People have described the display in the iPhone 6 Plus as having "3x retina."
My question is, What does 3x retina mean?
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Note that that article was based on the incorrect rumour of a 1704x960 display. The original iPhone display before the 4 had a pixel density of 163 ppi, the old retina displays doubled both horizontal and vertical resolutions to have a pixel density of 326 ppi. To keep the interfaces of apps without retina support the same size, each old size pixel is represented as 4 retina size pixels, in a 2x2 square. The iPhone 5 maintained the same pixel density, but got larger in one dimension, and what the article is talking about was the hypothesis that the iPhone 6 would have the same aspect ratio as the 5, but increasing the resolution in each direction by 1/2 in a 1704x960 display. Assuming the display was the same physical size as the iPhone 5, that would give a pixel density 3 times that of the original iPhones, thus "3x". So assuming one had ancient assets from a 1x display, a 3x display would need to use a 3x3 grid of its pixels per single asset pixel to keep the same size. The article is just showing that unlike the jump from the original assets to retina assets, the quality of interpolating assets from 2x to 3x isn't anywhere near as bad as from 1x to 2x as the resolution change is smaller.
As it turns out, the actual specs of the 6 and 6 Plus are completely different. The 6 has the same pixel density as the 5 but the screen is a bit bigger. The 6 Plus is both larger and somewhat higher density, though not what "3x" would imply. I'm not sure how the new phones would handle old assets. Neither of them are related by a simple ratio like the move from the old iPhone to retina. The aspect ratios are almost the same though, so I'm guessing old apps will default to filling the screen with some kind of interpolation.
I just found a relevant post here: https://stackoverflow.com/questions/25755443/iphone-6-plus-resolution-confusion-xcode-or-apples-website it seems that the way this is going to be handled on the 6 Plus is to use 3x assets which are then downscaled for the screen.
They've added more pixels than needed, because the screen resolution for the size of displays they use is already "retina". Which means that you can't see any individual pixels from a certain distance off of the screen. For iPhones it's like 30cm I believe.
Now that they've added more pixels then needed, they are capable of maximizing brightness and color ratio's, as well as contrast.
Update: Apple seems to be using a default resolution of 568x320 for assets (pictures, mostly). I suppose they call this retina for assets, so 3x retina would enlargen the picture 3 times, which makes 1704x960. I'm not quite sure about why this resolution is specifically chosen, but it might have something to do with the new 1080p display on the iPhone 6 Plus