This article was contributed by Billy Lau

Do you remember wearing the red and cyan 3D eyeglasses when you were a kid? Well, 3D tech certainly has improved by leaps and bounds, and is turning into the latest fad in the entertainment industry. 3D films have matured from campy to epic, with the most recent blockbuster being James Cameron’s Avatar. The technology that makes 3D possible, known as stereoscopy, was pioneered at the turn of the 20th century and popularized by the entertainment industry during the 1950s and onward. The biological basis of this three dimensional effect is called stereopsis (or depth perception, but we want to sound scientific), and relies on displaying slightly different images to each eye. Then, visual processing in your brain generates the sensation of depth. Stereopsis occurs naturally in everyday life, as your eyes receive a slightly different perspective of a single three-dimensional object because your eyes are in slightly different locations.

The main hurdle to perceiving depth from flat media (such as photographs or film) is that both eyes receive the same image. Because your eyes are getting the same information, the visual processing inside your brain interprets the scene as being flat. Over the past century, many techniques have been developed to overcome this problem and to generate a sense of depth in flat images. Here, we will briefly describe the science and engineering behind two of the most popular techniques and what new technologies are being developed.

Engineering the illusion of depth

A way to overcome the obstacle of both eyes seeing the same thing is to use a device (like a pair of eyeglasses) to selectively show each eye a different image. To do this, most methods use  anaglyphs: pictures composed of a superposition of two different images. For example, a color anaglyph is an overlay of two differently colored images (usually red and cyan) that are a little shifted apart from one another. By wearing a pair of glasses that contain special filters, each eye receives a slightly different image and this consequently creates the illusion of depth in the brain. The main advantage of this technique is that it is technologically quite simple, requiring only simple image processing of the picture and inexpensive color filters in eyeglasses. However, this technique can’t accurately deliver true colors and is unsuitable for productions where high-quality special effects are required.

In addition to color anaglyphs, other techniques also exist for separating images between the left and right eyes. For example, light can be filtered not only by color, but also by polarization. Because light has wave-like properties, this means that light can oscillate in different directions as it is traveling from its source to your eye. Polarization refers to the specific direction of this oscillation. One way to visualize this is to imagine waves traveling on a string from a source to your eye. These waves can be polarized in a linear way, where one wave front travels right behind the other like cars on a train. Or, the waves can be circularly polarized, whereby the wave fronts rotate around the string as they travel to your eye, like cars on a corkscrew-shaped rollercoaster track. Either linear or circular, the polarized light can be filtered by special polarization filters similar to the ones you can find in sunglasses used for driving or fishing. By having different polarization filters on the left and right lenses in a pair of eyeglasses, two slightly different separate images can be shown to each eye to create the illusion of depth.

What about the actual generation of the picture? In film, an alternating polarization filter is synchronized to each frame of the movie to control the polarization of the light as it is projected to your eye. In most films, each second is composed of 25 frames flashing by, so this alternating polarization happens too quickly for you to notice it. The matching polarization filter on the eyeglasses allows each eye to see a different image with every other frame and consequently creates the illusion of depth without loss of color fidelity. Recent popular movies that are shown in 3D, such as Avatar, use this technique.

Upcoming technologies: 3D without goofy glasses

While the engineering behind 3D films has come a long way, there are still some major obstacles to overcome before there could be mass adoption of this medium. One of these obstacles is that the special eyeglasses, in addition to being an awkward fashion statement, are often bulky and inconvenient. Newer display technologies are currently being developed that do not require any kind of special viewing equipment. These new technologies rely on directing different images to each eye straight from the source instead of presenting an overlay or alternation of two images that will be filtered later. This can be achieved by bending the left and right image slightly with lenses, or by permitting only the left and right image to come out of the display at a particular angle.

Another technology that is currently in development is eye- or head-tracking systems, in which the display changes the perspective of the image depending on the position of the eyes or head of the viewer. The effect would be like looking through a window, where the scenery changes depending on how you look through it (in contrast to the previously described technologies, where the perspective of a scene doesn’t change). Viewer tracking is of particular interest in the video game industry: it would allow for a more immersive experience and could accommodate a more active viewer or player.

The breakneck pace of research means that a new generation of 3D technologies will be released to market within the next decade. In fact, several major display companies recently announced LCD TVs with built-in 3D capabilities. Imagine watching a sporting event or a favorite TV show in 3D at home! Regardless of what particular technology will be created or become popular, it is clear that the entertainment industry is moving towards a more realistic picture-popping experience.

— Billy Lau, Harvard School of Engineering and Applied Sciences


For more information, please see:

Wikipedia article on stereoscopy: http://en.wikipedia.org/wiki/Stereoscopy

Technical Presentations at REALD: http://www.reald.com/Content/Presentations.aspx

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