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The 3D Renaissance

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The Renaissance of 3D:
How the New 3D Technologies Work

Bill Livolsi, February 5, 2010
ProjectorCentral.com

3D projection has been available in various forms for a very long time. The first mainstream 3D films made their debut during the 1950s. People at the time were convinced that 3D was the Future of Cinema. Films like Bwana Devil, Man in the Dark, House of Wax, and It Came from Outer Space thrilled audiences with this new technology. Despite a strong push, 3D didn't stick. The few 3D theaters that existed had a hard time with the expensive, complex equipment. Small mistakes could send the two-projector system out of synchronization and destroy the effect. Audiences were dissatisfied with the lackluster image quality and poor viewing conditions. For a number of years, 3D lay dormant.

3D made a brief comeback in the 1980s. This time the technology was a little better--theaters were able to show 3D with a single projector instead of two. This greatly reduced the occurrence of synchronization errors though an improper repair splice by a projectionist could still cause the movie to lose synch. This period also saw the birth of IMAX 3D. For a few years, 3D enjoyed a healthy resurgence and people were again convinced that it was here to stay. But with content like Jaws 3-D, Friday the 13th Part 3, and Amityville 3-D, audiences soon tired of the gimmick and the craze faded away.

Now, 3D has made yet another comeback with films such as The Polar Express, Coraline, Monsters vs. Aliens, and (of course) Avatar. This time around, there are several key differences which indicate that 3D might be here for good. With the advent of digital cinema, synchronization and timing issues have been eliminated. Decreases in the cost and complexity of cinema projection equipment have made it possible for 3D systems to be installed in many more theaters. And filmmakers are learning how to work with 3D, leaving behind the gimmicky effects of the previous generations in favor of more immersive, integral applications of depth.

Perhaps most importantly, it is now much easier to bring the 3D experience into the home. With active LCD shutter glasses, a compatible display, and a powerful enough computer, we are at the point where movies and video gaming can be seen in high quality 3D that closely approximates the commercial theater experience. Blu-ray players are on the way which will deliver full 1080p movies in 3D to TVs and projectors capable of diplaying them.

3D has been around a long time and has fallen flat before, so an assertion like "3D is here to stay" is not to be made without some consideration. With the recent technological advances in both cinema and home 3D projection, however, the stakes are higher than ever.

What is 3D?

In a nutshell, 3D technologies use two overlapping images to increase the viewer's perception of depth. One image is for the right eye, the other for the left eye. Content is filmed using a specialty camera system with two lenses which are separated horizontally, much in the way your eyes see two slightly different images. This creates the impression of depth, making the image appear to extend farther back from the surface of the screen and in some cases to pop off the screen towards the viewer.

There are several competing implementations of 3D technology, but they all accomplish the same objective: they show a different image to each eye. This is achieved through the use of different types of glasses which selectively show different parts of the picture to your left and right eyes. What differs among the methods in use is the way the two images are created and the way the glasses function to separate the images.

Anaglyph Method

"Anaglyph" is a fancy word used to describe the most basic style of 3D projection in use. The left- and right-eye images are superimposed (usually recorded onto the same film stock) and color differences are used to separate the two. The viewer wears a pair of colored-lens glasses (traditionally red and cyan) which force the eyes to differentiate between the two. The end result is the impression of depth. The anaglyph method is primarily used for home releases of theatrical movies--even if those movies did not use anaglyph in the cinema--because it is inexpensive and easy to use. Until now, if you have seen a DVD or Blu-ray labeled "3D," it uses the anaglyph method.

Advantages of Anaglyph 3D

Accessible. The anaglyph method is used for current 3D DVDs and Blu-ray discs because it requires no special equipment. You don't need a "3D-ready" projector or television. It will work on your current television or projector regardless of technology, frame rate, aspect ratio, resolution, or anything else. If you can watch a movie, you can watch an anaglyph movie in 3D.

Inexpensive. The DVD and Blu-ray discs in anaglyph 3D typically cost no more than their 2D counterparts and include four pairs of cheap cardboard-framed glasses. Since there is no extra expense to the consumer in glasses, video displays, or DVD players, it is by far the least expensive 3D system out there.

Passive Technology. We will discuss this in more depth shortly, but anaglyph 3D uses passive glasses. Passive glasses have no circuitry or electronic parts which makes them inexpensive, easy to use, and lightweight. This is ideal for children and larger audiences as instances of glasses breaking or "walking away" do not cause significant financial hardship.

