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DIY Backlight Strobing [must read for experimenters!]

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Re: DIY Backlight Strobing [must read for experimenters!]

Postby HeLLoWorld » 15 Jan 2014, 13:21

Yes, it's funny we need to further isolate the discretized "universe frames" even further, while real life is continuous, but that's because the frames stand for infinitely small moments, and the inbetween values we get are false, so best is to just send nothing rather than wrong; the mind will have less trouble filling the blank than dealing with errors :)
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Re: DIY Backlight Strobing [must read for experimenters!]

Postby HeLLoWorld » 15 Jan 2014, 13:26

Curtain is by analogy to the curtain that fall in theaters at the end of a play btw, but it seems not to translate.
Anyway even without having to explain these subtle effects, the mere fact the updates descend top down is intuitively not natural, the moving update line is maybe consciously or unconsciously perceptible. That's why I talked about random or algorithmic updates the other day too.
Last edited by HeLLoWorld on 15 Jan 2014, 13:27, edited 1 time in total.
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Re: DIY Backlight Strobing [must read for experimenters!]

Postby Chief Blur Buster » 15 Jan 2014, 13:27

HeLLoWorld wrote:Yes, it's funny we need to further isolate the discretized "universe frames" even further, while real life is continuous, but that's because the frames stand for infinitely small moments, and the inbetween values we get are false, so best is to just send nothing rather than wrong; the mind will have less trouble filling the blank than dealing with errors :)

Yes errors such as eye-tracking-based motion blur caused by your eyes being in different positions at the beginning of a refresh cycle & the end of a refresh cycle. This is now explained in my brand new post, Understanding Persistence: strobed & nonstrobed, CRT vs LCD.

This motion blur bigger problem to me, than the curtain effect / tilt effect, and is the rasion d'etre of Blur Busters -- It's why I created Blur Busters.
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Re: DIY Backlight Strobing [must read for experimenters!]

Postby HeLLoWorld » 15 Jan 2014, 14:59

Just realized that randomized or algorithmic pattern order of the update of pixels in one frame (for multiplexed oled for example, crt is not doable and lcd-tft do not blink individually) (also suggested by Carmack if I remember) would unfortunately be very bad for this reason : it would randomly distribute the age of each pixel across the screen, while the picture is from an instant timeframe. When tracking a moving background it would be a mess, far more than top-down where at least you got some local spatio-temporal coherence.

Which is to say, top-down seems ugly because it's non-isotropous, but it's the least sucking way of doing it.
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Re: DIY Backlight Strobing [must read for experimenters!]

Postby Chief Blur Buster » 15 Jan 2014, 15:05

HeLLoWorld wrote:Which is to say, top-down seems ugly because it's non-isotropous, but it's the least sucking way of doing it.

Yep, you got that right. Good on you for figuring that out so quickly!

Multi-scan artifacts are ugly. I've seen panning motion on an old Sony Jumbotron, and it's not pretty -- it creates sawtooths along vertical edges. (Newer LED jumbotrons are now too high-frequency PWM and more resembles sample-and-hold displays, at least during full brightness during daytime). The same thing happened with old Dual-Scan LCD's from more than 10 years ago; horizontal panning motion (that's refresh-rate-synchronized) often created a stationary tearline along the middle of the screen, because it was like two separate displays simultaneously scanning. Horizontal scanning results into what appears as two parallelograms stacked on top of each other, as a result.

If you must multi-scan an OLED or LCD as if they were multiple screes, then you should divide the screen in vertical strips, so that the scanning in in sync with each other. If you must refresh multiple rows of pixels simultaneously, then make the pixel rows contiguous (e.g. simultaneously refresh rows #1,2,3,4 then simultaneously refresh rows #5,6,7,8, then simultaneously refresh rows #9,10,11,12). This will eliminate multi-scan artifacts.
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Re: DIY Backlight Strobing [must read for experimenters!]

Postby HeLLoWorld » 15 Jan 2014, 15:22

wow, I realize, which is to say, sample and hold (speaking of lcd alone, not constant backlight) is actually good, since strobing saves us from the effects we just discussed, and strobing is the only thing that can save us (scanning oleds and crts are screwed because of the instantaneous forgetting behaviour), and strobing saves us because of the sample and hold behaviour (memory) of individual pixels between begin and end of scan where we finally flash them all.

