如何判断dpr的倍数

1. js中有window.devicePixelRatio可直接查看倍率。

2. 用css技术判断dpr的倍数

　　响应式设计：（媒介查询）通过设置断点来实现响应。

• max-device-pixel-ratio:最大设备dpr

• max-resolution：最大像素密度

//retina1x
@media (max--moz-device-pixel-ratio: 1.49), (-webkit-max-device-pixel-ratio: 1.49), (max-device-pixel-ratio: 1.49), (max-resolution: 143dpi), (max-resolution: 1.49dppx)
//retina2x
@media (min--moz-device-pixel-ratio: 1.5) and (max--moz-device-pixel-ratio: 2.49), (-webkit-min-device-pixel-ratio: 1.5) and (-webkit-max-device-pixel-ratio: 2.49),(min-device-pixel-ratio: 1.5) and (max-device-pixel-ratio: 2.49),(min-resolution: 144dpi) and (max-resolution: 239dpi),(min-resolution: 1.5dppx) and (max-resolution: 2.49dppx)
//retina3x
@media (min--moz-device-pixel-ratio: 2.5), (-webkit-min-device-pixel-ratio: 2.5),(min-device-pixel-ratio: 2.5), (min-resolution: 240dpi),(min-resolution: 2.5dppx)