LCD motion blur: Eye-tracking now dominant cause of motion blur (not pixel persistence) - Page 5

Contrast-enhancement of LightBoost photograph, showing ultra-faint pixel persistence that's still seen by human eye. It's almost below the noise floor. Most of the pixel persistence (transitions and overshoots) are hidden during the time period the backlight is turned off.


The camera is a Casio EX-FC200S. Any common film or digital camera can be used; they all yield the same motion blur result at the same camera exposure, if you properly synchronize the camera motion to the moving-object onscreen (within accuracy of +/-1 pixel during the duration of the exposure). I've confirmed there's no difference. Also tried an old Panasonic Lumix.
At the LCD panel level, the pixel transitions are taking longer than the backlight flash. However, the pixel transitions are taking place in the dark. Marc Repnow (scientist) reversed engineered LightBoost, and found it uses a vertical blanking interval of approximately 5 milliseconds. See his discoveries as well as his HardForum post. (Frames are buffered in order to allow accelerated scanouts, in order to have a long blanking interval). The refresh is scanned out fast (top-to-bottom) in total darkness, to allow a bigger idle time period between refreshes.

5ms -- backlight turned off; LCD is being refreshed in total darkness. Pixel transitions is occuring in the dark.
2ms -- backlight turned off; idle period, to wait for the last pixel transitions to settle in total darkness.
2ms -- backlight flash; fully refreshed frame seen by human eye.

This corresponds to one refresh cycle (8.3ms) at the 120Hz refresh. During longer strobe flashes, the backlight flash actually overlaps the next refresh slightly, causing slightly increased ghosting at the edges of the screen (a few percent more visible, only on the top/bottom 10% of the screen, and not objectionable). During shorter strobe flashes, the backlight flash completely fits within artificially-lengthened blanking interval between refreshes. Thus, by this technique, a good strobe backlight on a fast LCD, bypasses greater-than-99 percent of pixel persistence from becoming visible to the human eye.

There is definitely some vertical non-linearity in ghosting; e.g. top and bottom edges have more remnant pixel persistence than the center of the screen. That's an unavoidable situation caused by the top-to-bottom scanout of LCD, versus the all-at-once stroboscopic flash. But this no longer a motion blur bottleneck, unlike scanning which still has unavoidable light leakage between backlight segments (on vs off segments). Only strobe backlights (given a sufficiently fast LCD) currently make possible unbounded improvement in motion clarity.
Therein lies the confusion. The LCD has the same pixel persistence, during all these photographs. It's a photo of the same LCD. The LCD isn't accelerating pixel transitions by 10x. (But yes, it's accelerating the top-to-down refreshing, but the individual pixels still take the same amount of time to transition). The backlight is simply hiding the pixel transitions, simply by being turned off between refreshes.

There is little difference in the LCD panel's ability to change the speed of pixel transitions (overdrive tweaks) in the photographs between 60Hz versus 120Hz versus LightBoost. All of the photos were taken using the same camera on the same computer monitor. The law of physics in the LCD remain the same; it doesn't transition pixels faster between the 120Hz versus 120Hz LightBoost (aka the pixel persistence, from the point of view of the LCD law of physics, is unchanged). Marc Repnow mentioned to me that if you open up a LightBoost computer monitor and force the backlight to continuously shine during LightBoost mode, the motion blur instantly comes back -- including pixel persistence/overdrive artifacts -- which immediately comes back and becomes far more visible to the human eye. Further proof that the backlight being turned off, is successfully hiding pixel persistence. This A/B test of overriding the strobing, proves this.

The fact that the backlight is able to eliminate motion blur in this A/B test, also proves the following:
Edited by Mark Rejhon - 5/19/13 at 7:18pm

I prefer to educate hundreds of people publicly in public forums and blogs (efficient education), so I prefer not to educate one person privately (inefficient education) (unless it's paid work, of course!)

If sending PM, please keep your questions short enough that I can reply to them in less than 5 minutes. Otherwise, I prefer to discuss publicly.

