OS X @2X
Richard Gaywood wrote a great post today on TUAW about the math of Retina displays. If you haven’t already read it, please do, it was great, and I’m not going to rehash everything he wrote. He made some great observations, but it seems as though folks are missing some of the implications.
iOS was designed from the ground up for touch screen devies, and because of that, Apple took great care to create digital buttons that are easy to physically tap. In the iOS Human Interface Guidelines, they specify that 44 by 44 point buttons are about the smallest users can comfortably tap, and even then, they suggest that there be as much space as possible between buttons to make the tap target less crowded (there’s a sort of Fitt’s Law for touch screens — the bigger the button and more space between them, the less precise the user has to be in tapping.) The thing is, a 44 by 44 point button can be physically larger or smaller depending on the PPI of the screen it’s displayed on. If a 640 by 960 pixel iPhone screen were 4-inch diagonal instead of 3.5-inch, the buttons would be physically larger. Likewise, a 3-inch screen would make them significantly smaller, and thus impractical for touch screen use. There is a narrow range somewhere between 3-inch and 4-inch displays that would be practical for an iPhone or iPod Touch screen. And Apple is unlikely to ever move away from 640 by 960 pixels on those screens for the reasons outlined here: The problem with not going 2x with an iPad 2 Retina Display
But, OS X is not a touch OS, obviously. The mouse pointer has 1 by 1 pixel precision, thus the physical size of buttons and other UI elements can vary quite a bit without impacting overall user experience. And, in fact, we see a large range of display PPI in Apple’s current Mac lineup. From the 109ppi 27-inch iMac to the 135ppi 11-inch Macbook Air (which is actually 11.6 inches, an important detail when calculating PPI). The old 24-inch Cinema Display I still use is 94ppi. Also of note, OS X handles pretty much any aspect ratio you throw its way. Most of Apple’s displays are now 16:9 or 16:10, but a few crazy people like Craig Hockenberry prefer a portrait orientation (10:16) as shown here: Craig’s setup.
The point is, PPI is much less relevant on OS X than on iOS. To create Retina displays, Apple doesn’t have to build displays that are exactly 2X current displays, they just have to build displays that work well with OS X when running @2X. For example, the current 27-inch iMac is 2560 by 1440 pixels, which translates to 109ppi. Doubling that to 5120 by 2880 pixels is not strictly necessary. Such a screen might be incredibly difficult to manufacture, and therefore incredibly expensive. Instead, Apple could build a 3840 by 2400 pixel 27-inch screen that presented itself as a pixel doubled 1920 by 1200 pixel display. That’s effectively an 84ppi screen @1X and 168ppi screen @2X. Here’s what that would look like:
Compare that to the current 27-inch iMac which, at the same scale, looks like this:
What you should notice is that the text and UI elements are physically smaller on the current 109ppi iMac than they’d be on the hypothetical 84/168ppi 27-inch iMac. This may be frustrating to some users, but I actually prefer my old 94ppi 24-inch Cinema Display to any of Apple’s higher PPI displays. I like that the system default 12pt text is larger. The sacrifice is in the usable workspace, and that’s a matter of taste. I’ve been hearing from more and more people on Twitter that the 11-inch Macbook Air is surprisingly usable with OS X Lion, even though the workspace is a scant 1366 by 768 pixels. A display that presented itself as a pixel doubled 1920 by 1200 pixels, would be usable for developing Retina display iPad apps. A pixel doubled 2560 by 1440 pixel display might be preferable to some users, but shaving off those extra pixels could make the displays significantly more affordable.
I’m not going to go device by device, but you can take the same logic for every other display intended to run OS X… find a low, but comfortable PPI, then double it, rather than assuming every display will just be double its current resolution. Given the fact that some of Apple’s displays already mathematically qualify as Retina displays, I don’t expect Apple will be in a huge rush to replace every display in their lineup. Unless Apple has a major manufacturing breakthrough they’ve been sitting on, I think we’ll see a couple low PPI, pixel doubled screens this year and a slow transition of all Apple’s displays over the next few years.