https://www.vtechgraphy.com/android/180-%e5%b1%8f%e4%bd%94%e6%af%94%e7%92%b0%e7%b9%9e%e5%b1%8f-%e5%b0%8f%e7%b1%b3-mix-alpha-%e6%a6%82%e5%bf%b5%e6%a9%9f%e6%ad%a3%e5%bc%8f/