Alternate-frame sequencing

Alternate-frame sequencing (sometimes called Alternate Image, or AI) is a method of showing 3-D film that is used in some venues. It is also used on PC systems to render 3-D games into true 3-D.

Applications in film

The principle made its public debut remarkably early. In 1922, the Teleview 3-D system was installed in a single theater in New York City. Several short films and one feature-length film were shown by running left-eye and right-eye prints in a pair of interlocked projectors with their shutters operating out of phase. Each seat in the auditorium was equipped with a viewing device containing a rapidly rotating mechanical shutter synchronized with the projector shutters. The system worked, but the expense of the installation and the unwieldiness of the viewers, which had to be supported on adjustable stands, confined its use to this one engagement.

In recent decades, the availability of lightweight optoelectronic shutters has led to an updated revival of this display method.

The movie is filmed with two cameras like most other 3-D films. Then the images are placed into a single strip of film in alternating order. In other words, there is the first left-eye image, then the corresponding right-eye image, then the next left-eye image, followed by the corresponding right-eye image and so on.

The film is then run at 48 frames-per-second instead of the traditional 24 frames-per-second. The audience wears very specialized LCD shutter glasses that have lenses that can open and close in rapid succession. The glasses also contain special radio receivers. The projection system has a transmitter that tells the glasses which eye to have open. The glasses switch eyes as the different frames come on the screen.

Applications in gaming

The same method of alternating frames can be used to render modern 3-D games into true 3-D, although a similar method involving alternate fields has been used to give a 3D illusion on consoles as old as the Sega Master System and Nintendo Famicom. Special software or hardware is used generate two channels of images, offset from each other to create the stereoscopic effect. High frame rates (typically ~100fps) are required to produce seamless graphics, as the perceived frame rate will be half the actual rate (each eye sees only half the total number of frames). Again, LCD shutter glasses synchronised with the graphics card complete the effect. Aside from consoles, alternating frames to render 3-D images was used in some arcade games, most notably Sega's Sub-Roc 3D in 1982, Namco's Thunder Ceptor II in 1986, and Taito's 1987 racer, Continental Circus.