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Comment by user_7832 | Hacker News Reader

Just a mini-warning/FYI: running the 120hz test on my 60hz LCD IPad (Air 4) has caused that part of the screen with the crt effect to flicker even after leaving the demo. I don’t know what might cause this but it’s weird and worth a warning to anyone interested in trying this out.

(The flickering is more obvious when the control centre is opened, I managed to take a video of it but it’s only partially clear in it. It’s been about 5 minutes so far and I think the effect has reduced. I’m also quite perceptive to flickers so others might not notice it.)

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This is a well known effect. LCD cells must be driven with alternating positive and negative values (of the same magnitude) to maintain an average neutral value, otherwise you get some kind of offset buildup that will result in flicker.

If you alternate every other image with a different color value, you upset that balance.

It will slowly rectify itself for most displays.

Thanks for explaining, I've run into this on my phone in other contexts and I was starting to think my phone screen is having its last days. Turns out it's expected? Usually I run into this when the screen brightness is at the lowest setting.

Screen brightness is usually modulated by changing the strength of the backlight, not the values sent to the LCD cell array. So flicker induces by inversion doesn’t change when changing brightness.

(There are exceptions: one could dial up pixel values on a dark scene and dial down the backlight settings to save power. But that depends on the image content.)

I think some OLED displays don't have backlights?

On manipulating the backlight to display a dark scene more power-efficiently: my TV that does this, if there's a small region of constant color (e.g. a TV station's logo) its brightness wavers noticeably as the rest of the scene changes.

So Snow Crash[0] does affect both humans[1] and computers!

[0] https://en.wikipedia.org/wiki/Snow_Crash

[1] https://en.wikipedia.org/wiki/McCollough_effect

Damn it, now I have ro read another Stephenson book.

Snow Crash is the Hackers (1995 film) of Stephenson's body of work. It's so aggressively 90s and cyber techno that it seems somewhat adorably cheesy in retrospect, but there's an audience of Z-ers who are just now discovering it, and what it means to be excited about technology the way we were back then.

Google needs to retarget on being the Central Intelligence Corporation they were always meant to be.

Ah, is this why VRR displays start to flicker once your framerate drops too low? I had always wondered if it was a physical property of LCDs

That’s partially right. There’s also the issue of decay due to the LCD cell not being refreshed. It’s similar to not refreshing a DRAM cell.

But the flicker in VRR doesn’t only happen at lower frame rates. Some panels are more susceptible than others. It’s a headache.

It’s also a serious issue when doing 3D stereo on an LCD panel and having a static scene: alternating frames will display left and right views which may be pixels of different color.

> LCD cells must be driven with alternating positive and negative values (of the same magnitude) to maintain an average neutral value

This is called inversion and there are interesting web pages on the topic:

* http://www.techmind.org/lcd/

* http://www.lagom.nl/lcd-test/inversion.php

* https://pixelinversion.com/

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