How does 3D work?

Turning a flat image into one that pops out of the surface is all about depth perception. In order to appreciate depth, your brain needs to see two different angles of the same image just as we get with our binocular, two-eyed, vision. So, the main principle of 3D imaging is to send one angle of a picture to your left eye and a different angle of the same scene to the right, which your visual cortex will then put together as an object with depth. That's the theory. The last 169 years have been spent working out how the hell to actually do it.

Anaglyphs

Invented:
1850 by Wilhelm Rollmann
Usage:
3D photographs & comic books
Advantages:
Cheap and easy to produce
Disadvantages:
Looks terrible and will lead to headaches if viewing video

Anaglyph glasses
Talk to people about 3D glasses and it's probably the red/cyan ones from the anaglyph system that they'll think of. The real trick of 3D isn't superimposing two slightly different images onto a screen but getting your eyes to separate them again, and that's what the colour filters are all about.

The whole image in an anaglyph is made of two colour layers - one picture layer for one eye in reds and the other for the other eye in cyan shades. These two colours are used because they're opposites to one another. So, when the viewer looks at the resulting image through the red and cyan filters in the glasses, each eye sees one whole layer from a different angle to the other eye without seeing any of the other layer at all. The separate pictures from the two eyes are sent to the brain where the 3D stereo image is formed.

This technique is mostly used for static images rather than films, largely because it renders everything you view an annoying scape of either pure red or pure cyan. Despite popular knowledge it was not ever widely used in the cinema - even in the 50s.

Infitec (Interference Filter Technology)

Invented:
Designed by Daimler Chrysler, owned by Infitec GmbH since 2003

Usage:
Dolby 3D cinema

Advantages:
In colour and effective

Disadvantages:
Glasses are expensive and the system is unsuitable for home use


Infitec glasses
Infitec is one step up from anaglyths and is, in fact, often referred to as super-anaglyph. Instead of just splitting the two images into complementary colours, the points of view for each eye use narrow but different bandwidths of blues, reds and greens. So, the left and right eyes are actually seeing slightly different and distinct wavelengths of reds, blues and greens to each other, but it's not noticeable to human perception.

What it does mean is that you can send out two entirely separate and full colour images, which can be decoded by sets of glasses with the appropriate filters. The glasses can also correct for any slight colour differences, but because of their complexity are quite expensive to produce. Infitec is the technology used in the Dolby 3D system, as licensed from a German based company, and it works by putting special colour wheels in the projectors used which can select the wavelengths of light it lets through.

Polarised Light

Invented:
1936 by Edwin H Land

Usage:
RealD Cinema - 80% of all 3D films

Advantages:
Very effective and glasses very cheap

Disadvantages:
More complicated equipment required for home use


RealD glasses
By far the most successful way to make 3D work in the movies has been by polarising the two images. In the cinema, two projectors could be used to shine synchronised images at the same screen providing the right and left eye perspectives. The two projectors would have opposite polarising filters over the lenses to make sure they only let through light travelling in one orientation each. The audience then wears glasses with the same two polarised filters over their eyes, such that each eye can decipher one of the images from one of the projectors. Then, once again, the two distinct and different pictures are sent from the eyes into the visual cortex where the brain puts them together in 3D.

The easiest way to polarise the light in the past was to send it travelling linearly - either in the horizontal plane or the vertical one - but that meant that if you tilted your head at an angle, the light from the two images would bleed into each other and the effect would be lost. The way around this is to polarise light in a circular manner such that it moves either in a clockwise or anti-clockwise direction. Then it makes no difference which way you move your head so long as your glasses carry one filter for each direction.

The is how the RealD Cinema system works, which is the one most commonly used at the cinema today with films like Beowulf, Up and the hotly anticipated Avatar. The only difference is that RealD uses just one projector with the images for the right and left eyes alternating on the same reel at a speed of 144 time per second so that your eyes don't notice it.

The polarised light method means glasses are relatively cheap but it does require the projection of the image onto a silver screen, which is better at maintaining the light's polarised state rather than a white screen which would degrade it.

LCD Shutters (Alternate Frame method)


Usage:
XpanD cinema system, Panasonic 3D TV, DLP TVs

Advantages:
Very effective and very easier to set up

Disadvantages:
Very expensive glasses and will not work on current LCD TVs


LCD shutters
The way it's looking it's most likely that 3D will come to our TVs with the use of alternate image projecting and LCD shutter glasses. Glasses containing liquid crystals and a polarising filter have a property which means, when you pass a voltage through them, the lenses turn black, thus obscuring one eye's view through them. This can be done alternately in time with the frame rate on a TV or projector image as controlled by an IR signal from the screen. The idea is that the one display can be firing two separate images alternately for each eye as the other eye is covered up.

This makes the glasses much more expensive than any others we've looked at so far but it's a method that can and is currently used in both plasma TVs and DLP machines, both of which have high frame rates. It's going to be trickier with LCDs as they have slower refresh rates, but such is the pace in LCD advancement that it could well work here too.

Autostereoscopy (or no glasses at all)

Invented:
1840 by Sir Charles Wheatstone

Usage:
Magic Eye, side by side stereograms, Philips 3DTV

Advantages:
No glasses required

Disadvantages:
Only works from certain viewing positions and can cause headaches


Magic Eye
The simplest method of making a 3D image is by doing it yourself. Take two static images put them side by side and either look at them enough into the distance until they merge into one or go cross-eyed in front of them until you get the same effect. This idea was popularised more recently with the craze of Magic Eye images in the early 90s. Of course, it's not very practical with motion pictures unless you want a blinding headache and some serious eye damage but there are other autostereoscopic methods that the industry has come up with.

One way is to use lenticular lenses which provide a different view for each eye because of the refractions on the surface of the screen. It's the same technique as seen on holographic stickers with the fine ridges of cylindrical optics over the image, which make the subject appear to change state as you move it about.

Philips has used the same principle with its 3D LCD screens using a parallax barrier over the top of the display. The barrier is made of slits that only allow the viewer to see certain vertical lines of pixels from certain angles at any one time and therefore ensure that the viewer is seeing the two distinct images wherever they stand. The main issues with these methods are that there are dead zones where the effect doesn't work and it's also more prone to giving headaches and eye strain. It's very unlikely to be the future of mainstream 3D in the home.

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