Instead of using glasses to distort the viewer's vision, black stripes and twisted angles can also cause the 3D illusion when one's eyes each adjusts the images to a slightly different version of the picture. Up until very recently, this technology only worked by having the projector aimed directly at the viewer, as on a computer screen or an Ipod. Movie theatres have their projectors located in the back of the room, because it would take an enormous monitor to show the movies at that scale. Up until very recently this meant that glasses-less 3D was impossible in the cinema.
In Augusto of 2012, a South Korean research group published a detailed essay in Optics Express explaining a projector they developed that would be able to use the black stripes and twisted angles to show glasses-less 3D as well. Wired explained:
"The new method would allow movie theaters to keep their projectors where they’ve always been, behind the audience, and uses fairly simple optical technology. A special array sits in front of the projector and polarizes its light. A filter covering the screen then obscures different vertical regions of the screen, like the slats of venetian blinds. Each of your eyes, sitting at a slightly different angle, has some of the screen blocked and some of the screen visible. The movie has the right-eye and left-eye images interleaved in vertical columns with one another. The trick then is to have the light visible to your left eye contain the left-eye pixels and vice versa for the right eye."
The technology is not easily accessable at this time, but the researches believe it would be fairly cost-effective to modify existing theatres with the new technology. It is believed it could become widespread in as soon as five years.