https://blog.wongcw.com/2018/11/16/%e4%bd%bf%e7%94%a8php-%e6%8a%8a16-%e9%80%b2%e5%88%b6%e7%9a%84%e9%a1%8f%e8%89%b2%e4%bb%a3%e7%a2%bc%e8%bd%89%e6%8f%9b%e6%88%90rgb-%e6%95%b8%e7%b5%84/