I went to see Coraline this weekend. It is an animated movie made with stop motion animation (and it is supplemented by usual CGI effects). I like the medium of stop-motion very much, particularly because it is harder to achieve a good result. The most interesting thing about the movie is that it was filmed in 3D, which gives it a much more eerie feeling. The storyline was ok and there were various aspects that were very predictable. There are some elements of it that reminded me of my childhood: I used to be especially afraid of dogs and I met a lot of kids that didn’t fit in well. I also ended up eating more beets than I liked when I was growing up. Now I really love them, so I can understand a few obsessions that are portrayed in the movie. I think the thing I liked the most was that they did not use the 3D to do that usual ‘trick’ of having objects point towards the audience over and over again. There was a bit of that at the very beginning, but it soon faded away. (For an example of bad – or you could also say tasteless- uses of 3d, see this blog review of Beowulf)
Now, back to the 3D. Just to tell you a little bit about the technology involved (feel free to go to wikipedia for more info). The basic idea is that we have stereographic vision: we have two eyes, which is really good for measuring distances by angles of triangles. So ideally, each eye sees a slightly different image and our brain reconstructs a 3D map from two 2D-images.
The main technological barrier to doing movies that show a world in 3d, is that we project them on a screen, so we have just a 2D surface on which to do the work. So how do we put two images on a surface, so that each eye sees only one such image? This is is the technology problem of visualizing 3d. There are other ways to do this using holograms, but I’m going to leave that for the future.
The oldest solution to the 3d problem is to print two images on a piece of paper in different colors (red and cyan) and to use two colored filters so that each eye can only see one color distinguished from the background and not the other one: you give people glasses with a red and a green filter (the coloring can be done with acetate film and that is cheap). I had experience with a 3D geometry book that had that property when I was a kid. I don’t know how old the book was, but it was very cool.
The second way to do that is to realize that nature provided us with a way to generate two different images with light on a screen. This is beacuse light has two transverse polarizations, so that we can choose two polarizations of light to encode both images.
In old 3D film theaters, two different linear polarizations where used on a silver screen. You need a reflective material that does not damage the polarization, so the screen on which the movie was projected should be metallic in character. You then supplied people with linearly polarized glasses, and this was enough to filter the two images for each eye.
In this method there is bleeding of the image when your head is at a wrong angle (schematically shown in the figure).
If you tilt by an angle θ, then the intensity of the secondary image in the wrong eye is
So if you tilt your head by 10 degrees, you get a 3% bleed. Because this is quadratic in the angle, it is not too bad and people learn to compensate for it. However, if you are a purist, or have trouble keeping your head straight, you might complain.
However, the new technology is better. They use circular polarization of light instead: a left and right polarization can be rotated by any angle and they keep their information without bleeding. This is the case where polarizations can be thought of as complex vectors with two components (this is how one would describe it in quantum mechanics: two polarizations, and a complex amplitude for each). The idea is that the radius of each component describes the magnitude along some direction, and the relative phase of the two complex numbers indicates the phase shift between the polarizations. Circular polarization corresponds to a phase shift of π/2 between the horizontal and vertical polarization (both with the same amplitude).
The new technology is the glasses that you give people: they have a chiral polymer that only transmits one circular polarization in the optical range to your eye. With this new device, in the new 3D screens there is no problem with titlting your head: as changing the angle for a circular polarized beam keeps that polarization intact. Incidentally, in particle physics we would call this the helicity basis for the polarizations.
There is a third technology that people can use: glasses that become opaque many times per second and that are perfectly syncronized with images that alternate between left-eye and right eye. This is a more costly technology… but I believe some 3d ready televisions are banking on it. Time will tell if this is really going to be adopted.