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Toot

Written by fasterthanlime 🌌 on 2025-01-19 at 19:43

Hey, silly question: how do you know the real "display delay" when dealing with something like https://lib.rs/crates/egui on macOS?

I know what the audio delay is — the APIs report that. But when I make draw calls in egui, I have no idea how far in the future they'll show up. Should I assume one 60Hz frame? Two? Less? More??

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Written by fasterthanlime 🌌 on 2025-01-19 at 19:45

The audio delay (caused by playback buffer size) is about ~10-11ms on my Apple computers right now, I want to offset audio visualizations by that much to synchronize better.. but if the display is ~16ms late it would actually make things worse? I'm not sure I can tell when a frame is presented?

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Written by seri on 2025-01-19 at 19:48

@fasterthanlime This sounds like a beautiful beginning to a new 3h reading time blog post, I can't wait to read it.

May cool bear be with you 🙏

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Written by geist on 2025-01-19 at 19:50

@fasterthanlime I think this depends on a lot of intermediate factors that can ratchet up the latency, especially the display. John Carmack talks about that here, where he had to resort to a synchronized high speed camera to tell https://superuser.com/questions/419070/transatlantic-ping-faster-than-sending-a-pixel-to-the-screen

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Written by Tristam on 2025-01-19 at 19:56

@fasterthanlime have you poked at Apple’s GPU profiler? I seem to remember there was some frame latency data in there.

In general this is fairly hard to predict. It depends how many frames are in flight at any given time, and that depends on how loaded your system is…

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Written by Jackie Officecouch on 2025-01-19 at 20:43

@fasterthanlime there are various ways of getting the targeted presentation timestamp depending how the drawing is actually happening (e.g., CADisplayLink.targetTimestamp, or CAMetalDisplayLink‘s callbacks give it to you).

I don’t know how that fits into egui, and it likely depends on what’s driving the render loop

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Written by OneSadCookie on 2025-01-19 at 21:01

@fasterthanlime this is the problem CVDisplayLink theoretically solved… but it seems to be deprecated in Sequoia?

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Written by fasterthanlime 🌌 on 2025-01-20 at 07:57

@OneSadCookie yeah there's CADisplayLink and https://developer.apple.com/documentation/quartzcore/cametaldisplaylink now

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