3D TV Shoot-Out: Active Versus Passive Glasses Technology
By Raymond Soneira, DisplayMate Technologies
There are a number of very interesting (and frequently misunderstood) 3D imaging and visualization issues that need to be examined for both Active Shutter and Passive Glasses 3D TV technologies in order to straighten out the incorrect and confusing information about them. The full article at www.displaymate.com/3D_TV_ShootOut_1.htm provides an objective in-depth analysis of both 3D technologies, and this extract summarises the results and main conclusions.
We have plenty of measurement data, which provides lots of good objective evidence, but the most interesting and important part in evaluating 3D is the actual 3D imaging and visualisation itself, and that only happens inside the brain, so instruments cannot help with that part of the evaluation. We used lots of high-quality 3D content including 3D movies, photos, images and test patterns.
The Shoot-Out Set-Up
We examined four recent-model high-end 3D LCD HDTVs – two with Active Shutter Glasses from Samsung and Sony, and two with Film Pattern Retarder (FPR) Passive Glasses from LG and Vizio. They were set up in a Shoot-Out configuration for detailed simultaneous side-by-side comparisons as shown below.
All were simultaneously fed identical 1920x1080x24p frame-packed digital signal content using a Kramer Electronics 3D HDMI Distributor connected to a 3D Blu-ray player for movies and a PC with an NVIDIA 3D graphics board and NVIDIA 3DTV Play software to deliver our set of DisplayMate 3D test patterns, test photos and images. Normally, we first calibrate TVs for optimum picture quality before testing them. Since this article is only about 3D imaging we limited the set of adjustments to those that could influence the 3D conclusions.
The Main Issues
Both of these competing 3D technologies each have their own set of particular strengths and weaknesses. For Active Shutter Glasses the main issues are excessive flicker, image crosstalk and ghosting, insufficient brightness, problems with viewing comfort and cost of the glasses. For Passive Glasses the main issues are questioned resolution and sharpness, restricted viewing distances, angles and positions. You’ll see many of these issues mentioned in reviews and advertisements. But the most important issue of all is whether either of these technologies is able to provide an enjoyable and convincing 3D viewing experience.
Summary of Results
For the Active Shutter Glasses we found the flicker quite annoying and tiring, but the Passive Glasses were completely free of flicker. While not everyone notices the 60Hz shutter flicker from the Active Glasses, it is still possible to be affected by flicker and not be aware that it is present. Most people can sense flicker at 60Hz or even above. CRTs were well known sources of 60Hz flicker. In fact, most people ran their CRTs at 75 or 85Hz or above because of flicker, so 60Hz flicker is a firmly-established phenomenon – as a result, a significant portion of the population may be susceptible to flicker from Active Shutter Glasses.
Subliminal flicker, which is flicker just below the threshold of conscious detection, can also cause visual fatigue. There are good reasons for suspecting that a portion of the eye strain associated with 3D TV is the result of flicker and subliminal flicker from Active Shutter Glasses.
For most viewing angles and viewing positions, the Active Glasses also had considerably more Crosstalk and Ghosting, which are not only annoying but more importantly, interfere with the 3D imaging and 3D contrast. Passive Glasses also did considerably better with varying head tilt, which is very important during normal TV viewing. The Passive Glasses TVs delivered 3D images that were 2.5 times brighter than the Active Glasses. The Passive Glasses were also considerably more comfortable to wear, and cost less than 1/5th the price of Active Glasses.
On the other hand, Passive Glasses have a more restricted range of vertical angles and viewing distances, so you can’t watch 3D TV closer than about 6 feet from the screen, or watch 3D TV standing up closer than about 8 feet, or watch a 3D TV mounted high up over a fireplace without a tilt mount. None of these affect normal 3D TV viewing in our opinion, and none of them apply to 2D viewing.
In general, most reviews and evaluations agree fairly well with the above points, but our extensive measurements quantitatively show how much better the Passive Glasses perform under a wide range of typical viewing conditions.
Sharpness and Resolution with FPR Passive Glasses
By far the most controversial and misunderstood issue in 3D TV currently has to do with the sharpness and resolution delivered with Passive Glasses. Because they split the odd and even lines between the right and left eyes it’s easy to see why many people (and some reviewers) conclude that FPR technology delivers only half of the HD resolution. Although unsubstantiated, it still seems to have evolved into some sort of myth based on hearsay instead of actual scientific visual evaluation. Many people seem to get stuck on this particular issue and can’t get beyond it to think about what is really being seen in actual 3D vision.