Disadvantages of Anaglyph 3D

Color. The big problem with anaglyph 3D and one of the reasons people have a poor impression of 3D overall is poor color. When one lens is tinting everything red and the other is tinting everything cyan, color can seem a little out-of-whack. These days post-production houses are able to adjust the color such that it looks more normal, but everything still takes on a sort of shimmery look and colors don't seem as solid or definitive as they do in normal 2D.

Light loss. A filter by definition removes things which are unwanted or undesirable. For an anaglyph system to work only selected wavelengths of light are allowed to reach the viewer's eyes, so all other wavelengths are undesirable. As a result the image seen through anaglyph glasses is much dimmer than a comparable 2D image.

Cross-talk. Partly due to the design of the glasses and partly due to the use of color filtering, anaglyph 3D is more prone to cross-talk than other technologies. Cross-talk occurs when one eye sees a portion of the image meant for the other eye. It is distracting and obvious when it occurs and it breaks the viewer's immersion in the material.

Eyestrain. Anaglyph also tends to cause more eyestrain than other technologies. Some attribute this to the use of red and cyan filters which change the wavelength of light entering the eye. Cheap glasses do not have the ability to correct for this. As a result, one eye's focal distance is different than the other's and the eyes can have a hard time focusing. Coupled with the effect of 3D video, which is already taxing to the eyes, anaglyph can give some people eyestrain-induced headaches.

Polarizer Method

Light polarizing systems are used in commercial cinema and other high-end applications. These methods provide a high-quality 3D experience in commercial cinemas, made even better thanks to the prevalence of digital projectors. In a polarizer system, light from the projector or projectors passes through a polarizing filter which makes all the light waves oscillate in the same direction. Special filters on the viewer's glasses allow only the light meant for that eye to pass through. If you have ever seen a set of Venetian blinds, you already understand the concept--from certain angles you can see through the window clearly while from other angles your view is obscured. The use of different polarization for each eye allows for two discrete images to be projected, one for each eye, which creates the impression of depth.

There are two different polarizer-based systems currently used for commercial 3D projection. One version uses two projectors, each with its own polarizing filter, to project the left- and right-eye images. This system is used in IMAX 3D presentations. The other system, known as RealD, uses one projector and a fast-switching single polarizer to accomplish the same thing. This system flips between left-eye and right-eye images very quickly and the polarizing filter switches between clockwise and counterclockwise polarization in time with these switches. Again, polarized glasses allow the viewer's eyes to see only the information intended for them.

Advantages of Polarized 3D

Color. Compared to anaglyph systems, color is more accurate when using a polarized system. While there is some light lost because of the glasses, colors are closer to their original values. It is also easier to color-correct material used in a polarized system since the lenses have very little color to them. Flesh tones in particular look more realistic and believable when viewed on a polarized system.

Passive glasses. Like anaglyph 3D, polarized 3D uses passive glasses which are inexpensive and contain no electronic parts. Unlike anaglyph, polarized glasses' frames are usually made of plastic which makes them more durable and reusable than their paper-framed counterparts.

Cross-talk. Polarized systems have a lower incidence of cross-talk than anaglyph systems. Due to the properties of polarized light, it is almost impossible for the left eye's image to make its way into the right eye or vice versa. Systems using left-right polarization such as IMAX can lose the 3-D effect if you tilt your head too far to either side, but unless you are sleeping on your neighbor's shoulder, this should not be a problem.

Disadvantages of Polarized 3D

Light loss. In all single-projector 3D systems there is a significant reduction in luminance when compared to a 2D system. For the non-physicists in the audience, luminance refers to the light reflected from a surface (in this case, the screen) at a given angle (in this case, towards your eyes). This is not the same as illuminance, which is a measure of the light striking a surface per unit of area, typically measured in lumens per square meter and posted in all of our projector reviews.

In all systems except anaglyph, this loss is caused by the rapid switching necessary to display discrete left and right eye images. At any given moment while watching a 3D movie, one eye sees a projected image while the other eye sees nothing at all. As such, each eye only sees half of the light reflected from the screen, resulting in a luminance reduction of at least 50% right off the bat. I say "at least" because polarizers and glasses are not perfectly efficient. Polarization by its very nature only allows a portion of the projector's total light to reach the screen. There is some further light lost due to the glasses. The end result is a picture that appears much less bright than a 2D film from the same projector.

