Digital 3-D is a somewhat complex subject, and bears some discussion. The term "3-D" is used in digital cinema to describe stereoscopic images, where alternating left and right images are displayed on a 2-D screen, and a method for viewing these images is employed such that each eye only sees the intended images. The result is that the left eye sees a slightly different image from the right eye, providing the stereoscopic effect.

There are no standards in progress to date regarding digital 3-D, and DCI does not address digital 3-D in its specification. Because of this, there is no one single distribution method for digital 3-D content today. In response, SMPTE has formed a new working group to address the issues posed by digital 3-D content. It is expected that this group will pursue a modification of the 2-D delivery method as an interchange format for digital stereoscopic images.

While there are many techniques for creating 3-D images, there are only two classes of techniques for digitally projecting them. These systems can be identified as those using passive polarized glasses, and those systems using active "shutter" glasses. Both classes of projection systems can display any form of digital 3-D content, regardless of how it was created. The primary quality difference between the two methods of projection appears as a difference in image ghosting, where the left eye sees a little of the right image, and vice-versa. Active glasses are touted as superior in ghost reduction, due to a high extinction ratio (optical power ratio) between the left and right eyes. However, there are methods that passive systems can employ to improve this ratio. Another difference is that a polarized image must be projected when using passively polarized glasses. Projection of a polarized image requires the use of a silver screen to maintain the light polarization. There are varied opinions in the industry regarding the quality of images projected on silver screens. In terms of ownership, the primary difference between passive and active glasses is that passive glasses can be cheap enough to give away to patrons, and active glasses, being too expensive to give away, must be washed before reuse. In practice, passive systems are the most widely used in cinemas today. A popular digital 3-D projection method supplied by Real D™ utilizes passive glasses.

Ideally, only a single digital projector is used to display the 3-D image. The reason being not only economics, but also image quality. Use of a single projector will insure that the left and right images are properly registered on the screen, with no intervention required by the projectionist. However, the 3-D projection process is not efficient with light, resulting in a screen size limitation for single projection systems. This may force the decision to use two projectors for large screen presentations. Experience with image size in trade show demonstrations indicates when using the full 2K imager array with an anamorphic lens, images can be as wide as 14 meters with a low-gain screen without experiencing a degradation in quality. If using a normal lens and a low-gain screen, then the maximum image size will be smaller. Obviously, if using a high-gain screen, larger image sizes can be achieved. The only digital projection technology to date capable of displaying single-projector 3-D images is DLP Cinema. LCOS technology, which is being developed for 4K projectors, has not been capable to date of displaying stereoscopic images with a single projector.

Another noteworthy issue of digital 3-D is the "flash" rate, which is the rate at which the left and right images are "flashed" back and forth on-screen before displaying the next stereoscopic pair. The ability of digital projection to flash stereoscopic images improves the quality of digital 3-D presentations over film. However, keep in mind that flashing offers no improvement over motion smoothness. Motion smoothness is determined by the content's frame rate, not the flash rate. The flash rate improves the quality of the stereoscopic effect, and is an integer multiple of the content's frame rate. In digital cinema, the content for each eye is supplied at a 24 fps (frames per second) rate. The stereoscopic frame rate to the projector, therefore, is 2 x 24 fps = 48 fps. The demonstrations of In-Three™-produced content at ShoWest, Cinema Expo, and IBC used "double" flash rate (2 x 48 fps = 96 fps) digital projection, producing high quality stereoscopic images with no eye strain. Disney's "Chicken Little" was displayed in most theatres using a passive glasses system with triple flash rate (3 x 48 fps = 144 fps). DLP Cinema projectors, working at triple flash rate, cannot utilize the full 2K imager array, losing about 20% of the potential image resolution. Triple flash is also less light efficient than double flash. Combined with the fact that it cannot utilize the full lamphouse power applied to the imager array, triple flash reduces the maximum acceptable projected image size below that which is possible with double flash.