[mhenr18]

Let's say you have image assets in two sizes - 1x and 2x. If you want to show an image at 1.5x, you don't upscale from 1x, you downscale from 2x so that you have a crisper image.

If you're looking for a practical application of this, watch a YouTube video fullscreen that has 720p and 1080p options on a display that's 1680x1050 or anything in between the two video sizes. The 1080p version is better looking because it's scaled down, not up.

This is the concept behind running a 3820x2400 framebuffer and downscaling it to 2880x1800 for display - you're getting a crisper image because of the use of 2x drawing by the system for text and standard UI elements. Of course, third-party bitmaps aren't going to benefit from this until they're updated, but Apple developers have been through this before with retina displays on iOS devices.