MPRT Versus GtG Test Interpretation

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lizardpeter
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MPRT Versus GtG Test Interpretation

Post by lizardpeter » 14 Nov 2021, 21:28

I was trying out this test on various monitors but was a bit confused by exactly what is going on. It says "Observe the edge sharpness difference between the top and bottom halves of the moving bar! Fast pixel transitions will amplify the edge-sharpness difference between the top and bottom half. Slow pixel transitions will reduce the difference between top and bottom." So should I only be looking at the trailing edge or the entire rectangle? Also, for the bottom part, does the number of lines change depending on the refresh rate? On some devices I see only two lines and on others I see more. If the top and bottom look almost identical, does that mean MPRT and GtG are about equal? Does the bottom part represent the GtG and the top represents MPRT?
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Re: MPRT Versus GtG Test Interpretation

Post by Discorz » 15 Nov 2021, 05:59

lizardpeter wrote:
14 Nov 2021, 21:28
Its confusing yes, I assume it should say:

"Slow pixel transitions will amplify the edge-sharpness difference between the top and bottom half. Fast pixel transitions will reduce the difference between top and bottom."

Meaning top half can be slow/blurry or fast/sharp. Don't watch the bottom one, it's just there as a reference of how would instant 0 ms GtG look like (simulated inside sample-and-hold mode, spatial strobing). They need to be separated because your eye ends up seeing is MPRT blur (refresh rate based or strobe duration based) + additional Response Time blur (pixel transition speed).

column 1. 240Hz ,2. 144Hz, 3. 60Hz
ImageImageImage - Reference
ImageImageImage - MPRT + instant GtG
ImageImageImage - MPRT + slow GtG
Simulation, one pixel wide line panning left-right, transition 63-255-63, no OD, speed ∼960pps

You want to track both leading and trailing edge, but what people mostly do is track trailing only, which is ok. Same goes for ufo. In this case where something is panning from left to right, Trailing (left) edge shows Fall Response Time, Leading edge shows Rise Response Time curve. You must of already heard for both. What reviewers usually give u is both Rise/Fall average number (with 10 or 3% tolerance and other).

Image

I recommend you set background color to RGB 63 instead of full black. Transition from dark gray-white-dark gray (63-255-63) is a bit more demanding for IPS.
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Re: MPRT Versus GtG Test Interpretation

Post by Chief Blur Buster » 15 Nov 2021, 22:11

Great reply, Discorz!
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Re: MPRT Versus GtG Test Interpretation

Post by lizardpeter » 16 Nov 2021, 01:18

Discorz wrote:
15 Nov 2021, 05:59
Wow, thanks for the great response! That all makes much more sense now. The one thing I am confused about is your example with three columns for MPRT. On my 390 Hz monitor, the rectangles don't seem to get smaller with the decreased MPRT compared to, for example, my 120 Hz TV. Your example shows there being a major difference between The 240 Hz and 60 Hz tests. So the size of the box should be decreasing, right?
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Re: MPRT Versus GtG Test Interpretation

Post by Discorz » 16 Nov 2021, 07:56

lizardpeter wrote:
16 Nov 2021, 01:18
That's because I gave you different reference moving image, 1 pixel wide white line. The one on TestUFO is 32 pixels wide at 960pps speed and not white when gamma is applied. You can quick check by taking screenshot or pausing movement (unavailable). By increasing refresh rate only the edges of moving object will narrow down making it look closer to reference.

This is perhaps a better demonstration...

column 1. 240Hz ,2. 144Hz, 3. 60Hz
ImageImageImage - Reference 32px wide line RGB 146
ImageImageImage - MPRT + instant GtG
ImageImageImage - MPRT + slow GtG
Gamma 2.2
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Re: MPRT Versus GtG Test Interpretation

Post by Chief Blur Buster » 16 Nov 2021, 14:57

Great demonstration of MPRT blur, as well as MPRT+GtG blur. I can confirm accuracy of these simulated images match reality on many display models.

CRTs often resemble the topmost (zero blur)
OLEDs often resemble the middle (near instant GtG)
LCDs often resemble the bottommost (in sample-and-hold mode)

However, GtG relatively little from refresh rate changes compared to MPRT. So at lower refresh rate, the GtG is much smaller proportion of a refresh cycle (and GtG:MPRT ratio is extremely lopsided to MPRT). So at 60fps on a 240Hz 1ms GtG panel, MPRT+GtG looks almost the same as MPRT-only blur.

