Over time, the company's engineers have made these conveniences largely obsolete. iOS 6, for example, introduced a new technology, dubbed Autolayout, that allows developers to define their user interfaces by describing how the various elements relate to each other, independently of the screen resolution at which they are being displayed.
With the introduction of iOS 8, the operating system also gains the ability to use vector-based icons and images, which — particularly when coupled with the modern "flat" look of iOS 7 — scale to arbitrary resolutions much better than traditional, bitmap-based graphical assets.
Rabbit in the hat
Despite all these innovations, however, Apple didn't do away completely with whole-number resolution scales. Instead, the company decided to pull a bit of a magic trick: As far as apps are concerned, the iPhone 6 Plus's screen is equipped with a "virtual" screen whose resolution is 1242 by 2208 pixels, with images that are nine times the density of the original iPhone's.
This approach gives developers the opportunity to add "3x" assets to their software, which are easier to manage alongside the "1x" and "2x" images required to support older operating systems and devices, thus making their ongoing work easier to handle than would otherwise be possible.
Of course, this also leaves Apple with the problem that, virtual screen notwithstanding, the physical screen that will actually find its way on iPhone 6 Plus models will still be 1920 by 1280 pixels. To overcome this issue, the company added a dedicated hardware component that simply takes the images composed onto the virtual screen, resizes them in realtime, and renders them onto the device's real-life display (you can see this whole process beautifully illustrated in an excellent demonstration put together by the folks at software house PixelCut).
If this sounds like a recipe for grainy images and fuzzy text, you needn't worry: This same technique has been at work on Retina MacBook Pro models since their introduction; as the happy user of one of those computers, I can safely say that the end results are as crisp and as smooth as those of any recent Apple device — and it's probably no coincidence that company executive Phil Schiller referred to the hardware that handles the transition between virtual and real display "a desktop-class scaler."
Ultimately, the best news about all this complexity is that it is completely hidden from both end-users, who will enjoy the kind of graphical quality they have come to expect from Apple, and developers, who will be able to enjoy a simplified working environment and focus on building better apps instead of fiddling with screen resolutions. In typical Apple fashion, the company is sweating all the details so that we don't have to.
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