Why 960 × 640

Ever since I hinted last month that this summer’s next-gen iPhones would sport double-resolution 960 × 640 displays,1 there’s been a strain of incredulous feedback regarding this suggestion — emails from readers who don’t understand why Apple would do this. The gist of the feedback, more or less, is that the existing iPhone already has a nice high-resolution display, so why would Apple bother adding a more expensive display when the existing one is already so good?

Existing iPhone and iPod Touch displays offer 480 × 320 resolution, which, given a roughly 3.5-inch diagonal screen size, works out to about 162 pixels per inch. (The various iPhone and iPod Touch models have ever-so-slightly different physical sizes, so if you want to be truly pedantic, the pixels-per-inch on the various units released to date have ranged from around 160 to 163. The difference is insignificant.)

I put this table together in June 2007 showing the pixel-per-inch resolution of Apple’s then-current lineup of Mac displays; the highest was the high-res 17-inch MacBook Pro, at 133 ppi. (The optional high-res glossy and matte displays on the brand-new 15-inch MacBook Pros now match this.) Most others came in around 100-110. The iPad’s 1024 × 768 display has a resolution of 132 ppi — finer than most Macs, but noticeably cruder than existing iPhones and iPod Touches.

There are Android phones with 800 × 480 AMOLED displays. They measure around 3.7-inches diagonally, which works out to about 250 ppi. Small text is noticeably crisper on the Nexus One than on my iPhone 3GS. This Android advantage is mitigated, alas, by two factors. First, that Android only ships with the Droid family of typefaces — low-quality typefaces designed to look best on lower-resolution displays. The effect is sort of like when you print a screen-optimized font like Chicago. Second, it can be argued that because each pixel on these AMOLED displays is not capable of reproducing every color, they’re not really 800 × 480. These displays have 33 percent fewer sub-pixels than a display of the same size where every pixel contains three (red, green, blue) sub-pixels.

So 162 pixels-per-inch is indeed a very fine resolution when compared to most other electronic devices. But even today’s iPhone displays are crude when compared to print. Compare type on your iPhone or iPod Touch against the type in a glossy magazine. You can see pixels and anti-aliasing smudginess on an iPhone. You can’t see “dots” in the letterforms of a printed magazine.

The next-gen iPhone is shooting for that caliber of resolution — not merely to exceed the resolution of competing devices, but to rival the optical quality of print. A 3.5-inch diagonal display with 960 × 640 pixels works out to around 325-330 ppi. (Maybe even higher, if it’s more like a 3.4-inch diagonal.) That’s fewer “dots” per inch than on, say, a 600 dpi printer, but when combined with anti-aliasing, I believe the on-screen typography on the next-gen iPhones will be indistinguishable, or nearly so, from high-quality print. There will be four pixels packed into the space now occupied by a single pixel.

What This Means for Developers (Spoiler: I Don’t Know)

I’m not sure what the software story will be for iPhone apps to take advantage of this increased resolution. If existing unmodified apps run pixel-doubled, they should look identical with the naked eye to how they look today on existing iPhone displays — not at all like when iPhone apps are pixel-doubled on the iPad, because on the iPad, they’re being stretched not just in terms of pixels but also to fill a much larger physical area.

I’m also utterly unsure what the story will be for iPhone web developers. I don’t think that MobileSafari will be able to continue mapping the “px” unit to physical display pixels, because the disparity between the physical pixel size of old and new iPhones is going to be enormous. My guess, and this is just a guess, is that MobileSafari will switch to a system where “px” represent virtual pixels, which on existing iPhone map directly to physical display pixels, and which on new 960 × 640 iPhones map to 2 × 2-pixel squares that occupy the same physical dimensions as individual pixels on 480 × 320 displays.

I was hoping for clues to these resolution-scaling questions at the iPhone OS 4.0 SDK event earlier this month, but no luck. Apparently we’ll have to wait until WWDC in early June.