Back (page) on the big screen

What do James Cameron’s Avatar, Tim Burton’s Alice in Wonderland and a cinema advert for Doctor Who featuring Matt Smith as the travelling timelord have in common? They are screened in three dimensions. 3D movies are currently making a comeback, and computer science is helping to add that extra dimension of depth.

The mysterious floating sausage illusion

Make two fists, and then stick out your index fingers. Keeping the rest of your fingers clenched bring your hands up in front of your face and touch your two extended index fingers together. If you focus your eyes on the other side of the room and observe where your index fingers join a fleshy sausage appears. Moving your hands forwards and back will make this illusory floating breakfast product change size. It happens because each of your eyes has a slightly different view of the world and your brain computes like mad to combine these views together to give you a sense of depth. In the case of the sausage, there is enough similarity in the two out-of-focus views of your index fingers that your brain mistakenly matches them, and 3D sausage magic occurs.

The tagline: In a world divided by left and right, only a sausage could unite them.

Let there be red and blue light

The first wave of 3D movies used a method called anaglyptic stereo. The film was shot using two slightly separated but synchronised cameras, each camera recording a one eye view of the action. The two films were then developed and were tinted either red or blue. The audience had to wear special 3D glasses, one with a red lens the other with a blue lens. What this produces, as well as a grievous fashion nightmare, is the effect that the red covered eye only sees the red parts of the image and the blue parts get blocked, and vice versa for the blue filter covered eye.

The brain then does the sums to fuse these two images together, assuming the differences are caused by different distances, and 3D happens. This method was also used to make 3D comics because it was cheap and easy to do, but it really messed with the colours in the pictures.

The tagline: I see red, people.

Build a better pair of glasses

Polaroid sunglasses work because they block polarised light. The next wave of 3D technologies used this method. A two-lens camera system meant that the two separate images were squashed side by side on the same film frame. When played back through a single projector each of the squashed images was expanded by two special lenses, each lens having a different polarising filter on them. The projection had to be onto a special thin metal screen to keep the reflected polarisations perfect.

Each audience member wore special Polaroid glasses, where one polarisation was blocked from one eye, and the other polarisation was blocked from the second eye. This once again produced two different images, now in their proper colour, that the brain blended together to deliver that 3D experience. But the screens were expensive, seats at the side of the cinema couldn’t see the pictures properly, and your eyes and brain hurt after a while. So the second wave of 3D washed away until the large screen IMAX systems arrived and new computer technology made it possible to manipulate the images to be projected to reduce the viewers’ eyestrain.

The tagline: I’ll make them an offset they can’t eye fuse.

In future forget the glasses

Computer technology will open up whole new possibilities in 3D displays in the future. Methods include having lenticular arrays – a sheet of two types of tiny thin lenses which bend the light coming through them left or right. The 3D image is created by taking the two pictures and slashing them into tiny strips. You then switch on all the strips under left bending lenses, followed by all the strips under right bending lenses, and back again. If you switch between them quickly enough the brain doesn’t notice this flicker, but instead gets two different views through the left and right eyes, creating 3D. Technology like this is being developed for TV and mobile phones, so in the future depth will be as common as today’s breadth and height.

The tagline: In the future, every 3D film will be a slasher film.