Apple is finally getting serious about resolution independence on displays. In the Mac OS X Reference Library there is reference titled Resolution Independence Guidelines:
Resolution independence allows greater flexibility with high-density displays by allowing a Quartz point to map to any number of pixels (or fractions thereof). You can no longer assume a 1:1 correspondence between a Quartz point and an onscreen pixel…
Many years ago a typical LCD sported a resolution of about 72 PPI. A Quartz point had a scale of about 72 units per inch and one Quartz point mapped to about one pixel. Not any more:
One a 144 dpi screen, a 200 x 200 image is one quarter the size of a comparable image on a 72 dpi screen.
Of course in displays we typically use pixels per inch or PPI. But the point here is that developers cannot assume a 1:1 ratio between a point and an onscreen pixel. Apple is introducing The Scale Factor, which is a new parameter that determine the ratio between a point and an onscreen pixel. At the moment a scale factor of 1.0 corresponds to 72 PPI. A display with 144 PPI will result in a scale factor of 2.0. Look at the example below. The physical font size stays the same between the two displays but with the higher resolution 108 PPI display with a 1.5 scaling factor the font is made up of more pixels and is much smoother.
I’m very excited that Apple is moving toward resolution independence with its developers. This will in turn make higher resolution displays eminently more valuable. via Daring Fireball