Eigen: 求两个向量的夹角

//! Compute cosine of the angle between two vectors.
double computeCosineOfAngleBetweenVectors( const Eigen::VectorXd& vector0,
                                           const Eigen::VectorXd& vector1 )
{
    assert( vector0.size( ) == vector1.size( ) );

    // Get the cosine of the angle by dotting the normalized vectors.
    double dotProductOfNormalizedVectors = vector0.normalized( ).dot( vector1.normalized( ) );

    // Explicitly define the extreme cases, which can give problems with the acos function.
    if ( dotProductOfNormalizedVectors >= 1.0 )
    {
        return 1.0;
    }

    else if ( dotProductOfNormalizedVectors <= -1.0 )
    {
        return -1.0;
    }
    // Determine the actual angle.
    else
    {
        return dotProductOfNormalizedVectors;
    }
}