So sample and hold is desirable within one frame(as long as you can hide the frame at will), and undesirable between frames.

Unless (again, I know), realtime calculation of image pixel value is done taking in account the time passing between pixels blinks (think, a raytracer shooting pixel rays at moving objects :) (only way to have zero lag, ever, like an analog camera scanning pixels, sending them on air, and a tv drawing them as it gets them: only pure transmission lag but no frame-level buffering).
But that is a little far-fetched.
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Re: DIY Backlight Strobing [must read for experimenters!]

Postby Chief Blur Buster » 15 Jan 2014, 15:29

HeLLoWorld wrote:wow, I realize, which is to say, sample and hold (speaking of lcd alone, not constant backlight) is actually good, since strobing saves us from the effects we just discussed, and strobing is the only thing that can save us (scanning oleds and crts are screwed because of the instantaneous forgetting behaviour), and strobing saves us because of the sample and hold behaviour (memory) of individual pixels between begin and end of scan where we finally flash them all.

We can solve both simultaneously via using 1000fps@1000Hz.
See Why We Need 1000fps @ 1000Hz.
This is the only way to do low persistence sample-and-hold

1000fps @ 1000Hz has only 1ms persistence, without needing to strobe.

HeLLoWorld wrote:So sample and hold is desirable within one frame(as long as you can hide the frame at will), and undesirable between frames.
You don't have to hold the frame; the frame can fade out, such as phosphor decay. It soften flicker, and lowers flicker fusion threshold, while keeping average perceived persistence low. That's what CRTs tended to naturally do (by accident, by virtue of natural phosphor behavior).
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Re: DIY Backlight Strobing [must read for experimenters!]

Postby HeLLoWorld » 15 Jan 2014, 15:42

1000fps @ 1000Hz has only 1ms persistence, without needing to strobe.

Yes obviously there's a point where things are basically solved but if we cant generate that much fps, hiddend sample and hold + strobing is superior to crt/oled scanning if it's as wide as the frame time.

1000fps @ 1000Hz has only 1ms persistence, without needing to strobe.

If you can do that you might want to get less than 1ms persistence while you're at it! But it will take powerful leds :)
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Re: DIY Backlight Strobing [must read for experimenters!]

Postby Chief Blur Buster » 15 Jan 2014, 16:26

HeLLoWorld wrote:Yes obviously there's a point where things are basically solved but if we cant generate that much fps, hiddend sample and hold + strobing is superior to crt/oled scanning if it's as wide as the frame time.

If you can do that you might want to get less than 1ms persistence while you're at it! But it will take powerful leds :)

Obviously, strobing is a much simpler solution -- that way, you don't need quite as high refresh rates. In much the same way CRT 60fps@60Hz has always had less motion blur than LCD 120fps@120Hz (non-strobed). Other readers, who reads this post, and don't understand why this is the case for CRT versus LCD -- just read Understanding Persistence: Strobed & nonstrobed, CRT vs LCD -- it will help simplify understanding greatly.

That said, with 1000fps@1000Hz, you only have 1ms input lag for the entire frame (top and bottom edges), which is another bonus. And with only 1ms for the whole screen refresh, you don't need to worry about all-at-once flash presentation, so you can skip strobing. The scanning effect (curtains/tilting/skewing/compressing) would only be half as much at 120Hz as it is at 60Hz, and it would be only about one-sixteenth as much at 1000Hz (1/1000sec scanout versus 1/60sec scanout).

And even this is still not the final frontier -- you still get 4 pixels of motion blurring during 4000 pixels/second panning -- say, turning your head while wearing 4K VR goggles. For more information for the extreme-case situation where even 1000fps becomes a limiting factor, see this post.
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Re: DIY Backlight Strobing [must read for experimenters!]

Postby James Freeman » 15 Jan 2014, 16:57

In my case I'll be using the backlight strobing only in Movies, rarely I play games.

A movie frame is 24 and my monitor is 72Hz (OC'ed).
So, I shouldn't care for long vblank time, as 3 at the frames are exactly the same (no pixel change).

I'll just set the stobe so the center of the first frame is clear, and the other two will are just clear "by the way".
Or, strobe the Top only, because most of the movies are 1920x800 anyway. :D
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