Strobed LCD behaves similiar as CRT.RealNC wrote:There's still something I don't fully understand in regards to strobing and persistence. These methods are intended to make persistent displays behave like CRTs. Why is FPS important? Back in the CRT days, games had the same problems we have today; they often would fall below the vertical frequency of the monitor. Playing a game at 30FPS would look just as sharp on my 120Hz CRT as a 60 or 120FPS game.
So why is high FPS important for blur-free display on LCDs, if it wasn't important on CRTs?
You don't need high FPS for a strobed LCD, except to reduce flicker (as strobing is more noticeable than phosphor decay).
However, high FPS is often desirable on flickerfree displays
You have to understand persistence from the perspective of frame visibility length.
Higher framerate reduces persistence on flickerfree displays.
Higher framerate does not necessarily reduce persistence on strobed displays (if you don't change strobe length).
Persistence == frame visibility time == sample-and-hold (For the purpose of these discussions here, these mean the same thing)
Frames on a flickerfree display tends to have a persistence equal to frame length. (assuming GtG is not a major factor)
60Hz flickerfree = 1/60sec persistence = 16.7ms motion blur at 60fps @ 60Hz
120Hz flickerfree = 1/120sec persistence = 8.3ms motion blur at 120fps @ 120Hz
240Hz flickerfree = 1/240sec persistence = 4.16ms motion blur at 60fps @ 60Hz
Frames on a strobed display has a persistence equal to strobe length.
Strobing at 1/60sec flashes = 1/60sec persistence = 16.7ms motion blur (at any refresh rate)
Strobing at 1/120sec flashes = 1/120sec persistence = 8.3ms motion blur (at any refresh rate)
Strobing at 1/240sec flashes = 1/240sec persistence = 4.15ms motion blur (at any refresh rate)
Strobing at 1/1000sec flashes = 1/1000sec persistence = 1.0ms motion blur (at any refresh rate)
(Obviously, refresh intervals that are longer than the strobe, or you can't fit the strobes )
This is why flicker displays (CRT, plasma, LightBoost, black frame insertion) can produce less motion blur at lower refresh rates, than flickerfree displays. Nearly all flickerfree displays (including common LCD displays) are all high-persistence.
1ms of persistence translates to 1 pixel of motion blur during 1000 pixels/second motion
(This result has been so reliably repeatable in testing of recent monitors, I now call this Blur Busters Law)
Assumes: Strobed, instant transition, squarewave transitions, and framerate == refreshrate (== stroberate if strobed). Slow transitions, repeat strobes, strobe crosstalk, and phosphor decay fuzzies this up, but newer displays have become faster and more squarewave, and far more accurately resembles this equation. Several monitors I have sitting here, XL2720Z, VG248QE, XL2411T, VG278H, (and to an extent) FG2421, all follow this "Blur Busters Law" remarkably closely in tests.
Persistence (in X milliseonds), translates to X pixels of motion blurring during 1000 pixels/second framerate==refreshrate motion. So if you have 4.1ms of persistence in a panning test pattern at 1000 pixels/second, you will get 4.16 pixels of motion blurring. For 500 pixels/second, you get 2.08 pixels of motion blur, and for 2000 pixels/second you get 8.3 pixels of motion blur. Pursuit camera photographed motion blur trail lengths can easily be seen at PHOTOS: 60Hz vs 120Hz vs LightBoost as well as PHOTOS: LightBoost 10% vs 50% vs 100%.
Most motion blur on modern LCDs are not because of GtG transitions. The motion blur is caused simply because the display is high-persistence, and today's flickefree technologies are always high-persistence (unless you use motion interpolation). It is caused by eye tracking motion blur (aka persistence), as illustrated in the following motion test found at http://www.testufo.com/eyetracking
View the below animation on an LCD, with strobing off/LightBoost turned off:
1. Look at the stationary top UFO. 2. Next, look at the moving bottom UFO.
(Make sure the above is running at full frame rate, in a stutterfree browser such as Chrome with Aero mode enabled, or animation won't be accurate)
The motion blur you are seeing above in the moving UFO, is not caused by LCD GtG.
The motion blur you are seeing above in the moving UFO, is caused by high persistence on flicker free displays.
Modern LCDs such as those found in 120Hz and 144Hz gaming monitors, are rate at 1ms to 2ms pixel transition speed. This is a tiny fraction of a refresh cycle (1/60sec = 16.7ms, and even 1/120sec = 8.3ms). But we still see lots of motion blur compared to a CRT with a 2ms phosphor, because of the persistence effect explained in the animation at http://www.testufo.com/eyetracking
As you track eyes on moving objects, your eyes are in different positions at the beginning of a refresh than at the end of a refresh. Flickerfree displays means that the static frames are blurred across your retinas. So high persistence (long frame visibility time) means there is more opportunity for the frame to be blurred across your vision. That's why you still see motion blur on 1ms and 2ms LCDs, and this is why strobing hugely helps them (make them behave more like CRT persistence).
There's no motion blur during 60Hz strobing on an LCD either. But LightBoost is like a fixed-frequency CRT limited to refresh only at 100-120Hz since NVIDIA chose that rate for LightBoost for the flicker fusion threshold during 3D vision (60Hz/60Hz per eyes), and LigthBoost was originally designed for 3D Vision as well, not just to eliminate motion blur. That's the main reason for a higher strobe rate on LightBoost, even though 60Hz LightBoost works perfectly (emulated via software-based black frame insertion, http://www.testufo.com/blackframes when viewing this page on a LightBoost display, since it ends up suppressing every other strobe ...)
Makes better sense now?
NEXT POST: Detailed explanation of http://www.testufo.com/blackframes#count=3 to explain the relationship of persistence length / duty cycle affecting motion blur trail length.