Prior to retina display Macs, OS X implemented resolution independence as an arbitrary scale factor, with most of the UI elements being vector rendered. You could test it out with Quartz Debug. TextEdit, for example, scaled very nicely and remained crisp at arbitrary scale factors.
They didn't end up using this approach because the 2x mode is so simple, and will likely be the last scale factor really needed (especially with the additional scaling modes).
They didn't end up using this approach because the 2x mode is so simple, and will likely be the last scale factor really needed (especially with the additional scaling modes).