https://blog.wongcw.com/2020/08/09/%e9%9b%b7%e8%bb%8d%ef%bc%9a%e4%b8%8d%e9%87%8f%e7%94%a2mix-alpha%ef%bc%8c%e6%be%8e%e6%b9%83%e8%8a%af%e7%89%87%e8%a8%88%e5%8a%83%e9%82%84%e5%9c%a8%e7%b9%bc%e7%ba%8c/