3D is back in our cinemas and it’s big business. Cinemas can charge more for 3D movies, 3D movies earn more revenue and – along with monster screens – are another key differentiator for cinema, in this frighteningly modern age of Blu-ray, iTunes and Bittorrent (whatever that is).

Of course, 3D is nothing new, and I don’t want to use this space to reproduce Wikipedia entries about its history, nor whine about it being used badly or inappropriately, nor even do I want to assess its place as another colour for moving picture artists to add to their palette. I want to talk about something more fundamental – what is it?

The briefest of history lessons, just to explain why 3D is suddenly back in our cinemas, and to add a little context. 3D in this sense essentially means delivering two subtly different images to each eye. This can be done in various ways, with the most popular in the twentieth century being the red/blue anaglyph method. This sends a red-tinted image to one eye and a blue-tinted image to the other eye thanks to cardboard glasses. The method is cheap to implement but entails obvious compromise of colour within the images. LCD shutter glasses which blocked all light reaching each eye alternately were briefly experimented with but you can’t give out expensive items of electronic equipment free with breakfast cereal and they need both power and careful synchronisation with the projected image. Neither is ideal, which is why 3D was so niche for so long.

The reason for the current swathe of 3D movie is that two new technologies have caught up with an existing one. The glasses you get given at your local Vue or Odeon today are polarised, which effectively means that half the light enters one eye and the other half enters the other eye. No colour distortion, but a dimmer image, and you can’t just blast twice the light through 35mm film to make up for the shortfall, since twice the light means twice the heat and you’ll melt the celluloid. However, the advent of digital (filmless) projectors combined with hugely reflective metal screens means that polarisation can be deployed without the images being reduced to a murky gloom. Reflective screens + polarised glasses + digital projection systems = $$$ it seems.

And more is to come – 3D TV is on its way, with several lenticular (you know, like those corrugated postcards which seemed to move if you tilted them) systems which send a different image to each eye, provided you sit in just the right position and keep your head still.

But what exactly is the benefit of all of this?

Well, it isn’t 3D except in the very trivial sense that the images you are watching now have an apparent third dimension of depth to add to their existing dimensions of height and width. No, it’s stereoscopy or stereo imagery which is no more like actually being there than a stereo recording is actually like being at a concert.

Let’s start by considering how our brains interpret the three-dimensionality of the world at the moment. Information about how near or far things are comes from three sources.

  • Perspective. Things which are further away are smaller in our field of vision (Dougal) and parallel lines which recede from us appear to converge.
  • Parallax. Things which are nearer to us appear to cross our field of vision more quickly, whereas things which are further away cross our field of vision more slowly. This works even with stationary objects if you move your head. Try it now and notice that your monitor or laptop screen moves through your field of vision much more than the wall behind it.
  • Stereoscopy or stereopsis. The left eye sees a different version of the image than the right eye. The brain automatically combines both images into a single version of reality, giving each element of the image a “depth value”.

Of these three, perspective is present in any photograph of the world, and is obviously a part of any movie, 3D or not. The second, parallax, in the first sense is equally present in any live-action movie and, was a key reason for Walt Disney to create his multiplane camera in the 1930s. However, in the second sense, it is absent from all movies, 3D or not. You can’t move your head and see a different image as you could if you were witnessing the events live. The 3D stereoscopy is an illusion, one which is shattered if you try and interact with the image in even the most trivial way. We are not Luke Skywalker, able to walk around the projection of Princess Leia and see her from whatever angle we please. At least with your “RealD” glasses you can move your head without the image actually falling apart, unlike those lenticular TVs I mentioned earlier (disclaimer: this technology will obviously improve over time and I haven’t sat in front of one myself yet, so I know not whereof I speak – but that’s the prerogative of a blogger, is it not?).

Undoubtedly, 3D stereoscopy can enhance some movies. It can also ruin others (for some insights into what’s going right and wrong see this fascinating series of article by David Bayon on the PCPro blog). But it would please me if there were more realistic descriptions readily available for what is going on. Traditional “2D” cinema creates an illusion of depth through perspective and parallax. People with good vision in only one eye don’t live in flatland and if you close one eye, your ability to – for example – catch a ball will only be very slightly impaired. Stereoscopic movies enhance the depth illusion but do not complete it, and very odd effects can be created when an object which appears to be jutting out towards the viewer, then moves off the edge of the screen.

Stereoscopic movies are probably here to stay and the technology now exists to retrofit stereo into movies shot with a single camera, so we’ll be seeing more of them, not less. But they aren’t now, nor will they ever be as three dimensional as the room you’re sitting in right now. For that, you’ll have to go to theatre.