Pixel behaviour in sample and hold, in unchanging image.

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spacediver
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Pixel behaviour in sample and hold, in unchanging image.

Post by spacediver » 13 Dec 2014, 01:29

this thread gave me a silly idea, but before I explain it, can someone confirm the following:

In a sample and hold display, if you were to render 10 successive frames that consisted of a white screen, would all the pixels in the the 2nd to 10th frames be held at a constant luminance? i.e. there would be no pixel transitions during this defined period.

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RealNC
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Re: Pixel behaviour in sample and hold, in unchanging image.

Post by RealNC » 13 Dec 2014, 01:42

spacediver wrote:this thread gave me a silly idea, but before I explain it, can someone confirm the following:

In a sample and hold display, if you were to render 10 successive frames that consisted of a white screen, would all the pixels in the the 2nd to 10th frames be held at a constant luminance? i.e. there would be no pixel transitions during this defined period.
There's always some slight change in the pixels, since LCDs "decay" over time. Between every refresh, each pixel will change color until it is refreshed again. Note that this "decay" is not at all similar to the phosphor in CRTs. Those were fast to light up and decayed *very* fast to black. LCDs are slow to "fire up" and decay slowly. (Btw, if we had LCDs that would behave like phosphor, the whole motion blur issue wouldn't have existed in the first place ;))

The effect is very small and not visible, unless you have a very low refresh rate (this is one of the reasons why G-Sync avoids going below 30Hz.)
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spacediver
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Re: Pixel behaviour in sample and hold, in unchanging image.

Post by spacediver » 13 Dec 2014, 01:55

Well I'm asking specifically about frames that don't change pixel value:

for example, see image below:

Image (taken from the following open access paper: http://www.plosone.org/article/info:doi ... ne.0012792 )

If you look at the middle row, there is an overlap of the decaying previous frame and the rising upcoming frame. But this is when the two frames contain nonoverlapping pixel information (in previous frame there is a disk, and in upcoming frame there is a surrounding ring). In my scenario, all pixels in both frames are solid white. I want to confirm that you don't get those overlapping functions in such a scenario. I'm assuming that in my scenario, the 2nd to 10th frames will be described by the SOF (sum of frames assumption) depicted in the top row. In other words, the voltage at each pixel will remain at a constant level until instructed to change value, and if the same pixel information holds across multiple frames, then there should be no decay and rise in between frames, right?

spacediver
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Re: Pixel behaviour in sample and hold, in unchanging image.

Post by spacediver » 15 Dec 2014, 01:22

bump :p

can anyone confirm my assumption?

flood
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Re: Pixel behaviour in sample and hold, in unchanging image.

Post by flood » 15 Dec 2014, 05:33

in general no the luminance is a wavy pattern.

it could be due to temporal dithering, especially for 6bit panels.
could also be due to what's discussed here:
http://forums.blurbusters.com/viewtopic.php?f=5&t=1627

here are some nice plots
http://display-corner.epfl.ch/index.php/ASUS_VG248QE
http://display-corner.epfl.ch/index.php ... RIS_FG2421

spacediver
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Re: Pixel behaviour in sample and hold, in unchanging image.

Post by spacediver » 15 Dec 2014, 06:15

thanks for the links, look forward to absorbing them. My main question is whether the overall pattern (whether it is steady luminance, or temporally dithered) persists over multiple frames in my scenario.

Another way of asking the question:

On a sample and hold display, if you were to show 10 successive frames, each of which had an identical full white screen, and you were to record a region of this display with a photodiode and oscilloscope, for the interval between the onset of frame 2 and the offset of frame 10, would there be any periodic changes in the data that indicated the transitions between frames?

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Re: Pixel behaviour in sample and hold, in unchanging image.

Post by Chief Blur Buster » 15 Dec 2014, 08:44

spacediver wrote:On a sample and hold display, if you were to show 10 successive frames, each of which had an identical full white screen, and you were to record a region of this display with a photodiode and oscilloscope, for the interval between the onset of frame 2 and the offset of frame 10, would there be any periodic changes in the data that indicated the transitions between frames?
Yes. For about 2-3 frames.

1. Show an image
2. Display blank screen for a few refresh cyckles
3. Image will still faintly in refresh #2 and super-faintly in refresh #3
etc.

It's extremely faint, and depends on whether or not you have overdrive enabled or not, and it goes below the noisefloor pretty quickly, so you wouldn't be able to see beyond certain frames.

A VERY good test is http://www.testufo.com/blurtrail .... I usually use this color setting, and some settings of monitors show as many as 3 ghosts behind.

Also, looking REALLY closely at http://www.testufo.com/ghosting with the window dragged low (with the UFOs moving at the bottom edge of screen), you will often see 3 images. That's persistence over 3 refresh cycles.

