Alternate-frame sequencing

Alternate-frame squencing (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 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.

This system is not generally used anymore in venues in favor of polarization. It is used, however, in home 3-D movie systems.

Applications in gaming

The same method of alternating frames can be used to render modern 3-D games into true 3-D, although it has been used to give a 3D illusion on consoles as old as the Sega Master System and Nintendo Famicom. Here, special software/hardware to used generate two channels of images, ofset 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 frames). Again, LCD shutter glasses synchronised with the graphics card complete the effect.