This is actually one of the chief advantages of using a two-projector system. Each eye gets the benefit of the full light output of one projector, though light loss due to polarization and glasses still apply. The end result is a much brighter picture, all else being equal. That last part is important because all else is not likely to be equal. The most widespread commercial implementation of a two-projector system is IMAX, which also uses a much larger screen than most RealD theaters. The projectors used can vary in their lumen output. Polarizing plates can vary in their efficiencies, as can glasses. There are too many variables to make any definitive judgement about which system is "better," but each has its advantages.

Expensive. While the glasses themselves are inexpensive, the rest of the system is not. It requires at least one high-end digital projector paired with a special processing device to manage synchronization, at least one polarizing filter, and a silver screen (traditional white screens cannot hold polarity). Two-projector systems obviously require two projectors and two polarizers.

Interference Filter 3D

Another implementation of 3D in commercial theaters uses a technology called interference filters, made by the German company Infitec. This system uses one projector and does not require a silver screen. Infitec 3D compatible projectors have a special color wheel inserted between the lamp and the imaging device itself that divides the primary colors into discrete sections. Think of it this way: where there once were red, green, and blue, there are now red-1, red-2, green-1, green-2, blue-1, and blue-2. Special interference filter glasses allow the left eye to see only the "1" sections while the right eye sees only the "2" sections. The glasses use additional filters to correct color perception so what your eyes see is as close as possible to the original film. You may have seen this technology already: it is known as Dolby 3D in commercial cinemas.

Dolby 3D is not nearly as widespread in cinema projection as polarized 3D systems are, but the world premiere of Avatar at the Empire Cinema in Leicester Square, London, was shown in Dolby 3D.

Advantages of Interference Filter 3D

Passive glasses. Passive glasses are a common element in 3D systems designed for commercial theaters and other large venues both for their ease of use and for their relatively low cost. Interference filter 3D glasses are more difficult to produce than are polarized glasses. As such, they are built to last. The lenses are made of glass and the heavy-duty plastic frames even have a slot for an anti-theft device. They are more resistant to scratches and scuffs than the more flimsy polarized lenses.

No silver screen. Several decades ago, silver screens were used for all movies, but they fell out of favor. Silver screens have more restrictive viewing angles than do white screens, so when projectors became bright enough to light up a white screen sufficiently, silver screens fell by the wayside.

What was true about silver screens then is still true now: brightness falls off significantly for viewers sitting off-center. In a commercial theater this can include a significant portion of the audience. Since interference filter 3D does not use polarization it does not need a silver screen, thereby improving the viewing experience for everyone in the audience. People along the sides of a theater get more consistent screen illumination while everyone in the audience gets color fidelity on par with non-stereoscopic film.

Disadvantages of Interference Filter 3D

Light loss. Any single-projector 3D display method suffers from significant light loss compared to 2D display on the same projector. This does not mean that single-projector interference filter and polarizer systems are always the same brightness, or that dual-projector implementations are always brighter than single-projector systems. Single-projector polarized systems, interference filter systems, and shutter glasses systems all share this limitation.

Specialized equipment. It is possible to install a single-projector polarized system like RealD on an existing DLP Cinema projector, which many commercial cinemas already use, without actually altering the projector itself. Since the Infitec 3D mechanism works inside the projector, theaters need to either purchase cinema projectors pre-fitted with the Infitec color wheel or have the wheel installed inside their existing DLP cinema projectors by a trained engineer.

Expensive glasses. The major advantage of passive glasses is that they are inexpensive, so one might expect Infitec 3D glasses to be inexpensive as well. They are not. Most of the manufacturing cost of Infitec glasses is in the lenses themselves - the glass must be given several coatings to help them differentiate between the wavelengths used. The most recent Dolby press release related to 3D glasses has their wholesale price at $27.50 per pair. Obviously this represents a significant expense to a participating theater, so great lengths are taken to ensure the glasses are safely returned by theater patrons. One Dolby 3D theater I attended required that I hand over a picture ID in exchange for the glasses.

LCD Shutter Glasses Method

LCD shutter glasses are the first high-quality 3D implementation suitable for home use. In systems that use these glasses, the video display shows alternating left-right images very rapidly--up to 120 frames per second. The viewer wears a pair of active LCD shutter glasses which alternately block the left or right eye. Much like the effect of a DLP color wheel, this happens so quickly that your brain melds the two images together and creates the impression of a single image with 3D depth.

