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.