Older 33ms LCDs could ghost for 6 to 7 refresh cycles, possibly even as many as 10, see below:

Image
Look at all the ghost trails!
Persistence over multiple refresh cycles!
33ms LCD taking over 100ms real-world.

(Observe: LCD pixel response can be slower than a refresh cycle.
See, LCD response has nothing to do with refresh rate).

Today's 120Hz LCD's barely ghost beyond 1 refresh cycle, but I've seen them ghost to 2-3 refresh cycles if you look really closely in some synthetic motion test patterns like Blur Trail or Ghosting test (especially in BENQ Z-Series strobed mode, which doesn't use scanline-optimized overdrive).

Each subsequent LCD refresh pass, "push" the pixel ever closer to their final color value. The first refresh pass pushes it to 99% accuracy, and then the next refresh pass push it to 99.9% accuracy (faintness very hard to see), then the next refresh pass pushes any ghosting to below detectability by human eye but an ultrahigh precision oscilloscope may see it. And then the next refresh pass pushes it below equipment noisefloor - with the noise of refreshing, dithering, and inversion obscuring any "tiny faction" (e.g. say, 0.00001% persistence) of persistence that may be left over.

But bottom line, persistence CAN cover multiple refresh cycles.
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spacediver
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Re: Pixel behaviour in sample and hold, in unchanging image.

Post by spacediver » 15 Dec 2014, 09:53

Chief Blur Buster wrote:
But bottom line, persistence CAN cover multiple refresh cycles.
I understand that, but why would persistence be an issue in my scenario?

Remember, in my scenario, each successive frame contains the same information (all pixels are white). I'm not concerned with what happens after the white changes to a different color. I'm only concerned with the pixel behaviour during these white frames (in sample and hold context). Now if the rise time took over 1 frame, then I can see how the first few frames might not be the same as the latter frames.

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Re: Pixel behaviour in sample and hold, in unchanging image.

Post by Chief Blur Buster » 15 Dec 2014, 10:35

spacediver wrote:I understand that, but why would persistence be an issue in my scenario?
I am not 100% sure I get what you're trying to say because I don't always understand things spoken in a specific way.
...but you could visit my place and allow me to show you what I mean.

So let me try to explain behavior, so you can do informed scientific testing:

My post will help you extrapolate behavior based on:
"How does image retention/persistence work through successive solid color/white refresh cycles, after an image?"
"How does image retention/persistence work if several preceding refresh cycles were solid color/white, rather than just a few?"
"How does image retention/persistence work with successive images going back and fourth?"
"How does image retention/persistence work based on how many preceeding(or succeeding) refresh cycles were solid color/white?"

When looking at motion tests under specific conditions (that most scientific people don't test for), I can really tell that there is a stepped behavior to persistence. It's the type of stuff more easily seen by human eye than an oscilloscope -- consider an oscilloscope trying to map over 8-bit of greyscale -- 256 greyscale levels, but that sometimes the human eye can tell apart far, far, far, more than 256 luminance levels if they are ultra-wide dynamic range, from the very dim (eyes adjusted to dark, faint light), all the way to blindingly bright (like staring directly at the sun). So if you mapped over such a wide dynamic range, your eyes can really tell far more than 256 luminance levels, allowing eyes to adjust. An oscilloscope graph only 1 inch tall, a 1/256th difference in height would be hard to see, so oscilloscope graphs are currently not too useful for this specific use case, here unless you're using a very tall/logarithmic graph, or knowing what you are looking for & zooming into a specific section at different stages of LCD response, rather than mapping the whole LCD response.

Normally looking at test patterns without strobing, it's hard to see the stepped behavior (multiple-hump-curve effect). So let's include photography.

Ghosting without strobing
Image
- Lots of motion blur, and you see a ghost trail to the left. The stepping effect of the voltage kicks is extremely hard to see, but if you put an ultrahigh-precision photodiode oscilloscope (accuracy far more than necessary to tell apart adjacent shades), a stepping effect of each refresh cycle should become visible.

Ghosting as part of Strobe Crosstalk
Image
- No motion blur, but the existing ghost trail is now "chopped up" in the form of strobe crosstalk. You see the stepped effect much more clearly, thanks to the human eye. Most of this is caused by the strobing itself, but this is an excellent illustration of how each successive step gets closer and closer to the final color value, until closeness is so close that it falls below the vision noise floor...

....So now that out of the way, let's forget about oscilloscopes because most normal use of the oscilloscope won't always reveal what the human eye sees, in a strobed blur trail test (and I doubt most scientists have tested strobe backlights much, which amplifies the human eye's ability to see any imperfections in LCD response leaking between refrehes)

So now that out of the way, let's start with an explanation which might help you extrapolate your scenario (that I may not fully understand), based on what I explain.

