[SOLVED] Scanlines Illusion

I’m new in the shaders world and i don’t know much about it. Using some shaders from libretro i have noticed that the scan lines seem to disappear from static objects when the game camera moves. When it’s moving left-right the vertical lines seem to disappear and when moving up-down the horizontal lines seem to disappear. They do not really disappear it’s mostly an illusion, they are there and if i focus i see them clearly.

Can someone confirm this effect of camera movement or it’s just my dumb eyes? I do not have a CRT screen on my hands right now and i can not check if this effect is present on the hardware too.


I think @MajorPainTheCactus has confirmed that this is an illusion. Feel free to correct me if I’m wrong.

You can see the effect in this page:


If you focus your eyes on the yellow UFO at the top, you can clearly see the lines. If you instead focus on the moving UFO at the bottom, the lines will disappear as if they’re not there.


Thanks! I think this is my answer. The brain it’s a weird thing.


Just to add a little to @RealNC’s answer is that you may (probably will) see this effect more on a LCD than a CRT.

LCD’s are what’s called ‘hold and sample’ displays that is the display retains the image until the next refresh cycle.

CRT’s on the other hand are what’s called ‘impulse’ or ‘strobed’ displays and the phosphors on them are lit up for only 1-2ms and the rest of the time they are black - your brain making up the rest.

What happens with a sample and hold display is that your eyes have enough time to move a noticeable amount during a refresh cycle and so your brain smeers each frame slightly. Impulse displays don’t do that (as much).

So you have the persistence of vision happening at multiple levels. A few LCDs/OLEDs now have what is called backlight strobing which can simulate to a degree what a CRT does but it comes at the cost of brightness. We need very bright screens (1000-2000nits?) to overcome this pitfall.


I’m able to take advantage of BFI with 350 paper white (HDR 700). It looks very nice and bright in a moderately lit room.

HOWEVER, if there is a lot of flashing or a lot of white on the screen at anytime, the white pixel and BFI don’t seem to get along and results in strange dark strobing on the bottom of the screen. This can be seen here: https://1drv.ms/v/s!AmQUGfXPPtq7gw3q-vVZo46cCI5G

Took me a bit to figure it out.

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No idea what that link contains. It requires a login.

I’m able to open it in privacy mode. It’s just a video hosted on my onedrive instead of youtube or something. You don’t need an account.