Choice in 3-D Digital Cinema
by Michael Karagosian
©2007 MKPE Consulting LLC All rights reserved worldwide
Published in the October 2007 MIT Newsletter "Ask the Professor"
Digital 3-D is the buzz today in cinema. Innovation in both content production and in presentation has elevated stereoscopic 3-D to a quality of audience experience never before possible. On the content production side, live stereoscopic capture and editing techniques make possible productions like the upcoming U2 3-D movie. Improvements in computer graphics rendering, such as that developed by Sony ImageWorks, has led to visually stunning 3-D imagery seen in Beowulf. Conversion of live 2-D productions into 3-D, perhaps best known through In-Three's demonstration clips of the Star Wars series, will make possible the re-release of popular 2-D blockbuster movies in 3-D.
Advances in the presentation of stereoscopic images through digital projection, however, have made the resurgence of 3-D possible. The Texas Instruments' DLP Cinema technology makes it possible to project stereoscopic 3-D images with a single projector and with a quality level not possible with 35mm film. In contrast, LCOS projectors, such as the Sony SRX-R220/R210 cinema projectors now in trial installations, require two projectors to present 3-D images. By outfitting either the DLP or LCOS projector with a 3-D presentation kit and glasses (available from a few companies), the exhibitor has the opportunity to present 3-D movies worthy of a premium charge to audiences.
All digital 3-D content is distributed in a 48 frame-per-second (fps) format. In the stereoscopic format, images for the left eye are distributed at 24 fps, and similarly, 24 fps for the right eye images. (The sum of these image rates equaling 48 fps.) However, there are other factors which affect the single-inventory nature of 3-D distributions, leading to disparate distribution methods. To overcome this, Digital Cinema Initiatives (DCI), a coalition of the six major motion picture studios, announced in April its draft specification for 3-D content distribution, stating that all 3-D presentation methods must utilize a common distribution format. While this ideal has not yet been realized in existing 3-D systems, the goal is technologically feasible. To accelerate progress, the Society of Motion Picture and Television Engineers (SMPTE) is now standardizing a single 3-D distribution format. As importantly, the various providers of presentation systems either now or will in not-too-distant future offer systems that support single inventory 3-D content.
DLP technology can project 3-D images with a single projector by presenting the stereoscopic left/right image pairs sequentially. This means that a left image is presented, and then a right image is presented, and never will both a left and a right image appear on the screen at the same time. However, presenting left/right images to the audience at a 48 fps rate is less than ideal as the sequential nature of the images are perceivable and distracting. To overcome this, sequential projection requires that the stereoscopic pair of images are "flashed" on screen. This involves, within the time frame of 1/24th of second, the repetition of a left/right sequence three times before presenting the next left/right sequence. This process is called "triple flash." With triple flash, the rate in which images are presented to the audience is a speedy 3 x 48 fps, or 144 fps. The triple flash rate is a property of the projector, and is the flash rate employed with all add-on technologies for presenting 3-D images in the theatre.
Several add-on kits and glasses for 3-D presentation are now available. These can be categorized by technique: polarization, spectral division, and shutter glasses. While all three techniques can be used with DLP Cinema projectors, only polarization and spectral division work with dual-projection systems. Where the 3-D add-on technologies differ is in the method employed to direct left images to left eyes and right images to right eyes.
Polarization is the most widely used technique today. It involves optically encoding each left image with a particular direction of light polarity, and each right image with an opposite direction of light polarity. In the Real D system, the encoding takes place at the projector using an electronically controlled polarizer, which Real D calls the "Zscreen." Images are decoded when the audience wears complimentary decoding polarized glasses. To allow head movement without upsetting the decoding quality of the glasses, Real D uses only circularly polarized filters in its system. Polarization alone, however, does not offer sufficient protection from crosstalk, also referred to as extinction ratio, with stereoscopic images. The audience experiences such crosstalk as a ghost in the motion picture. To enhance the ability of its polarization method to reject ghosting, Real D employs a "ghost busting" technique, which requires pre-processing of the images prior to projection. In its early systems, Real D's ghost-busting is applied prior to distribution. In future systems, ghost-busting will be applied in real-time by means of a processing box in the playback system.
Spectral division technology optically encodes left and right images by projecting each with a differently filtered spectrum of light. In the Infitec spectral division technique licensed by Dolby Laboratories, the light is filtered such that the left spectrum appears as white light (or near-white light), as does the right spectrum. In this way, this technique is importantly differentiated from the older, much lower quality, anaglyph method of using red filters for one eye and blue filters for the other. In Dolby's implementation, the light path in the projector is modified with a filter wheel to achieve spectral division of the stereoscopic images. Prior to projection, some color-balancing is applied to the image signal inside Dolby's digital cinema server. Complementary spectral division glasses are worn by audience members for decoding the images so that left eye images are seen only by the left eye, and right eye images are seen by only the right eye. To accomplish this, Dolby's glasses employ some 50 layers of thin-film coatings to create the appropriate optical interference filters. As interference filters require the light to pass through at a 90-degree angle, the glasses are curved to allow for eye movement without losing decoding quality at the viewer.
Shutter glasses, promoted for cinema use by Nuvision, take direct advantage of the sequential nature of the projected images. No special optical encoding is required with shutter glasses. To "decode" the sequential images, the audience wears glasses that allow only one eye to see the screen at any one time. By synchronizing the shutter-nature of the glasses with the flash rate of the projector, the audience correctly sees only left images in the left eye, and right images in the right eye. Synchronization of glasses takes place through infrared transmission inside the auditorium. To achieve the shutter action, the glasses must have battery-powered electronic circuitry in them that drives the liquid crystal (LCD) lenses.
The three methods described have important points of comparison. To preserve the polarized nature of the projected light in the auditorium, the polarization method requires the use of a silver screen. In contrast, the other methods, both spectral division and shutter glasses, work well with a normal mat white projection screen. Polarization, however, allows the use of very-low-cost glasses, such that they can be given away to the audience members. Both spectral division and the shutter method require expensive glasses that must be recycled (and thus regularly washed) for the method to be economical.
Manipulating projected light for the presentation of 3-D images has its price: all methods severely reduce the amount of light that reaches the eyes of the audience, typically around 85-88%. To the exhibitor, the light level determines the maximum screen size possible to present an acceptable 3-D image. Fortunately, it is acceptable to project 3-D images at significantly lower light levels than 2-D images, typically around 4 foot-lambert (ft-L), versus the standard 14 ft-L for 2-D. To compensate for low light levels, a high gain screen can be employed. Silver screens, of course, are very high in gain. It's understandable, then, that the polarization method, which requires the use of silver screens, adapts well to large screen applications, as indicated by IMAX's choice to use polarization for its 3-D presentation systems.
There is good reason for the recent resurgence in cinematic 3-D, spurred on by recent advances in both content production and in digital cinema presentation. For exhibitors, excellent choices exist with 3-D add-on technologies for 2-D digital cinema systems, all revealing high quality 3-D images to the audience. However, each 3-D add-on system presents a unique set of tradeoffs, clearly leaving the choice of system to exhibitor preference.