There's a stepped behavior in LCD response. Each voltage pass (LCD scanout) is a kick of momentum that pushes the LCD pixel closer to its final color value. Overdrive may cause overshooting (coronas) or undershooting (ghosting) but often both (aka squarewave ripple)

So picture it this way. (I'm picking numbers out of thin air, only to explain the point -- but the numbers would vary)
Several refresh cycles were white, but now next refresh has an image.
1 - first scanout pass kicks the LCD pixels suddenly towards the image. Pixels manage to reach 98% of the way to their final value, in a curving behavior. This is easily seen in oscilloscope graphs.
2 - second scanout pass kicks the LCD pixels suddenly from 98% to 99.5% accuracy and it keeps costing slowly (e.g. in the scale of 0.001). The scale of ~1/60th differences in luminances -- this can still be barely human visible, for certain shades, especially when testing blur trail in strobed mode -- but a 1/60 height difference is already hard to see in a 1-inch-tall oscilloscope diagram
3 - third scanout pass kicks the LCD pixels suddenly from 99.5% to 99.99% accuracy -- this may still be human visible, as it's roughly a 1/200th difference in luminance, and tests have shown that people can tell these apart in certain conditions. (tip: strobed test, using medium-scale greyscale, since accuracy is more variable with midrange greys than with full-black and full-white). At this level, a 1/200 difference in height might be extremely hard to see in the oscilloscope, even if it may still be visible to human eye
4 - fourth scanout pass kicks the LCD Pixels suddenly from 99.99% accuracy to 99.999% accuracy; well below the noisefloor of other LCD phenomenae, inversion, signal noise, etc...

Between each voltage push, is a curve going towards the final pixel value. During voltage, a new acceleration occurs (so if you zoom oscilloscope to that tiny slice, and expand height). This is so faint, though, and wouldn't show up in oscilloscope graphs except when displayed inverse-logarithmic (I think that's the term) and if everything was so crisp/accurate that noise was a tiny fraction of the difference between adjacent greyscales (e.g. greyscale shade 253 greyscale versus shade 254). The dim greys are easier to tell apart (e.g. greyscale shade 3 and greyscale shade 4), but we also run into the 6-bit TN panel limit, so a lot of shades may look the same at the low end -- see Lagom Black Level -- make sure bright light sources are off, and you maximize your browser window to hide the bright UI elements -- then you can often tell apart adjacent greyscale levels easily. This is hard to see in an oscilloscope graph, unless you expand the low parts of the graph logarithmically (inverse log).

However, I would say on an ultra-sensitive ultra-accurate oscilloscope (accuracy far more sensitive than adjacent colors -- e.g. more than, say 10x more sensitive necessary to accurately tell apart adjacent colors, such as greyscale 235 versus greyscale 236), on a logarithmic graph (assuming no 'noise' from inversion, dithering, etc) that expanded more and more as fractions got smaller and smaller (I think "inverse logarithmic" is the phrase -- I hope I got it right) you would see a multiple-humped graph. You'd have to clamp off/cut off the log scale at the bottom, since it would be infinitely tall as the number approached smaller fractions. You'd suddenly reveal the multiple-hump-curve appearance of the multiple LCD pixel momentum events, as each scan pass kicked the LCD pixels ever closer and closer to final value. You won't see this in typical oscilloscope tests of LCD response, and most people don't "think from the Blur Busters perspective" like I do, and because I'm targetting the mainstream with Blur Busters, my language isn't very scientific. So sometimes I need help from people like you, peer reviewers, to tidy up my language --

Another thought experiment to understand this better: Think of it like expending exactly "X" of energy to accelerate, everytime you approached the speed of light. One acceleration event goes to 90% of speed of light, next acceleration event goes to 99% of speed of light, next acceleration event goes to 99.9%, etc. Vanishingly smaller and smaller points of returns. You needed several refresh cycles for a 33ms LCD to get these points of diminishing return, but now only need about 1 refresh cycles of good well calibrated overdrive on a ultra-well-calibrated LCD, to already get to the diminishing return.

It doesn't matter whether the pixel started at color X, and the next is color Y. Multiple refreshes of white followed by stimuli. Or stimuli followed by multiple refreshes of white.

And if your concern is related to "how does those partially-refreshed pixels behave for the next pixel change", then picture of it this way. If you had a solid white screen followed by an image, and the pixels was only 98% accurate (e.g. 98/100ths all the way to an exact shade of grey - e.g. a grey shade slightly too light), then the behavior of that LCD pixel tends to be the same as if the pixel was already at a specific lighter grey color. So the next refresh will begin the momentum from that level, as if the screen was already intentionally refreshed to that slightly lighter shade of grey. On typical 1ms LCDs, the LCD momentum coasting is already flattened out, so pushing the pixel at the next scan, will be as if the pixel was already at its slightly lighter shade.

