It seems like just about every week there is a new 3D technology being announced or released. This is due to the fact that there are so many possible ways to present stereoscopic media to a viewer, but none of the existing solutions are perfect, so there is clear room for improvement. There has been a lot of creative innovation in this regard over the last few years, which is one of the reasons I am so interested in this subject, but the variety of competing solutions can be quite confusing, especially to someone unfamiliar with any of the options.
There are a number of strategies that can be employed to present separate images to the viewers’ eyes. The first is to mount separate displays in front of each eye, but this limits size and resolution, and only serves a single viewer. All other options involve displaying both images on the same screen, and filtering what each eye sees.
Shutter glasses can be used with displays that have high refresh rates, to filter alternating frames from one eye or the other. A 120hz display can provide 60 frames to each eye, which is enough to create smooth motion and make the blinking shutter imperceivable. The disadvantage is that each viewer requires a pair of expensive active shutter glasses, which all require charged batteries or some other source of power.
Polarization has been used in many different variations of 3D display solution. The most straightforward is to use dual projectors, each projecting onto the same silver matte screen, with opposite polarizing filters in front of the lenses. Complementary filters are used in cheap passive 3D glasses that each viewer wears, filtering out light from one projector or the other, from each eye. One issue with this method is that perfectly aligning two projectors on a large screen can be very challenging. To solve this, RealD took this process a step further, and created a polarizer for a single projector that alternates the polarization angle at the same rate that shutter glasses usually blink. The result is that the “active” part of the filtering for a 120hz projection is done once, and each viewer can use cheap passive 3D glasses to separate the views for each eye, while experiencing the full resolution results offered by shutter glasses.
Polarization can also be used within passive 3D LCD displays, but currently with these solutions, any pixel is permanently polarized either right or left. As long as the image sent to the screen conforms to the pattern, usually horizontal interlaced, it can send imagery from the correct angle to each individual pixel. This requires a 1:1 image to pixel ratio, in order to ensure that the display places pixels from the correct angle in the correct locations. The disadvantage is that usually half of the image resolution to each eye is lost to the filtering process. Displays that have this type of built in polarization can be viewed with cheaper passive 3D glasses. In my opinion, any passive 3D is also easier on the viewer’s eyes, especially over periods of extended use, since there is no alternating flicker.
The cheapest, but lowest quality option for viewing stereoscopic depth information on regular displays, involves using color information to separate the images for each eye. This is called anaglyphic processing, and the benefit is that nearly any standard full color display device can be used, but it comes at the cost of most or all of the original color information being discarded. Anaglyphic video can either be generated on the fly during playback from two separate streams, or rendered out into a single stream to be edited, usually as an offline version. Viewing anaglyphic imagery, or even just wearing anaglyphic glasses for any significant period of time is usually not comfortable.
On the other end of the spectrum, there are certain displays available that use lenticular filters to isolate the separate views, and focus them on each eye. This is similar to the process used to create depth and motion effects on static 2D surfaces, called lenticular printing, but using this method for video requires the underlying display to be extremely high resolution. With stereoscopic source, this results in certain areas where the effect is null or reversed, and requires the viewer to be in just the right spot for maximum effectiveness.
The next step to solve that issue is to replace the alternating left and right views projected from the lenticular screen, with a series of progressively different views. That way any viewing angle within reason can be accommodated for, without gaps where the illusion breaks down. Instead of recording every variation of viewing angle, these perspectives are usually generated on the fly from a single recorded perspective, usually with the help of a Z-depth map. The Z-depth channel is like an alpha channel, but instead of storing values of transparency info, it records how far an object on screen is from the camera. This information can either be captured at record time with specialized hardware, or be generated from an image analysis process, with a bit of interpolation.
At the end of the day, each option has certain pros and cons, and multiple display types could be used at different stages of a single project. As long as you have both full streams of data available, you can adapt your content for any display. Once you render it into a display specific format, like interlaced or anaglyphic, certain image content may be lost that cannot be recovered for use on other display types, without going back to your original source. Because of this, it is highly preferable to have a system that allows you to display stereoscopic content in 3D without having to pre-render into a dedicated display format. As computers become more powerful, and better software is developed, that capability is becoming much more common, greatly aiding the stereoscopic post process.