https://blog.gongquiz.com/2020/12/01/%ea%b5%ac%ed%98%95spherical-type-%ec%a0%80%ec%9e%a5%ed%83%b1%ed%81%ac%ec%97%90-%eb%8c%80%ed%95%9c-%ec%84%a4%eb%aa%85%ec%9c%bc%eb%a1%9c-%ed%8b%80%eb%a6%b0-%ea%b2%83%ec%9d%80-%ea%b0%80%ec%8a%a4/