via John Gruber. David Barnard:
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″ iMac is 2560×1440 pixels, which translates to 109ppi. Doubling that to 5300×2880 pixels is not strictly necessary. Such a screen might be incredibly difficult to manufacture, and therefore incredibly expensive. Instead, Apple could build a 3840×2400 pixel 27″ screen that presented itself as a pixel doubled 1920×1200 pixel display. That’s effectively an 84ppi screen @1X and 168ppi screen @2X.
Let’s get this one thing out of the way: Is the existence of HiDPI modes a clue to future Retina-class displays on Macs? It depends on your definition of a Retina display on the Mac. But I submit the answer is generally no, because in general we think of a Retina display as having double the number of pixels horizontally and vertically. The truth of the matter is HiDPI modes have little to do with Retina displays on OS X.
HiDPI modes make use of 2x the number of horizontal and vertical pixels to render screen elements compared to normal DPI modes. And the link from HiDPI modes to pixel-doubled Retina displays stems from the expectation that displays will have double the horizontal and vertical pixels to be classified as Retina. This is wrong. Right now there are several Macs that already are Retina-class. Read Richard Gaywood for a solid analysis and a list of Retina-class Macs.
We need to disassociate HiDPI from pixel-doubled Retina displays in OS X. A Retina display does not mean double the resolution (ppi) of a non-Retina display. We stumbled into this expectation because of what happened in iOS. Pixel doubling and its association to Retina happened when the 3.5-inch 480×320 LCD in the iPhone 3GS doubled the number of pixels to 960×640 in the iPhone 4. This will happen again when the 9.7-inch 1024×768 iPad 2 goes to 2048×1536 in the next iPad. So the association with pixel doubling and the Retina classification will unfortunately get stronger, but this will not happen on devices running OS X. There is no need. To get to Retina only incremental improvements are necessary. In The Resolution Gap I suggest possible pixel formats for a future lineup of Retina-class MacBooks, and not one require pixel doubling.