的版本。可能還有一些不好的事情。例如,我認爲應該更改爲使用MatrixBase
而不是實際的Matrix
。這可能會讓它優化並更好地決定何時需要分配存儲空間。這也使用命名空間internal
這可能是皺眉,但似乎有必要使用Eigen的NullaryExpr
這似乎是正確的事情。有可怕的mutable
關鍵字的用法。這是必要的,因爲當在NullaryExpr
中使用時,Eigen認爲應該是const
。 依靠提升也有點煩人。看來在C++ 11中,necessary functions已經成爲標準。在課堂代碼下面,有一個簡短的使用示例。
類eigenmultivariatenormal.hpp
#ifndef __EIGENMULTIVARIATENORMAL_HPP
#define __EIGENMULTIVARIATENORMAL_HPP
#include <Eigen/Dense>
#include <boost/random/mersenne_twister.hpp>
#include <boost/random/normal_distribution.hpp>
/*
We need a functor that can pretend it's const,
but to be a good random number generator
it needs mutable state. The standard Eigen function
Random() just calls rand(), which changes a global
variable.
*/
namespace Eigen {
namespace internal {
template<typename Scalar>
struct scalar_normal_dist_op
{
static boost::mt19937 rng; // The uniform pseudo-random algorithm
mutable boost::normal_distribution<Scalar> norm; // The gaussian combinator
EIGEN_EMPTY_STRUCT_CTOR(scalar_normal_dist_op)
template<typename Index>
inline const Scalar operator() (Index, Index = 0) const { return norm(rng); }
};
template<typename Scalar>
boost::mt19937 scalar_normal_dist_op<Scalar>::rng;
template<typename Scalar>
struct functor_traits<scalar_normal_dist_op<Scalar> >
{ enum { Cost = 50 * NumTraits<Scalar>::MulCost, PacketAccess = false, IsRepeatable = false }; };
} // end namespace internal
/**
Find the eigen-decomposition of the covariance matrix
and then store it for sampling from a multi-variate normal
*/
template<typename Scalar, int Size>
class EigenMultivariateNormal
{
Matrix<Scalar,Size,Size> _covar;
Matrix<Scalar,Size,Size> _transform;
Matrix< Scalar, Size, 1> _mean;
internal::scalar_normal_dist_op<Scalar> randN; // Gaussian functor
public:
EigenMultivariateNormal(const Matrix<Scalar,Size,1>& mean,const Matrix<Scalar,Size,Size>& covar)
{
setMean(mean);
setCovar(covar);
}
void setMean(const Matrix<Scalar,Size,1>& mean) { _mean = mean; }
void setCovar(const Matrix<Scalar,Size,Size>& covar)
{
_covar = covar;
// Assuming that we'll be using this repeatedly,
// compute the transformation matrix that will
// be applied to unit-variance independent normals
/*
Eigen::LDLT<Eigen::Matrix<Scalar,Size,Size> > cholSolver(_covar);
// We can only use the cholesky decomposition if
// the covariance matrix is symmetric, pos-definite.
// But a covariance matrix might be pos-semi-definite.
// In that case, we'll go to an EigenSolver
if (cholSolver.info()==Eigen::Success) {
// Use cholesky solver
_transform = cholSolver.matrixL();
} else {*/
SelfAdjointEigenSolver<Matrix<Scalar,Size,Size> > eigenSolver(_covar);
_transform = eigenSolver.eigenvectors()*eigenSolver.eigenvalues().cwiseMax(0).cwiseSqrt().asDiagonal();
/*}*/
}
/// Draw nn samples from the gaussian and return them
/// as columns in a Size by nn matrix
Matrix<Scalar,Size,-1> samples(int nn)
{
return (_transform * Matrix<Scalar,Size,-1>::NullaryExpr(Size,nn,randN)).colwise() + _mean;
}
}; // end class EigenMultivariateNormal
} // end namespace Eigen
#endif
下面是一個使用它的簡單程序:
#include <fstream>
#include "eigenmultivariatenormal.hpp"
#ifndef M_PI
#define M_PI REAL(3.1415926535897932384626433832795029)
#endif
/**
Take a pair of un-correlated variances.
Create a covariance matrix by correlating
them, sandwiching them in a rotation matrix.
*/
Eigen::Matrix2d genCovar(double v0,double v1,double theta)
{
Eigen::Matrix2d rot = Eigen::Rotation2Dd(theta).matrix();
return rot*Eigen::DiagonalMatrix<double,2,2>(v0,v1)*rot.transpose();
}
void main()
{
Eigen::Vector2d mean;
Eigen::Matrix2d covar;
mean << -1,0.5; // Set the mean
// Create a covariance matrix
// Much wider than it is tall
// and rotated clockwise by a bit
covar = genCovar(3,0.1,M_PI/5.0);
// Create a bivariate gaussian distribution of doubles.
// with our chosen mean and covariance
Eigen::EigenMultivariateNormal<double,2> normX(mean,covar);
std::ofstream file("samples.txt");
// Generate some samples and write them out to file
// for plotting
file << normX.samples(1000).transpose() << std::endl;
}
而這裏的顯示結果圖。
使用SelfAdjointEigenSolver
可能比一個Cholesky分解慢了許多,但它是穩定的,即使協方差矩陣是奇異的。如果你知道你的協方差矩陣總是滿的,那麼你可以用它來代替。但是,如果您很少創建分佈並從中抽樣,那麼這可能不是什麼大問題。
你是否需要給它一個數字,比如10,而不是「int」? – Akanksh 2013-05-03 14:29:45
我認爲它希望你使用int值作爲第二個模板參數,就像EigenMultivariateNormal –
spiritwolfform
2013-05-03 14:30:04