Originally Posted by Chronoptimist
Variable Refresh Rates are incompatible with low-persistence strobing in its current state, so you aren't going to see it used with VR. It's theoretically possible to combine the two if we get some breakthrough that makes displays at least an order of magnitude brighter than they are today, but unlikely to happen.
How is the current frame rate related to the image persistence and thus OLED peak brightness?
If your current framerate is 90hz (~11.1ms frame time), you show each frame 50% of the time, or for ~5.55 ms.
If your current framerate is 30hz (33.33 ms), you show each frame 50% of the time or for 16.66ms.
If your current framerate is 60hz (16.66 ms), you show each frame 50% of the time of for 8.33ms
The peak OLED brightness of a 50% strobed is 2X. That number doesn't depend on the current framerate, it's a fixed number with a unit in lumens. All that's necessary to do VRR + strobed is the ability to modulate the amount of time a signal is shown to the OLED. Are you saying they can't do that? I'm not an OLED engineer so I honestly don't know.
Then again, with PWM being fairly easily achieved for LED backlights, I'm still surprised they haven't sorted out ULMB + VRR yet.
ULMB of 50% or 10% frame time simply means the PWM frequency is determined by the peak refresh rate, not the lower refresh rates.
Put it this way, if you can strobe fast enough for a 90hz HMD, you can definitely strobe fast enough for a 60hz HMD, or 32hz, or 78hz, or whatever number in between. VRR is simply treating the HMD as if it has a different spec, from one frame to the next.
The "toughest" strobe time to handle is the highest refresh rate. I don't see any technical reason you couldn't combine ULMB or strobed OLED with VRR. I mean, with PWM I definitely don't. Not sure about OLED. You can definitely modulate the backlight of your LCD for 16ms this frame and for 8ms the one after.
With VRR you'd have to show each frame starting at the beginning of the frame duration as "ON" and for the latter part as "OFF", to both minimize input latency and to allow for a variable "ON" vs "OFF" time. If your framerate fluctuates wildly you should be able to apply an alpha smoothing filter in order to predict the current frame's duty cycle and avoid flickering.
Any local brightness variations occurring above 60hz should be smeared in your retina, for the same reason that FRC works.