Friday, February 26, 2010, 10:00 pm PT (01:00 am ET)
Inside Apple's iPad: iPhone OS vs Mac OS XApple's new iPad is facing criticism for using the streamlined iPhone OS rather than the full Mac OS X, but Apple has a number of reasons for aligning the iPad with the iPhone rather than its desktop Mac platform.
While Apple's new iPad was widely anticipated to run the iPhone OS well before the company officially announced it, some observers have decided that it would make more sense for the tablet device to use the full version of Mac OS X instead, in order to enable the device to run a wide array of existing desktop applications.
Such a strategy would have resulted in the iPad looking more like Microsoft's Tablet PC and OMPC devices, which run Windows XP/Vista/7 with some extra software that enables stylus and or touch features.
Resolution problems in mobile devices
A primary problem with running a desktop operating system on a mobile device is that it would devote a lot of the available screen real estate to elements designed to support a mouse-based interface. While the iPad offers the same 1024x768 display resolution as the first iMac models, it squeezes those pixels into a 132dpi, 9.7" screen rather than the original iMac's 15" (13.8" viewable) display.
As screen pixels get packed together more densely, the targets of interface elements that were originally designed for a mouse pointer get smaller. But the iPad's multitouch screen is designed to be navigated by fingers, which are larger and less precise at pointing than a mouse cursor.
This means that in order to be usable, interface elements such as buttons, the menu bar, and window frames all have to be drawn larger than they would be on a conventional desktop computer at the same resolution. But since the resolution available on a mobile device isn't any greater, the user would end up with much less usable area for content, such as their browser page.
Rethinking for mobile constraints
Rather than delivering the iPad as a slower notebook with less usable area for content (which is essentially what Tablet PCs deliver), Apple used the iPhone as a starting point for creating its interface.
The iPhone already uses a relatively low but dense resolution, so the finger-optimized human interface guidelines Apple developed for it make much more sense on the iPad. Rather than being a dumbed down notebook, it's a scaled up iPhone.
In addition to thinking about usable screen resolution and interface target size, Apple also devoted a lot of thought into how the iPad would handle a multitouch interface, evolving upon the model set by the iPhone.
Applications created for the Mac OS X desktop all assume the use of a mouse or trackpad to navigate. While a few touch elements could be added to the mix as Microsoft has done with Windows 7 and as Adobe is working to do with Flash, all existing desktop software would have to be rewritten to take advantage of this new functionality, because existing titles simply wouldn't work as designed without a mouse pointer.
Cocoa Touch: no mouse pointer
For the iPad, Apple used the new Cocoa Touch platform created for the iPhone to deliver an interface that isn't just coated with a thin layer of touch functionality, but is designed from the ground up to be piloted by fingers. The differences are subtle, but significant.
With a mouse pointer, the user is constantly pointing at targets, and can select or open icons by clicking. In a multitouch environment, the system usually isn't aware of what a user is pointing at until his or her finger makes contact with the screen, which typically needs to be recognized as a click.
It quickly becomes complex (and increasingly less intuitive) to try to mimic mouse behaviors with touch. Rather than trying to emulate mouse pointer behaviors via touch and various combinations of gestures, Apple designed Cocoa Touch to intuitively respond to finger touches the way users would naturally expect to interact with the screen.
This resulted in Cocoa Touch being an entirely new platform developed around touch rather than a poin