How Does 3D Cinema Work?

1. How do 3D movies work?

The basic idea is very simple. Close one eye. Go on- do it! Now close the eye you have open, and open the eye you had closed. What do you notice?

Each eye sees a different picture. The brain 'overlays' these two pictures on top of each other to produce a single image that has clearly defined depth. this is why it's hard to grab something when you have one eye closed- you are not seeing in 3D anymore!

So, in order to make a movie 3D, All you need to do is make sure that each eye 'sees' a slightly different picture- just like in real life. This used to be done with those red and blue tinted glasses. The same image was placed onto the film twice, and each was coloured with one of the colours on the glasses. Basically, one image was tinted red, the other was tinted blue.

This meant that each lens effectively filtered out the bit of the picture that was not meant for the eye that it covered. So each eye saw something different, and the brain combined the images to a 3D picture.

Now we have two new kinds of 3D technology for movies and the cinema- active 3D and Passive 3D. Most 3D cinemas currently use the passive version, so we will discuss that first.

Passive 3D movies:

In this case, all the active technology is in the display. The principle is exactly the same as it was 20 years ago when we where all trying to use the red and blue glasses- but this time we use a thing called polarised light. Don't worry, I'm not about to get all star trek on you! This is actually quite a simple 3D system.

Light travels in waves. Now, these waves can undulate (move up and down) in almost any direction. They might wave left or right, or they might wave up and down, or at any angle! The light we see is a mix of all sorts of light waves, all undulating at different angles.*

In a passive 3D cinema, two images are projected onto the screen at the same time. Each image is projected with light that is only polarised in one direction. The screen has special reflective qualities that preserve this polarisation- in most screens this would not be the case. The wonderful thing is that our eyes can't tell the difference between light that has been 'polarised' and regular light.

The glasses then complete the effect. One lens filters out all the light that is undulating in one direction, and the other lens filter out all the light that is undulating in the other direction. So, just like with the old red and blue 3D glasses, each eye gets a different image, the brain combines the two separate images, and you get a 3D movie!

This is the simpler (in purely mechanical terms) of the two systems.

*This is a bit of an over simplification. The polarisation is actually whether the light corkscrews clockwise or anti clockwise. The important thing to remember though is that each seperate image on the screen has a unique property that the other does not.

Active 3D.

This 3D technology is currently being pushed by the Nvidia graphics card company as a means for people to play videogames in 3D. The basic idea is still the same- to make sure that each eye 'sees' a different image. This set up requires two things: A display that can run at at least 120hz, and a set of active 3D glasses.

This system works because each 3D 'lens' in the glasses is in fact a one pixel screen. This can either be set to on (totally blanked out), or off (totally transparent). The display switches between the image intended for each eye back and forth very quickly. It shows the image for the left eye, then the right, then the left and so forth- at no time do both images appear on the screen at the same time!

The trick is that the screens in the glasses also switch back and forth at the same speed, so when the image on screen is for your right eye, the 3D glasses block your left eye. When the image on screen is for your left eye, the 3D glasses block out the right.

Just like with TV and animation in general, this changes happens so quickly that we don't notice it (this is why your display needs to have such a fast refresh rate), each eye receives a different image, and a 3D illusion is created!

Which is better?

The active 3D system is more freely available to most home users (but it still isn't cheap), but at the moment it only works for videogames. There are designs in the pipeline for polarised light televisions, but it will be some time before they are commonplace (At least a year, probably two). In short, you won't be buying 3D movies that use the same technology as the cinema for some time yet!

However, I think that in the end, the passive system will win out. It's far less fiddly, and many consumers just want 3D technology that will work 'out of the box'. The active system simply can't do that, and also requires batteries for the glasses e.t.c.

So I can't watch 3D movies at home?

Not yet. Putting on this polarised glasses and watching 2001 will not make it 3D! The technology is mainly being developed to drive people back into the cinema, so the movie makers will be holding onto the technology as long as possible. But, it is only a matter of time before this technology makes it into our homes.

How are the dual images created?

When using CGI, the developers simply make two versions of the same shot (I say simply, but it does mean almost twice the work!), one at a slightly different angle. When using cameras, a special 3D camera must be used which has two lenses, each recording a slightly different picture.