Advantages of Shutter Glasses

Color. While glasses vary, most have lenses without significant tint, which puts them on par with the polarization method for color accuracy. The glasses we have been using have a greenish tint to them but other brands we have seen do not.

Cross-talk. LCD glasses are perhaps the best of the bunch when it comes to cross-talk because the eye not being used is actively blocked by the shutter mechanism. Earlier versions had problems with ghosting, where part of the previous image would be retained on the screen after a new image had appeared. Newer, faster displays have all but eliminated this artifact.

Relatively inexpensive. The prices of 3D projectors and televisions are already very low. Even today, a projector capable of 3D at 720p is available for less than $800. While there are ancillary costs involved, such as a computer with a powerful graphics card and a set of glasses for each viewer, for home use with small audiences it is certainly affordable.

Disadvantages of Shutter Glasses

Active glasses. This is the main difference between LCD shutter glasses and other systems: the mechanism that controls which eye sees what is embedded in the glasses, not the content or the projector. LCD shutter glasses are great for small audiences of responsible adults, but they have several downsides. They are expensive compared to the cost of glasses for all other technologies ($100 to $150 per pair at the time of this article's publication). They are complex, with batteries and electronics and fragile LCD lenses. If they are dropped, stepped on, or run out of batteries, it can disrupt the 3D experience. And if you have a lot of friends who want to come see the Super Bowl in 3D, it can be costly to supply everyone with their own pair.

Synchronization. An active shutter system needs to have a way to synchronize the shuttering of the glasses with the content on the screen. Some systems use an infrared emitter. Texas Instruments' system for projectors and televisions, called DLP Link, uses synchronization pulses built into the projected image itself. While the two implementations use different sources for the pulse information, the pulses themselves are the same.

There is a difference between a left-eye pulse and a right-eye pulse, which is why all glasses synchronized with a given TV will be in synch with each other as well. But the projector has no way of knowing which frame is intended for which eye, leading to an interesting problem. These pulses can become reversed, leading to one eye seeing what is intended for the other. In other words the left-eye pulse will be associated with the right-eye image. This is known as the "pseudoscopic effect" and it is quite disconcerting the first few times you experience it. The effect is a strange sense of the image being inside-out, as if you are looking at a hollow 3D image from the wrong side. It is a common occurrence, so manufacturers have planned for it - it can be remedied by toggling an option in the display, or a computer's video card driver, or sometimes in the software itself. Still, it is not as foolproof as other systems.

Flicker. Some people have reported seeing flicker when using shutter glasses. Flicker is what happens when the refresh rate is not high enough, leading to a sort of stop-motion effect. It is true that, since each eye only receives half of the refresh rate, projectors used with shutter glasses must be very fast. To this end, modern implementations use at least a 120Hz refresh rate to deliver 60 frames per second per eye. We did not encounter any flicker during our testing so we cannot comment on the phenomenon's prevalence or seriousness.

Light loss. Shutter glasses systems use a single projector which switches rapidly to display different images, so there is a minimum 50% reduction in light output compared to 2D just as in other single-projector systems. The glasses reduce perceived brightness as well. While not all glasses are the same, the models we have tested decrease perceived brightness by roughly 60% regardless of the display or the content. The upside is that they also reduce black levels and contrast is increased significantly.

Conclusion

Commercial theater 3D is now firmly established, thanks in no small part to James Cameron's Avatar. Several more big releases are coming soon to 3D theaters near you, including Tim Burton's Alice in Wonderland (March), Shrek Forever After (May), and Toy Story 3 (June). These should continue to whet the consumer's appetite for more 3D entertainment in commercial theaters as well as in the home.

Last month an avalanche of 3D video products debuted at CES in Las Vegas. The momentum building toward a revolution in 3D-enabled video gaming and movies for the home looks upstoppable. Blu-ray players capable of delivering full 1080p films in 3D will hit the market by this summer. Lower resolution projectors capable of displaying some forms of 3D content from computers are already here and they can be used with active LCD shutter glasses for a higher quality viewing experience than traditional anaglyph. 3D-enabled 1080p home theater projectors will begin to appear this fall. All of this will be fascinating to watch. 2010 will be remembered as the year that 3D came home.

(05/25/19 - 02:04 AM PST)
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