But it’s not that simple, because we watch TV from a far enough distance that the lines are not resolved and we know that the brain combines the images from both eyes into a single 3D image (the one we actually see) in a process called Image Fusion. The 3D TV images have only horizontal parallax from the horizontally offset cameras, so the vertical image content for the right and left eyes are in fact identical – but with purely horizontal parallax offsets from their different right and left camera viewpoints.
So there isn’t any 3D imaging information that is missing because all of the necessary vertical resolution and parallax information is available when the brain combines the right and left images into the 3D image we actually see. That is the theory and fundamental principle behind 3D Image Fusion for FPR TVs – so next we actually tested it to see how accurate it is and how sharp the 3D images actually appear.
Sharpness and Resolution with Active Shutter Glasses
Active Shutter Glasses also have 3D image sharpness issues, but they instead arise from left-right image crosstalk that can blur fine detail and muffle the 3D image depth and 3D contrast. This results from the limited response time of the LCD screen and the LCD shutters on the Active Glasses. Since both 3D technologies have 3D image sharpness issues, we needed to test them both to see how well they actually do.
Testing 3D Image Sharpness
Because the 3D images are created in the brain, instruments cannot be used to measure how sharp or muffled they appear on a given 3D TV – that can only be done with human vision by actually viewing 3D content – but it can be done in a very precise and analytical manner. What matters here is the actual 3D visual performance NOT an analysis of the display hardware diagnostic performance the way it is normally done for 2D displays.
We performed a series of quantifiable sharpness tests by using what is in effect a Reverse Vision Test where we determine 3D image sharpness by how small a text can be read on a given 3D TV at a given distance when viewing regular Blu-ray movie content. If there is image fusion we should be able to read particularly small-sized text (6 to 10 pixels in height) with the Passive Glasses, but if the Passive Glasses only deliver half the resolution, as some claim, then it will be impossible to read the small text on the FPR TVs. The primary source for our tests was the Blu-ray documentary IMAX Space Station 3D because it has very high quality 3D imaging shot by NASA with an IMAX stereo camera without artificial effects or special effects, and the spacecraft has lots of labels and printed signs with small text on the instruments and walls that are great for detailed quantifiable sharpness comparisons.
3D Sharpness Results
The 3D sharpness tests were all done at the closest recommended 3D viewing distance of 6 feet. In all cases the small text (6 to 10 pixels in height) was readable on the FPR Passive Glasses, which definitively establishes that there is excellent 3D image fusion and the Passive Glasses deliver full 1080p resolution in 3D. Again, if the Passive Glasses only delivered half the resolution, as some claim, then it would have been impossible to read the small text on the FPR TVs. So those half-resolution claims are manifestly wrong – no ifs or buts!
Furthermore, in all cases the small text was actually sharper and easier to read and fine details easier to resolve on the FPR Passive Glasses than on the Active Glasses because of the Crosstalk, ghosting and Response Time issues that reduce 3D image sharpness and 3D contrast in Active Glasses TVs. We also compared the small text 3D visual sharpness to the 2D sharpness by repeatedly turning the 3D mode on and off for each of the TVs and watching in 3D with glasses and then 2D without glasses.
In all cases the images were sharper in 2D than in 3D, but the differences were much smaller with the FPR TVs than with the TVs with Active Shutter Glasses. In fact, the small text 3D visual sharpness on the FPR TVs were only slightly less than in 2D, reinforcing our conclusion that the Passive Glasses deliver 3D image fusion with full 3D 1080p resolution and are visually sharper in 3D than Active Glasses because of the crosstalk, ghosting and response time issues mentioned above.
Some reviewers have evaluated 3D TVs by analysing the combined display hardware performance for the right and left channels instead of the actual 3D visual performance tests that we have done. That simply leads to incorrect conclusions in the case of 3D vision because of image fusion in the brain. In fact, based on our own extensive display diagnostic tests, it is clear that the FPR TVs have been optimised for the best 3D visual performance when viewing natural photographic and video content instead of the best hardware diagnostic performance – that is most likely why they perform so well with 3D vision.
On the other hand, there are instances when 3D image fusion may not work well with FPR. They arise when the brain is unable to properly match up the right and left image content or when there is fine computer pixel and line graphics, but they were very seldom noticeable in all of the video Blu-ray content we used for the Shoot-Out.
