G2O explain

First
https://www.cnblogs.com/CV-life/p/10286037.html

The second part
https://www.cnblogs.com/CV-life/p/10449028.html

 

 

other information

<https://www.jianshu.com/p/e16ffb5b265d>
<https://blog.csdn.net/heyijia0327/article/details/47686523>