https://blog.wongcw.com/2020/11/18/%e6%8a%95%e8%b3%87%e5%9f%ba%e9%87%91grayscale%e8%87%aa%e7%a8%b1%e6%af%94%e7%89%b9%e5%b9%a3%e6%8c%81%e5%80%89%e8%b6%8550%e8%90%ac%e6%9e%9a%e5%83%b9%e5%80%bc83-5%e5%84%84%e7%be%8e%e5%85%83/