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Comment by marknadal | Hacker News Reader

Why does when I scroll my eye perceives the background pattern as flashing different colors like purple (?)

Is this a perceived effect, or something generated by my monitor framerate/refresh, or intentional?

How is this possible, if/when the pattern is purely grey when standing still/static?

Was wondering the same thing and just used a screen recorder to try and find out. Still can't answer why it happens, but in the recorder, if I flip from one frame to the next, the flash happens and it's impossible to freeze frame "mid-flash."

This means its either an optical illusion or something with the monitor refresh rate. My money is on the monitor.

I think it’s a Moire effect from the patterned background being resized and no longer 1:1 with actual device pixels. Changing zoom level and then scrolling on my iPad changes the appearance of the flashing.

How is this possible, if/when the pattern is purely grey when standing still/static?

I don't know if that's what you're seeing but there is a well-known, poorly-understood effect just like it.

https://en.wikipedia.org/wiki/Benham%27s_top

Perhaps record video (with phone), see what happens in the playback. Or look at the screen pixels with a loupeor magnifying glass. Human visual system is kinda weird so could be either.

Looking at my monitor pixels with a good loupe, the gray is made of red+green pixels in a Bayer pattern. So the scrolling is making a purple effect, probably in our aforementioned weird human visual system.

update: Bayer pattern is wrong term. The MacOS background is a red+green checkerboard. The HN background is a vertical stripe of red+green.

There was an application for the early B&W Macs that used motion to try to create the perception of colour. It didn't really work for me but is probably the same effect.

My guess: scrolling a checkerbox pattern requires turning on and off the rgb components of the pixels at a high speed. The blue components are the slowest so they linger for longer.

The part of an LCD that turns on and off is separate from the layer that imparts color to the subpixels. If the blue subpixels have a slower response time than adjacent subpixels, then there's something quite odd about the display's design.

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