While "big" phones are nothing new for the rest of the mobile industry, the iPhone 6 Plus represents a radical departure from Apple's historical love for small screens.
Still, the company's first foray into "phablet" (ugh) territory has a distinct Apple flavor to itself--not only in the aesthetic choices that define its design, but also in a myriad little details--starting, for example, with the dimensions of its screen.
High-density screens have been the name of the game since Steve Jobs first introduced the iPhone 4 back in 2010. That handset's Retina display, with its 326 points-per-inch (PPI) resolution, has graced the screen of every single smartphone that the company has manufactured since, even as the screen itself has grown, first to four inches with the iPhone 5, and now to 4.7 inches with the iPhone 6.
When it comes to the iPhone 6 Plus, however, Apple shaken things up a little: The new Retina HD screen on the largest iPhone model is not just bigger, but also--at a whopping 401 PPI--denser.
If this seems a bit odd, the company's choice can be easily explained by a number of factors. First off, the iPhone 6 Plus screen is 1920 by 1080 pixels--dimensions otherwise known as the standard resolution of 1080p HD television. This makes for easy rendering of the majority of existing video content, and probably helps the company realize economies of scale by taking advantage of commoditized display parts that are more readily available than a completely custom solution.
Of course, this still doesn't quite explain the reason for the change in screen density; after all, Apple could have simply made the iPhone 6 Plus screen a little bigger to give its display the same 326 PPI resolution as its predecessors.
The likely answer here is simply that market forces dictated this choice: A 5.5-inch phone is still small enough that most users will be able to hold it with one hand, but not so large as to start encroaching into tablet territory and risk cannibalizing sales in that business.
The right size at all costs
While picking 1080p resolution and a 5.5-inch screen may make sense from a business point of view, these choices are forcing Apple to break what has, so far, been a cardinal rule of its mobile displays.
When the company first introduced high-density screens, it did so by giving Retina displays four times as many pixels as its predecessors. This nicely round number meant that existing content could be scaled up easily, leading to reasonable graphics quality without taxing the device's hardware. It also made things easier for developers, who could use the same layouts for both regular and Retina models by simply making their graphical assets twice as tall and wide.
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.