Generally
- The slower the LCD GtG
or
- The higher the LCD refresh rate

The more likely MPRT vs MPRT+GtG will look different. This is also why 360Hz LCD panels aren't yet fully 1.5x better than 240Hz LCD panels, even though they are somewhat better.
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Re: MPRT Versus GtG Test Interpretation

Post by Discorz » 23 Nov 2021, 14:19

Chief Blur Buster wrote:
16 Nov 2021, 14:57
I noticed. Refresh rate and moving speed are almost same thing. But can't figure out why. This is confusing because curve has same shape throughout refresh rate range (without applied overdrive). As refresh or speed change it stays consistent but 'scales in other axis'? Wouldn't that mean response times don't matter? Just push highest refresh rate possible and you're good? I'm missing something here.
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Re: MPRT Versus GtG Test Interpretation

Post by Chief Blur Buster » 23 Nov 2021, 16:03

Discorz wrote:
23 Nov 2021, 14:19
Chief Blur Buster wrote:
16 Nov 2021, 14:57
I noticed. Refresh rate and moving speed are almost same thing. But can't figure out why. This is confusing because curve has same shape throughout refresh rate range (without applied overdrive). As refresh or speed change it stays consistent but 'scales in other axis'? Wouldn't that mean response times don't matter? Just push highest refresh rate possible and you're good? I'm missing something here.
Check out www.testufo.com/blurbusterslaw for a great demonstration of Blur Busters Law.

- Assuming GtG near 0, the motion blur of all 3 UFOs is identical.
- A motion twice the speed at twice the frame rate has the same motion blur.
- Try it on a fast-GtG 240Hz LCD display or 120Hz OLED display.
- When GtG is nearly 0 (with GtG90% less than a refresh cycle), Blur BusteLaw looks like it scales linearly.

When GtG=0, display motion blur math undergoes a beautiful "E=mc^2" style simplification which I've deemed Blur Busters Law.

Blur Busters Law can also be read in a frametime perspective on sample-and-hold displays, because 60fps looks the same blur at 60Hz, 120Hz, 240Hz.

1ms of frametime translates to 1 pixel of motion blur per 1000 pixels/sec

Half the frame time = half the motion blur
Twice the speed = twice the motion blur

It all beautifully balances out at www.testufo.com/blurbusterslaw

When GtG nonzero, it's more of a muddy curve.
When GtG is zero, it's linear and not a curve.
That's why Blur Busters Law is a simple linear math relationship!

The good news is that modern displays have near zero GtG. Like 240Hz 1ms IPS LCDs with great overdrive tuning, that are running at only 60Hz or 120Hz. GtG is so fast that it's an insignificant error margin when you underclock a high-Hz LCD so vastly massively, that it behaves like GtG=0 because most of GtG is so tiny of a frametime. By the time GtG is 90% then 95% then 99%, the object has moved ahead only one or two pixels -- creating ultratiny GtG ghosting that may not even be human visible (below human visible noisefloor). The MPRT blurring so dominates the blur.

Let's take the 60fps on an ultra fast 240Hz IPS LCD with one of the fastest pixel responses and best overdrive tuning. Now, imagine 16.7ms of MPRT blurring and 1ms-3ms of GtG90% blurring, and you get the idea of how almost all the blur you see at 60fps on a 240Hz IPS LCD, is pretty pure MPRT blur. It then look similar to eyes more like OLED behavior rather than LCD behavior.

However, there's a minor technicality
Also, 60fps at 240Hz also means a 4-pass refersh, so LCD GtG of 60fps at 240Hz sometimes is slightly faster than LCD GtG at 60fps at 60Hz on the same 240Hz LCD, because the pixel gets four voltage boosts per refresh cycle, slightly speeding up GtG a little bit. This does not always happen, but it's a subtle behavior, sometimes not human visible, but it does happen to some LCDs whose overdrive tuning is more optimized to 240Hz than 60Hz. Multipass refresh cycles (e.g. low frame rates on high Hz) sometimes have a way of re-kicking the GtG soccer ball faster to its goal line. Imagine kicking the soccer ball 4 times instead of 1. The problem is that the monitor chips (scaler/TCON) metaophorically have to be a faster soccer player, for GtG to speed up instead of GtG to slow down. It's a double edged sword that varies LCD to LCD. But the GtG heatmap of the XG2431 is probably better for 60fps-at-240Hz-on-240Hz-panel than 60fps-at-60Hz-on-240Hz-panel, though I have not seen reviewers test this effect yet (testing the same frame rate at higher/lower Hz).
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