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Comment by ridiculous_fish | Hacker News Reader

I worked on the Cocoa frameworks back when the hope was arbitrary scaling. The short answer to your question is "rounding." At 1.5x, a box of length 3 occupies 4.5 physical pixels. Either one side of the box will be blurry, or the box must get resized to align with physical pixels, which can cause mis-centering of text or icons, or unexpected clipping or truncation.

There was also a lot of custom layout code in existing apps, both first and third party, which just didn't work at arbitrary scale factors because it was never tested.

Yet another complication is that apps are expected to handle scale changes, e.g. if the user drags a window from one display to another. Android doesn't have to deal with this.

After years of trying to make it work, Apple basically gave up and did the scaling in WindowServer.

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> I worked on the Cocoa frameworks

The thing I love about HackerNews is how you can casually run into people who worked on things you've been using every day for the last 10 years.

> The short answer to your question is "rounding."

Yeah, on Android, you're supposed to round yourself. View dimensions are integers, so you have to, but graphics APIs take floats, you don't strictly have to, but you want to anyway. Even though there's no such thing as pixel-perfect on AMOLED displays thanks to their subpixel layout. When you use XML and specify `25dp`, the system does the rounding.

> Android doesn't have to deal with this.

Oh it does! Though it deals with this in a lame way: your activity with all its views gets destroyed and recreated for a "configuration change" unless you've explicitly specified that you can handle this specific kind of change. Density doesn't change often, but the possibility is certainly there.

I remember how around 2013 there was this bizarre device. It was a phone, but it came with a tablet you could dock it into. The tablet contained only a screen and a battery. The screens on the phone and on the tablet had different densities. There were bug reports about my app not handling these transitions correctly, presumably because I was dumb back then. I can imagine something similar happening with the new hotness, foldable devices, where the external and internal screens could have different densities.

And while I have you, one question. Why does macOS have the coordinate system origin in the bottom left? I've always found that bizarre, especially when iOS does use the usual top left.

Ah yes, the Hi-DPI days. I left Apple before all of that was sorted out. I assume all of the code dealing with scaling was just removed from the frameworks?

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