A set of six generalized densities:
- ldpi (low) ~120dpi
- mdpi (medium) ~160dpi
- hdpi (high) ~240dpi
- xhdpi (extra-high) ~320dpi
- xxhdpi (extra-extra-high) ~480dpi
- xxxhdpi (extra-extra-extra-high) ~640dpi
- xlarge screens are at least 960dp x 720dp
- large screens are at least 640dp x 480dp
- normal screens are at least 470dp x 320dp
- small screens are at least 426dp x 320dp
l : m : h : xh : xxh : xxh = 2 : 3 : 4 : 6 : 8;
比如mdpi对应图片为48*48px,则hdpi为72*72px,xhdpi为96*96px,xxhdpi为144*144px;xxxdpi为196*196px;
参考 ：http://developer.android.com/guide/practices/screens_support.html