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学习四元数的小demo：
https://github.com/AbnerCSZ/Eigen_Quaternion_demo
1.
从两个向量a,b中得到四元数，ab无需相同单位，也不用规范化。
FromTwoVectors (const MatrixBase< Derived1 > & a,const MatrixBase< Derived2 > &b)
2。
Quaternion ( const Scalar & w,const Scalar & x,const Scalar & y,const Scalar & z )
Note the order of the arguments: the real w coefficient first, while internally the coefficients are stored in the following order: [x, y, z, w]
3。
http://eigen.tuxfamily.org/dox/classEigen_1_1QuaternionBase.html