## How 3D movies really work

In a comment on my earlier post about 3D movies, Phillip Helbig writes

In the latest Physik Journal (magazine of the German Physical Society), there was a two-page article on 3-D techniques. You mentioned three: colour, linear polarisation and the quarter-wave-plate model (your hypothesis was correct; that's how it works). The last is definitely the best of these three, but shares this problem with linear polarisation: the reflected image has to be polarised, so the screen has to be mirror-like, not just a white screen.

It’s nice to know that that’s what’s really going on!

Phillip goes on to say

There are two other techniques. One projects frames at twice the normal rate, altenately for each eye, and the glasses contain infrared-controlled LCD shutters which alternate at the appropriate rate. Probably the best system, but the glasses are more expensive.

I actually used a system that worked on this principle, way back in the 1980s, when I had a summer job working in Don Wiley‘s lab at Harvard.  It was a special computer with a huge monitor that rapidly switched between two images, along with a pair of glasses that plugged into the computer and alternately switched from opaque to transparent so that each eye was presented with one of the images.  This must have been a special-purpose, very expensive system back then.  I don’t remember how fast the switching was;  I’m sure it wasn’t fast enough to show movies and have it look good.  The lab used it to visualize big biological macromolecules.

Phillip again:

Another one is quite interesting: for one eye, use three primary colours, and for the other eye, use three OTHER primary colours. The filter for each eye only lets through the primary colours intended for that eye. (For a given perceived colour, there are many ways of mixing it out of narrow-band "primary" colours".

I’ve never heard of this.  What a cool idea.  The next time I teach our mathematical methods class, I’ll use it as an example of vector spaces and projection operators:

The spectrum of a light source is a vector in an infinite-dimensional vector space, but we only see color in a three-dimensional space.  Your visual system is in effect projecting from the big space down to the small space, so of course there are many different spectral shapes that are perceived as the same color.  The filters in front of each eye are additional projection operators.  By a clever choice of those operators, you can use different parts of the original, big vector space for each eye, and still present the entire three-dimensional space to each eye.

I bet you can mock up a not-very-many-dimensional model of how all this works.

### 8 Responses to “How 3D movies really work”

1. This Wikipedia article mentions all of the techniques, though not in much detail, and some additional ones (though normally not used in the cinema): http://en.wikipedia.org/wiki/3-D_film .

(The article in Physik Journal is available online only to members of the German Physical Society (who probably get the paper version anyway). It’s in the December 2009 issue. Being in German, it probably wouldn’t be of much use to most readers of this blog.)

Of course, all of these use only the stereoscopic effect. Other effects by which we notice that our visual world is three-dimensional—such as being able to focus selectively at different distances, being able to move and thus see around an object etc—can be reproduced on a flat screen probably only via holography.

For more realistic cinema, check out http://www.youtube.com/watch?v=TCq_nzlou0Q ; the punch line is worth the wait.

2. Michael T. says:

The method for 3D display mentioned by Phillip is known as Spectral Mulitplexing. By using six, preferably laser light sources, of RGB and R’G’B’ with a 20nm or so difference between the two sets. Triple notch filters are used for left eye red eye glasses. From what I remember, Hughes Aircraft advanced this idea about twenty years ago for use in flight simulation.

3. Ted Bunn says:

Thanks for educating me!

As Phillip points out, spectral multiplexing has the advantage of working with just a plain old white screen, which would be nice from the movie theater’s point of view. I wonder how it works from the point of view of the filmmaker, though: obviously traditional photographic film can’t produce the required narrow-band outputs. I suppose you could have two ordinary projectors with the required filters in front of them. The only downside I can see there is that you’re throwing away a lot of light at the projector, so you need an intense light source and waste a lot of energy.

4. I’m guessing here, but I suspect that, at least theoretically, all of these systems work with both analog and digital projection, but the spectral multiplexing might be better suited to digital projection. I don’t know the details, but I suppose that the colours are created by some sort of display with three primary colours and that this is then optically projected to the screen. So, one could have one display with one set of primary colours, and another with another. Digital processing could probably convert colours from a normal (digital) “film” (hard-disk or flash memory, probably) to one or the other system.

5. Tim Savage says:

Ted, we went to see the 3-D version of “Cloudy with a Chance of Meatballs” yesterday, so I tried your experiment of turning the glasses upside down. I’m sorry to report that you were wrong. It doesn’t reverse the depth, it just flattens things out a bit. That is to say, the things that are supposed to be forward are still forward, just not as much so, although a little bit more than in a traditional 2-D picture. It does give you a headache if you try it several times, though.

6. Awesome share man. I was eager to know from where those 3D effect come and this post has helped me to a certain extent.
I have heard that some companies are launching 3D TVs that do not require goggles.

7. “I have heard that some companies are launching 3D TVs that do not require goggles.”

Yes. These make use of the fact that the eyes see the screen at slightly different angles. Two images are projected so that one reaches one eye, one the other. That means you have to sit right in front of the screen.

8. hello! this is really helpful for me. thanks.