People can see in three dimensions because of parallax. Our two eyes give the brain two slightly different viewing angles that it combines to produce depth.

It means that we can use flat images to produce the illusion of depth, whether it be the overlapping images of a sequence of air photos to study the topography of Earth and other planets, the use of computers in 3-D visualization for design and modeling, 3-D movies at the theater or 3-D television on the horizon.

Filmmakers realized the potential for 3-D movies early in the 20th century when “;The Power of Love”; played at the Ambassador Hotel Theater in Los Angeles on Sept. 27, 1922.

The early 3-D films used “;anaglyph”; format, which created the 3-D illusion by projecting images from parallel cameras in two monochromatic colors. Cardboard glasses with red and cyan filters allowed each eye to see only one color image.

There were many drawbacks to the technology.

Two movie cameras had to capture images in sync, which was nearly impossible because the movement of film through the camera can create distortion.

It required a projector for the left and right images that had to align them perfectly, doubling the amount of film and requiring an intermission to change reels because theaters had only two projectors.

It created disorientation and headaches for many people, and the popularity did not extend much beyond the horror and space-invasion films of the 1950s.

Three-D movies made a brief comeback in the 1980s using linear polarization. In this method the right and left images are projected through Polaroid filters that are oriented at right angles. The glasses used for viewing have Polaroid lenses, each oriented to filter out the other image. These allowed for color images, but tilting the head from perfect upright alignment allowed the image from one eye to 'leak' through to the other eye.

Today's 3-D movies are filmed digitally, which eliminates the sync problem, and use projected images and plastic lenses that are circular polarized, eliminating the “;tilt leakage”; problem.

Another method of separating the two images is LCD shuttering, in which left and right images quickly alternate in sync with glasses that alternately blacken. Computer games have used LCD shuttering for a long time because the computer can control the shuttering via a direct connection to the glasses.

Infrared-controlled LCD shuttering now allows glasses to be controlled from the front of the theater, and the new 3-D televisions have an infrared-radiation emitter in the TV that controls the shutters.

Not everyone can appreciate 3-D movies. An estimated 4 percent of people are physically incapable of seeing 3-D, and research suggests that as many as 56 percent have problems with binocular vision that present difficulties in seeing 3-D, which could explain the headaches that many viewers report.