1080p PenTile is not true 1080p
I just did the math
Full HD means 1920 by 1080 and it gives us 2073600 pixels since 1920*1080 = 2073600. On a traditional 1080p RGB panel (like on the HTC One), it’s the same but on a Pentile display, things are a bit different. See, each pixel on a RGB panel is made of 3 subpixels. On a PenTile display (like the one on the Galaxy S4), it is made of two and the missing subpixel in each individual pixel can be found in the adjacent pixel, which means on a standard 1080p RGB panel, there are 6220800 subpixels (2073600*3 = 6220800), but on a 1080p PenTile display, it’s 4147200. How?
Since 1 subpixel is missing from each pixel, we subtract 2073600, which is the number of missing subpixels on a 1080p Pentile display from 6220800, which is the total number of actual subpixels on a 1080p RGB display and we come up with 4147200. To get the total pixel count, we divide it by 3, because technically each pixel is made of 3 subpixels. So, 4147200/3 = 1382400.
So we now got the actual total pixel count. But how do we convert it to resolution? It’s easy, we divide the horizontal aspect ratio by vertical and then vice versa.
16/9 = 1.77 and 9/16 = 0.56. Then, we multiply each by the total pixel count:
1382400 * 1.77 = 2446848
1382400 * 0.56 = 774144
Then we take a square root of both:
sqrt(2446848) = 1564.24
sqrt(774144) = 879.85
Rounding each number, we come up with 1564 x 880