iPhone 7 Plus background image size

Question

What are the proper dimensions of a background image on iPhone 7 Plus? I have seen answers which indicate 1242x2208 which gets downsampled to 1080x1920 on-the-go. I cannot understand, why 1242x2208 is the appropriate size, since 2208px/0.66 (equivalent to 2/3) is not equal to iPhone 7's height (1334px). I know I can make my image be 1334px, it wouldn't be a big problem. But I don't understand the math behind it.

Answer 1

Logically the device accepts input of 2208px x 1242px for rendering which divided by 3x retina factor ends up as 736 x 414 logical points frame. The downscaling by 1.15x for the physical 1920 x 1080 pixels happens in the iPhone hardware rendering pipeline. This allows Apple keeping the same frame coordinate system for 1x 2x 3x screens.