https://blog.wongcw.com/2019/09/24/%e5%b0%8f%e7%b1%b3mix-alpha%e7%99%bc%e5%b8%83%e5%bc%95%e8%80%81%e5%a4%96%e7%86%b1%e8%ad%b0%ef%bc%9a%e5%b7%a5%e6%a5%ad%e8%a8%ad%e8%a8%88%e8%b6%b3%e4%bb%a5%e8%ae%93%e8%98%8b%e6%9e%9c%e8%b5%a7%e9%a1%8f/