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Comment by jrochkind1 | Hacker News Reader

> It will probably take a little while for Mac apps to be updated to support the ProMotion display. I found that some apps scrolled text in the same buttery-smooth way that apps do on my iPhone and iPad, but others didn’t. I was able to find a few Catalyst-based apps that have already been written to support 120Hz displays on iPad and iPhone, and they looked spectacular.

It surprised me that an app would have to be updated to support a better display. Anyone know the details, and nature of the update that has to be done?

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I think it depends on the framework - as part of the ProMotion library isn't automatically running everything at 120 FPS but dynamically scaling the framerate up/down based on demand and requirement so that it can balance the additional power consumption that 120fps drives.

Because of this, apps that don't update for ProMotion might be capped at 60hz (or something like that) so that the laptop doesn't have to run everything at 120hz if one app isn't updated.

It's not just a better display, it's a software system that dynamically adapts the framerate based on what's happening with the content at a given moment

I am still a little surprised it requires app support, but not shocked

For iPhone 13, you need to 'opt-in' for 120 FPS by adding a key to your app's Info.plist file. This is probably something similar.

I don't know about macOS, but on the Web introduction of 120Hz has been a compatibility issue. Lots and lots of implementations assumed `requestAnimationFrame` fires 60 times per second (treating it as a "tick" for game loops or anims), so the event frequency had to be capped.

I don't know the details, but the idea behind ProMotion is that the display dynamically changes refresh rate based on need. So, apps would need to tell the OS when they want a higher refresh rate to display smoother motion.

Anyone know the details, and nature of the update that has to be done?

I don’t know what is happening specifically with these apps (we’d have to look at their source code on a case by case basis) but to hypothesize:

Since 60hz has been a de facto standard on the Mac for decades I can imagine that a lot of apps may have a hard-coded 60hz timer for updating their window contents. This would completely negate the advantages of a 120hz on things like scrolling smoothness. These apps would need to be updated to not assume 60hz and instead update their window contents when requested to do so by the system. Furthermore, some applications which generate their window contents on demand (based on their internal data structures) may need new algorithms if they are too slow to update at 120hz. I know that many applications (including web browsers) I’ve used over the years can’t even keep up with 60hz updates when scrolling through a complicated document.

It is about power consumption.

They could, if they wanted, automatically opt existing applications to use 120 Hz refresh. But the power usage would be terrible, and it would be up to application authors to manually go in and tune it down (which many would not do, or would take too long).

So instead they keep them at the current refresh, and have them "opt-in" to higher. Which I'm imagining Apple is hoping application developers will do responsibly and only ask for 120 Hz when it is beneficial (although big YMMV).

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