Resolution independence, or the lack of it, is one of those nagging problems most users don't even realize have a name. But the concept is simple: user-interface elements like icons, buttons and window borders on the same OS should be the same physical size no matter what screen you're using.
From the 1980s until just a few years ago, the gold standard for computer screen resolution was 72 dots per inch (dpi). This wasn't an accident.
"When the Mac first came out, one of its great WYSIWYG features was that a pixel on the screen was supposed to be equal in size to a printer’s point: 1/72″," says Mac blogger Dr Drang. "Back then, onscreen rulers matched up quite well with physical rulers, and 12-point type on the screen looked to be the same size as 12-point type on the printed page. But those days are long gone."
Manufacturers can fit an ever-larger number of pixels onto screens. This is generally a good thing, as it makes images sharper, clearer and more like physical objects. But it also makes anything defined by its pixel-count resolution smaller.
Operating systems, including Mac OS X, began to move away from 72dpi in the middle of this decade. "The old assumption that displays are 72dpi has been rendered obsolete by advances in display technology," Apple said in 2006, in a developer overview of OS X 10.5 Leopard. "But it also means that interfaces that are pixel-based will shrink to the point of being unusable. The solution is to remove the 72dpi assumption that has been the norm."
Leopard and then Snow Leopard were supposed to do away with pixel-defined resolutions, allowing developers to draw user interface elements using a scale factor. But while screen resolutions kept getting sharper, resolution independence never quite came.
That is, it never quite came for the desktop. For iOS, resolution independence is essential, mostly because the UI elements need to match our bodies. On the desktop, if icons get smaller, well, pointers and cursors get smaller too. Your fingertip is always the same size.
But even on the iPhone and iPad, resolution-independence is only partial. Yes, icons might register at the same size, but images within the application don't. Developers who built a pixel-defined app for an older model iPhone find those apps not looking quite so sharp on the higher resolution of a retina-display iPhone 4 or blown up onto the larger screen of an iPad.
For Dr Drang, the absolute size of interface elements matters less than their variability. "On an 11-inch MacBook Air, a 72-pixel line—which would measure 1 inch long against an onscreen ruler—is just 0.53 physical inches long. On a 21.5-inch iMac, that same line is 0.70 inches long. User interface items, like buttons, menu items, and scroll bars are 30 percent bigger on the iMac than on the Air."
Application developers are necessarily conflicted. Keeping UI tied to pixel counts saves them work rewriting their apps. On the other hand, they can't count the physical uniformity of experience across every device. Desktop publishing and design pros also have to factor in differences in size from the screen to the page, or one screen to the next. Images and text all materialize differently.
"Microsoft has universal settings to change the size of UI elements," Dr Drang adds. "Even X Windows allows you to set a screen dpi for fonts. Apple has nothing. With screen resolutions increasing at an accelerating pace, this has to be addressed soon."
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