One problem we'll have when scaling up our UI is that the images' quality will be decreased. A 256 x 256 pixels sprite scaled up to 512 x 512 will look blurry.
In order to avoid having blurry sprites, we could simply have all our assets in high-definition sizes. On the one hand, they will always look good, even on high-resolution displays. On the other hand, displaying a 1024 x 1024 sprite on a small screen with an effective final size of 128 x 128 will result in a waste of the device's memory.
Hence, it is useful to create, for example, three atlases for different resolution ranges:
SDAtlas
: This is for screens up to 800 x 480. Example devices include:iPhone 3G and 3GS 480 x 320
HDAtlas
: This is for screens from 800 x 480 to 1600 x 1200. Example devices include:iPhone 4 and 4S: 960 x 640
iPhone 6: 1334 x 750
iPad 2: 1024 x 768
Galaxy S2: 1280 x 720
SHDAtlas
: This is for screens from 1600 x 1200 and above. Example devices include:Nexus 5: 1920 x 1080
iPad 3: 2048 x 1536
Nexus 7...