3D became popular following the introduction of digital cinema in part because of its ability to operate with a single unmodified digital projector. The 3D effect takes advantage of the stereoscopic nature of human vision. A 3D production delivers a left-eye series of pictures, and a right-eye series of pictures. In practice, both sets of images are packaged in a single track file, as defined in SMPTE ST 429-10 DCP Stereoscopic Picture Track File. When packaging stereoscopic content, a stereoscopic frame pair is considered a single frame, with each frame carrying a left-eye and a right-eye image. In practice, stereoscopic content is normally played at a 48 frame-per-second (fps) rate, or 24 fps-per-eye. A stereoscopic package as such would be referred to as 24 fps 3D.
At the projector, the left-eye and right-eye images of 3D content can be processed sequentially or simultaneously. In order for the audience to correctly perceive the stereoscopic effect, left-eye images must be directed to the left eye, and right-eye images directed to the right eye. There are three techniques for performing this selection, all of which require the use of glasses.
The most straight-forward technique is that of shutter glasses. Shutter glasses are designed to allow light to pass to only one eye at a time. By synchronizing the shutter mechanism of the glasses with the playout of left-eye and right-eye images, the correct images are directed to each eye. The shutter process must take place quickly so as not to be detectable. To accomplish this, the sequential content is played using a technique called “double-flash” or “triple-flash.”
In triple-flash, left-eye and right-eye images are displayed sequentially by the projector at 3 times the rate of playout. (Double-flash requires 2 times the rate of playout.) When counting the linear image rate of 48 fps (2 x 24 fps), the projector displays 144 images per second. This is illustrated below for frames 0 and 1.
Figure P-15. Triple-Flash Sequential 3D
Another technique for selecting images when viewing is polarization. Circular polarization is the most common 3D selection technique found in cinema. The 3D polarizing element is placed after the projector lens, and the audience wears matching polarized glasses designed to select the appropriate image for each eye. For sequential 3D projection, the polarizing element is synchronized with the flash rate of the projector, changing the state of polarization according to whether the image being displayed is left-eye or right-eye. It is also possible to employ polarization with simultaneous 3D projection, where left-eye and right-eye images are displayed simultaneously. Simultaneous 3D projection is possible with dual projectors and specially configured single projectors.
A third technique for selecting stereoscopic images is color-separation, also known as spectral filtering. In this technique, image selection is performed by allowing the left eye to see one set of RGB primaries, while the right eye sees a different set of RGB primaries. This requires a single projector capable of generating 6 primaries (2 x R, 2 x G, 2 x B), or two projectors having two different sets of 3 primaries. Depending upon the projector configuration, left-eye and right-eye images can be projected sequentially or simultaneous. If sequentially, the images are flashed. The illustration below shows left-eye images being projected simultaneously using primaries RGB(L), and right-eye images projected using primaries RGB(R).
Figure P-16. Color-Separated 3D (shown for simultaneous 3D presentation)
All three methods are employed in cinemas, although the most popular system is polarization, due to the low cost of glasses and the ability to recycle glasses. Shutter glasses require active circuitry, powered by a battery, and cleaning after each use. Similarly, color-separating glasses require somewhat expensive filter coatings on the lenses, as well as cleaning after each use. It’s worth noting that 3D color-separation, using RGB laser-illuminated projectors configured with 6 primaries (6P), delivers the most light-efficient 3D of any of the techniques described.