How G-sync switches between ON and OFF based on frame time?

Talk about NVIDIA G-SYNC, a variable refresh rate (VRR) technology. G-SYNC eliminates stutters, tearing, and reduces input lag. List of G-SYNC Monitors.
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Arx
Posts: 14
Joined: 12 Jul 2020, 16:53

How G-sync switches between ON and OFF based on frame time?

Post by Arx » 21 Jul 2020, 16:42

When the frame rate fluctuating above and below the monitor refresh rate, I wonder, what is the mechanism to dynamically active G-sync.
(This topic is based on the following setting: G-sync is enabled, and V-sync is disabled, and frame rate is not capped.)

Let's assume a case where our frame rate changes, and based on frame rate, we have 3 stages:

<stage 1>:
frame rate is below the refresh rate
At this stage, G-sync is active, and monitor's vertical scan is synchronized with rendering buffer

<stage 2>:
frame rate is above the refresh rate
At this stage, G-sync is suspended, and monitor scans as fast as it can, and rendering buffer is updated in the middle of a full scan. (tearing)

<stage 3>:
Continue from stage 2, suddenly the GPU takes more time to render frames, and frame rate drop below refresh rate again.
You might think, okay, now we turn on G-sync immediately, easy right?
Wait a second, when this slower rendering frame goes into front buffer, the monitor is at the middle point of vertical scan. So, we cannot just start vertical scan again from the very top of screen!
Then how would G-sync do in this case? Should it is still be suspended and let tearing happens (for lower input lag but more tearing)?

I draw a picture of the above stages, you can find it in the link below:
https://docs.google.com/spreadsheets/d/ ... edit#gid=0

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Note that:

This topic is not to discuss how to setup G-sync properly. I know people prefer to use G-sync with frame rate cap below refresh rate, some of them even would enable v-sync to further eliminate tearing.
This topic is just to discuss the mechanism behind the G-sync.

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