Human beings, with few exceptions, view the world from two places at once: our eyes. Our eyes are separated by approximately 2.5 inches and each retina can only generate a two-dimensional image. But we live in a three-dimensional world so our brains have evolved to combine the two-dimensional images from each eye into a crisp three-dimensional model of the world around us.

A key part of this process is binocular disparity. This refers to the fact that because each eye has a slightly different viewpoint, the two views are slightly displaced from each other. It is central to our ability to judge distances because the amount of displacement between the two images is, after all, a function of how far whatever we are observing is from our eyes.

Since our brain is used to taking two 2D images and combining them to make a 3D model for us, it should be easy enough to persuade it to do so with any two images displaying the expected degree of binocular disparity. Mercifully for b-movie film producers and, more recently, LCD display manufacturers, it turns out that it is actually very easy to fool the brain like this.

It's pretty simple to present the brain with two "stereoscopic" images, but making sure that each eye sees the relevant image has proved more taxing. The best cinema 3D (if "best" is an appropriate word) uses polarising glasses to filter the two images and create the 3D effect. Sadly, the punters never really took to the idea of wearing the glasses, polarised, green and red or otherwise, mainly because they look so incredibly naff.

The problem for computer screen and TV manufacturers, then, was to create a plausible 3D image without using specs or - as with early attempts, two separate screens. Sharp, along with most other display companies, started looking into developing a 3D display some time ago: the company's European lab began its research back in 1992. Dimension Technologies Inc (DTI) meanwhile began its research in the mid 80s and now holds several patents covering technology for switchable 2D/3D screens.

Early prototypes consisted of double screen systems but by 1994 Sharp's researchers managed to get it down to one panel. The next trick to crack was a screen that could display both 3D images and 2D images: all but the most frighteningly dedicated gamers would be reluctant to have a set up that meant changing monitors just to check email.

So how does it work? Think about that 3D cinema experience: differently polarised images play on screen together. Viewers wear special glasses with lenses that allow opposite polarisations through.

In a simple 3D screen, a very fine grating, called a "parallax barrier", is placed in front of the LCD screen. It does the job of the polarising glasses, directing light from each image slightly differently so that at a so-called "sweet spot" about 20 inches in front of the screen the two images are separated just enough that the brain will create a composite 3D image. The downside of this is that since the parallax barrier is a permanent feature of the monitor, then it's always in 3D mode - not much use for day-to-day use, as noted above.

In Sharp's screen, the parallax barrier is created by a second LCD screen - known as the "switching" LCD - which is off in normal 2D mode. When this screen is activated, it polarises in stripes so that it is impenetrable to light from the first LCD screen, generating opaque grid lines in front of the normal screen. The following graphic is from Sharp's background to the process:

The limiting factor on this method is that you have to have your head in exactly the right spot to see the 3D image, so no peering over shoulders allowed. Multiple viewer screens are not much more complicated to make: the parallax barrier is finer and cross hatched instead of striped. However, making the images for such a screen is much harder, since instead of two pictures, the 3D image is created by combining at least four viewpoints.

For the time being, then, gamers will have to make so with single viewer screens, and will have to learn to play Quake without trying to peek round corners. ®

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