Resolution Independent UI can be used to provide the user with either detail (more pixels per point, but fewer points on the screen), or real-estate (fewer pixels per point but more points on the screen). To do so the graphics system and user interface require an additional parameter to govern this preference. We call this additional parameter “scaling factor”. In future release of Mac OS X we expect this parameter to be settable by the user the same way a user can today go to the Displays Preferences Panel to change the screen resolution.

Let’s assume that on a 1600×1200 pixel display we want to target the basic “look” of our current user interface at 1024×768, which assumes a 72 virtual dpi. To take up the same fraction of the display, the UI would need to be larger by a scaling factor of 1600/1024 = 1.5625. Note that from a distance, this desktop would look identical to today’s 1024×768, but upon closer inspection the 1600×1200 display would be much more detailed. If instead, we wanted to make the display look like today’s 800×600 displays, we would run our 1600×1200 display at a virtual dpi of 144 with a scaling factor of 2, which would be highly detailed using 4 times as many pixels to represent the user interface.

(Apple Confirms Resolution Independence in Leopard, YAHOOO!)