Based on our extensive lab measurements and visual test comparisons between 3D TVs with FPR Passive Glasses versus 3D TVs with Active Shutter Glasses, we found that the Passive Glasses TVs delivered substantially and demonstrably better all-round 3D imaging, 3D contrast and sense of 3D depth, better 3D sharpness, better overall 3D picture quality, immersion and realism, and freedom from 3D ghosting, image crosstalk, and flicker. This was true in all but a small number of situations.
One of my favourite examples for demonstrating the differences between the 3D TV technologies is at 10:47 in IMAX Space Station 3D, which shows a protruding glove and orange pipe in front of a deep equipment area with lots of fine image detail. Press ‘pause’ on the player. With Passive Glasses you feel that you are right there in the Space Station with a convincing, clear and realistic 3D image that has crisp 3D detail and good 3D contrast throughout, and without any noticeable visual artefacts.
With Active Glasses there is so much large-scale crosstalk that generates ghosts and poor 3D contrast, and small-scale crosstalk that produces fuzzy 3D, that the image looks quite phony – and then there is the annoying flicker from Active Glasses. There are plenty of comparable demonstrative examples in the wide range of 3D content that we viewed. Visually the differences between these two 3D technologies are enormous when compared side-by-side – FPR Passive Glasses TVs provide a substantially higher quality 3D visual imaging and 3D visual experience than the TVs with Active Glasses.
The Passive Glasses were quite comfortable and, more importantly, free from the annoying flicker that many people (including the author) experience with Active Glasses. The annoying picture flicker, crosstalk and ghosting from Active Glasses are the main reasons why many people have previously shunned 3D TV. The lab measurements showed Passive Glasses to perform much better than the Active Glasses, but what genuinely surprised me is that for the first time I really enjoyed watching 3D content with Passive Glasses.
Almost everyone that I invited to the 3D Shoot-Out left with the same feeling and many remarked ‘This is cool’ or ‘This is great!’ Avatar was the most requested title for people that came to see the 3D Shoot-Out and everyone was just as thrilled by it on the FPR TVs with Passive Glasses as in the movie theatres. Everyone that watched the roller coaster scene in the 3D Blu-ray movie Despicable Me commented on the sensational 3D experience of an actual roller coaster ride.
Almost Holographic 3D
One of the most fascinating visual effects of 3D TV is how the 3D image changes as you change your viewing position. If you are looking at a still image in 2D and change your viewing angle by walking left to right in front of the TV, the image of the TV picture produced by the brain stays the same as you move. But when you do that in 3D the picture appears almost holographic because the brain continuously reworks the perspective geometry of the image as you change your viewing position. As a result, people sitting at different locations will see somewhat different perspective geometries of the same 3D image.
The effect can grow to be quite large for images with significant depth. It sometimes seems as if you might be able to see additional things that are currently obscured by shifting your viewing position even more, but of course that never happens, you only see an increasingly shifted perspective view. It’s one more interesting facet of 3D TV viewing.
The Real Magic of 3D
If you read the consumer 3D movie reviews on Amazon.com people often focus on how big the 3D effects are – which is great for demos and impressing friends, but that wears off soon as a gimmick (and it also causes eye strain). The real magic of 3D in my opinion is when I am watching well-produced typically subtle 3D content with Passive Glasses and then feel that I am actually present in the scene, walking on the beach along with the people in the video, for example – an emotional response that results from convincing 3D visual input.
Following our ‘Recommendations for 3D TV Viewing’ I experienced virtually no visual fatigue, and absolutely no headaches, dizziness or other adverse effects while watching with Passive Glasses. The Passive Glasses are very lightweight, inexpensive and comfortable – they’re easy to pop on and off and it’s easy to forget that you are wearing them. The Passive clip-ons are great for people with prescription glasses. The magic of providing a comfortable, convincing, and realistic third dimension to TV viewing is what will make this 3D technology catch on and become successful in the future.
3D TV has finally come of age and arrived as a fun and pleasant enhancement to watching traditional 2D movies and TV content.
Dr. Raymond Soneira is President of DisplayMate Technologies Corporation, which produces video calibration, evaluation, and diagnostic products for consumers, technicians, and manufacturers. He is a research scientist with a career that spans physics, computer science, and television system design, and he has authored over 35 research articles in scientific journals in physics and computer science, including Scientific American.
Copyright DisplayMate Technologies Corporation. This article extract appears with kind permission of Dr. Raymond Soneira and the full details can be read at www.displaymate.com/3D_TV_ShootOut_1.htm
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