If your concern is related to vision science, then describe a specific test case (in layman's language, if possible). Typically, if you've been already displaying a specific image for several refresh cycles, the persistence won't factor into an inaccuracy. If it's about displaying whiteness, followed by a single frame of image, then back to white frame, what you're getting is a partial transition to the image (pixels never quite reaching their final value) and then back. On slow LCDs, this may mean an image that's only 10% ghostly. On faster LCDs, this may mean an image about 98% or 99% as strong as the original. Conversely, if it's a sequence of single-refresh-cycles images displayed in rapid succession, they will ghost into each other. And completeness of transition of each pixel can also depend on whether it's a "slow" color (e.g. dark grey or light grey, one of the two is often slower than others, especially on VA) or a "fast" color (e.g. solid white or solid black). If it's a 1-bit monochrome stimuli (no antialiasing, etc) you've got the best case scenario as black-to-white and white-to-black transitions are often more accurate, have less pixel ripple at the ends, and can often go directly without overshoot, since it's a totally of all LCD molecules being aligned one direction or another -- without possibility of going below black or above white). You'd want bidirectional overdrive, for fast-to-black and fast-to-white. Most new 120Hz+ monitors as well as scientific models like Viewpixx/etc, I believe, already use bidirectional overdrive (don't vouch me on this). For other scenarios, you will have to extrapolate based on my explanations.

Different displays will be different. And overdrive muddy the questions a lot. The first pass may have an overshoot past. The next voltage pass (if pixel color value unchanged between the subsequent refresh passes) is not overdriven. So in this case, you've got an overshoot (and ripple) followed by a multiple-hump effect of each voltage push towards the correct color value. Overdrive lookup tables is often only 1-refresh-depth, making an assumption that the pixel colors are of perfect final value on the next pass. Let's say a specific panel's overdrive quirk causes undershoot during upwards transitions but overshoot during downward transitions. (Or vice versa. Or even for certain color values -- overshoot occurs on certain colors but undershoot on other colors!). So if you've got a stimulus then back (light grey then dark grey then back to light grey), you may have a double or amplified overshoot, such as the final pass (light grey) overshooting far more severely than if the refresh sequence was darkgrey-darkgrey-darkgrey-lightgrey, because lightgrey-darkgrey only transitioned partially to darkgrey. But the overdrive back to lightgrey assumes the pixels were already fully transitioned to darkgrey. Since you're starting off from a lighter-than-expected pixel, the overdrive would then more heavily overshoot than usual, creating a much stronger corona than usual. Yes, yes, it's a headache of complexity.

Now, I don't think I can answer any further questions about this -- as I think I've explained it out as much as I could -- you will have to extrapolate from here onwards....

...If you need to measure this effect for scientific purposes I really suggest you buy equipment as I don't have the equipment to measure this. You need something accurate enough that's more than an order of magnitude more accurate than the luminance difference between two adjacent greyscales of 8-bit, AND you need inverse logarithmic graphing (hope that term is correct) to clearly show the multiple hump curve effect of the multiple voltage kicks of subsequent scanouts. Or if it's an issue of understanding the stepped effect, the option is test the same test on a strobed display to check out the phenomenae, while I drew pencilled graphs of what's really happening on the screen. Then scientists can follow a precise procedure to get it graphed on paper.

It could muddy certain vision science questions such as the "do you see a bright flash optical illusion when the image flickers to dark and then back?" question, because overdrive might very well be responsible for a surge of brightness, rather than trying to measure a human brain stimulus. So, I can understand why you might be askng these kinds of questions, because various behavior of LCDs can affect science. So one wants to back it all up with verification measurements from a high-precision oscilloscope (knowing that bumps not visible on normal non-logarithmic graphs, as explained previously, may actually be seen by the human eye, knowing the logarithmic nature of the human eye's dynamic range).

I may split this thread off to a separate thread, in Area 51, to keep the simplicity of the "motion blur in old photography".
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blargg
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Re: Pixel behaviour in sample and hold, in unchanging image.

Post by blargg » 15 Dec 2014, 14:29

Chief Blur Buster wrote:There's a stepped behavior in LCD response. Each voltage pass (LCD scanout) is a kick of momentum that pushes the LCD pixel closer to its final color value.
Doesn't the LCD hold the voltage between passes? What causes there to be a kick each pass if the same voltage is held each time? Perhaps the answer connects to another question I had about all this: the AC waveform. Each pass the polarity of drive for each element is reversed, correct? This would explain the kick, and why each scan it gets a little closer to the final color, even though the voltage is the same. Though I'm still unclear on how inverting polarity doesn't require lots of time for the element to twist, yet merely changing voltage does. Somehow once something is twisted one way, it can quickly twist the other way the same amount it seems (maybe it's like twisting something hanging up one way, then letting it unravel and retwist the same amount the opposite direction, like a pendulum of sorts).

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