I hereby invite you to do the following steps:
1. Buy a LightBoost monitor. Supported monitors are listed in the LightBoost HOWTO.
2. Download a motion test such as PixPerAn. (Or use Blur Busters Motion Test if you have an account).
3. Run the motion test. Observe the motion blur with your eyes.
4. Enable LightBoost.
5. Run the motion test again. Observe the lack motion blur with your eyes. The ghosting/overdrive almost completely disappears*

*Some LightBoost monitors (e.g. VG278HE instead of VG278H) don't do as good as job at eliminating visibility of pixel transitions (e.g. 95% gone instead of 99% gone). I also have a BENQ XL2411T, which does an excellent job of hiding pixel transitions.

If you want to actually run a true scientific A/B test with exactly the same LCD refresh pattern (the LightBoost-tweaked overdrive algorithm), then do this too:
6. Open the LightBoost monitor and make the backlight shine continuously while in LightBoost mode.
7. Run the motion test again. Observe the return of motion blur (and ghosting/overdrive artifacts) with your eyes. (unchanged LCD panel pixel persistence)
8. You can run A/B test by disconnecting/reconnecting the backlight strobe feature, while keeping the LCD panel refresh behavior unchanged. Even while the monitor is still running!
9. Observe that whenever the backlight continuously shines, there's a lot of motion blur and ghosting (like regular 120Hz). But if you let the backlight strobe once per refresh, the motion blur (and most LCD transition artifacts disappear -- including most of ghosting/overdrive disappearing).

This proves: "Recently, with today’s faster LCD’s, pixel persistence now only has a minor factor in motion blur."
For Blur Busters Blog purposes, "LCD pixel persistence" means "physical pixel transitions within the LCD panel, independently of whether backlight is ON or OFF". Some sites, including prad.de and PixPerAn, has tended to use this terminology. Others standardize on "pixel transitions" terminology. If this is the confusion, then now we're clear!

That's all, folks. I gotta focus on family and work!
Edited by Mark Rejhon - 5/19/13 at 2:45pm

Confusion is resolved over PM.
"pixel" = A single controllable LCD element; independently of whether it's seen by human eye or not (e.g. backlight turned off)
"pixel persistence" = The transitions of an LCD element from one state to a different state; independently of whether it's seen by human eye or not (e.g. backlight turned off)

These terminologies has precedent:
"pixel" can be used from the perspective of a digital technology, e.g. an array of CCD pixels, even if the CCD chip is not currently capturing pictures, or when an LCD is not currently visible (e.g. backlight off).
"pixel persistence" has long been used in many contexts, such as PRAD's Pixel Persistence Analyzer.

That said, Blur Busters is going to address this to fix potential terminology confusion, such as posting of a Blur Busters Glossary Page, to keep things consistent. The phrase "pixel response time" is more common nowadays, and may be clearer.

"Recently, with today’s faster LCD’s, the LCD panel's native pixel response time now only has a minor factor in motion blur"
Edited by Mark Rejhon - 5/19/13 at 7:25pm

What's so exciting to me about LCD today, is that it became possible for motion clarity of LCD to become unbounded from LCD's own panel limitation (provided certain stringent requirements are met). When these requirements are successfully met, the LCD panel's native pixel response is no longer has a factor in motion blur. Pixel transitions completed in total darkness, all the way to below human vision noise floor at normal viewing distances, before the backlight is strobed.

By effective purposes (to the human eye vision), it becomes a true impulse driven display to the human brain. Doesn't matter if parts of the technology is intrinsicially unavoidably sample-and-hold (NRZ), due to the teamwork between the LED backlight & the LCD panel, is all that matters.

That said, other limitations apply (e.g. imperfect black levels are an unavoidable limitation, and faint vertical non-linearities in picture quality due to different ages of pixel transitions, due to the LCD scan-out versus full backlight strobe). Some of the transitions are faintly visible, as seen, and some GtG are worse than others, but are otherwise impossible to see in most scenery (ultra-faint 3D crosstalk too). There can be interplay between the strobing and LCD inversion, causing amplified inversion artifacts (but not on all LightBoost monitors; it's a non-issue on the XL2411T, while far moreso on the VG278HE). Also, some LightBoost monitors are better than others. Older LightBoost monitors such as XL2420T, have more visible ghosting, while newer ones like VG248QE, can have nearly perfectly hidden pixel transitions (at least across a large middle band across the screen, with slight faint ghosting at top/bottom edges).

The pursuit camera photos truly show how successfully the panel's own pixel transitions can be hidden. For many competitive FPS video gamers, the remaining (and created) artifacts are not issues.
Edited by Mark Rejhon - 5/21/13 at 7:55am