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The feeling of immersion is greatly enhanced by stereoscopic display, which makes images look three-dimensional. But contrary to common belief, you don't have to see a stereo image to feel immersed. Just think of the IMAX theater (the 2D one, not 3D). Haven't you felt immersed in the movie? That is because the screen is of such a huge size that it occupies your entire field of view! A very large, sometimes curving around the viewer, screen is a widely used VR device.

But stereovision (sometimes called 3D vision) is not to be underestimated. If you have ever been to the Universal Studios show Terminator, you know just what I am talking about. Aided by stereovision, the feeling of immersion can be so strong that one literally loses perception of what's real and what's just an image. How does stereovision (also known as binocular vision) work? In order for us to see things in stereo, each eye has to see a slightly different, shifted picture. Put a finger in front of your face and look at it with one eye and then with the other. Notice how the two views differ a little bit? This is called the viewing parallax; without it we can’t have the depth perception.

On the screen, like on a piece of paper, we get depth cues from the perspective drawing (you know, the objects farther away are drawn smaller etc.). To see a screen or paper image truly in stereo we must use some special device, and the image has to be drawn twice to create the parallax. The simplest device is a head-mounted display: there is a small screen for each eye, showing a shifted image (View-Master stereoscopic viewer is the best example of that).
 

Here's what happens with two overlapping Polaroid glass pieces. Since each ace only lets through light in one plane, when oriented at right angles to each other they completely block light. One sheet of Polaroid glass lets through light waves that are oriented in the horizontal plane, as shown in the top right image, while the other allows through only light waves that are polarized in the vertical plane.

A more complex device is stereo glasses, and red-and-green glasses are one kind you probably have seen. On paper (or screen) one image is drawn in red and the other, slightly shifted, in green. When you put on the glasses, your "red" eye can only see green image and the "green" eye - the red one. So, each eye sees the same image but slightly shifted, which is the recipe for stereovision! If you happen to have a pair of glasses like that, you will see that the cube on the picture "stands out" from the screen. (Actually, it stands out if your right eye is green, and seems to be inside the screen if green is the left eye. Can you explain why?)

Red and green glasses have been used for 3D movies. This is not too exciting though, because we only see one color. Naturally, we want to see color images. To be able to do that we need another kind of glasses: "passive" polarized or "active" LCD shutter glasses. The first ones are the kind used in movie theaters. The image on the screen is again projected twice, but in orthogonally (at right angles to each other) polarized lights. If you think of light as a beam, polarizing it means cutting only one slice from that beam -- say, vertical or horizontal. So, one image comes from the horizontal slice, the other from the vertical slice. The glasses are made of a special material which lets through only one slice of light; each lens can be oriented whichever way we want, so one is placed to pass only the horizontal light slice, the other, only the vertical light slice. That’s how each eye sees its own picture.

"Active" glasses are a much more complex device. The idea is that computer generates two images, one at a time (so far, we had both images at the same time, remember?) many times per second. A typical setup is to display each image 48 times in one second, flashing them one after another. If you look at the screen in this case, all you can see is a double image, just like before. But the glasses are made so that each lens, made of liquid crystals, opens and closes in synch with the flashing of the images. So, when the left image is shown, the left lens is open and the right one is shut, and vice-versa. In our example each lens will open and shut 48 times a second.