https://www.zhihu.com/question/19714540
https://www.cnblogs.com/tenderwx/p/5245859.html
http://blog.csdn.net/abcjennifer/article/details/7622228
卷积
https://www.zhihu.com/question/22298352
https://zhuanlan.zhihu.com/p/19759362
https://www.cnblogs.com/bithuaning/p/6924978.html