Windows XP at 2560x1600 is ok on my 30" monitor, but it would be next to unusable on a 15" display. The Icons didn't scale to the resolution, so they would have been tiny.
2560x1440 on my 27" iMac is ok, but I have to use CMD-+ on almost all web pages. Only in Safari is Magic Touchpad zooming smooth and iOS like, but I'd like to use Chrome.
NeXT had bitmap icons (TIFFs) displayed using display postscript. The problem with vector graphics in icons is that you'll still need multiple versions for different resolutions or the icons will look terrible at most resolutions, and then it's horribly inefficient to render them on-the-fly all the time so you'll end up caching them in memory and rescaling them... and -- oh look -- bitmaps.
Well, eventually it's going to come out as a bitmap—they'll still end up as pixels on the screen. Vector icons just give you the flexibility to do it really really well. Cache them, prerender them, I don't care what works best, just match the icon DPI to the screen DPI and we're good.
His point is that it's not good enough to simply match the DPIs and render. Small icons need actual visual differences, not just scaling, in order to really look good. Some icons completely change their character at small sizes, because what looks good when small is not the same as what looks good when large, regardless of DPI. So going pure-vector doesn't completely solve the problem, although it certainly can make it easier.
There is that, and there is also the quality of how aliasing is handled by real time vector art renderers. We just aren't their yet to be relying on vector art for icons (even though they start out that way in Illustrator).
That's true at low-DPI, but not so much at high-DPI. I wonder if a bundle of low-DPI bitmap + a vector for high-DPI would work better than the standard practice of bundling multiple bitmaps...
I think it's still true, although less so. Certainly a certain amount of fiddling takes place in an attempt to align to pixels etc., but sometimes icons just need to change at different physical sizes. Small elements may need to be enlarged to remain visible and such.
Quartz2d is effectively "Display PDF". Bitmap icons are necessary (at least in the icon portfolio) as they can degrade more gracefully at lower pixel counts.
Lots of screen real estate though.