Eigen的欧拉角

旋转矩阵转欧拉角

Vector3f ea = mat.eulerAngles(2, 0, 2); 

这个表示等效于

mat == AngleAxisf(ea[0], Vector3f::UnitZ())
     * AngleAxisf(ea[1], Vector3f::UnitX())
     * AngleAxisf(ea[2], Vector3f::UnitZ()); 

使用的函数说明见

http://eigen.tuxfamily.org/dox/group__Geometry__Module.html#ga17994d2e81b723295f5bc3b1f862ed3b