Friday, January 8, 2010

3D cinema explained

Avatar 3D is amazing.

Do you wonder how the 3D works? I did. Turns out it is distributed in several ways. Most cinemas use Dolby 3D or RealD. I got to see Avatar 3D in RealD, and it was nice!

The most important requirement to achieve actual depth perception is to get a different (distinct) picture to the each of the left and right eye, thus achieving stereopsis. This can be achieved in a variety of ways, but in cinema, the most obvious way (if not sole way, at this point in time), is to have the spectators wear glasses that separate two pictures that are displayed on top of each other on the same screen.

One way is to use colored glasses. The really old-skool is to use red/green or red/blue. This ends up loosing out on the blue or green component. Thus, a newer way is thus to use red / green-blue (cyan). These separate color inputs is somewhat fused in the brain, thus giving "full colors", but the effect is rather annoying, and you can literally feel the tearing in your brain as it tries to restore the 3D information from the spatially slightly differing images (but which have massively different color contents) and at the same time fuse the colors, into one single full color 3D image.

Dolby 3D uses an extended version of this colored glasses idea: it lets both eyes see red, green and blue - only different variations of the colors. The glasses is thus very specific in the bands they let through: One band of red, one band of green and one band of blue for the left eye, and another band of red/green/blue for the right - without having an overlap. According to the WP article, "Difference in color perception for the left and right eye are corrected within the glasses through additional filters" - this just needs to correct the relative strength of the r/g/b on one eye, and average on the other, since the eye doesn't distinguish between any two versions of red as long as they don't affect the other colors. To avoid using two projectors, a special color wheel that alternates between these two sets of red/green/blues is installed, so that each eye sees alternatively a black frame, or the image dedicated to it. A benefit of this system is that the screen can be any white surface, as it does not depend on any polarization effect of the light. Also, head-tilting does not pose a problem.

RealD, on the other hand, use polarized light. This way of separation needs a literal silver screen - yes, it is not just a figure of speech, there was actual silver involved. And now, with the needs of polarization-based 3D cinema, there is again. The obvious idea here is to use vertical polarization on one eye and horizontal on the other. This idea have been employed many times throughout the history of cinema. However, this has a problem when the viewer tilts his head: Since both glasses now are out of alignment, both eyes will see both images, effectively killing the 3D effect as if not wearing the glasses at all. RealD have fixed this, and this initially baffled me when checking out the glasses - by holding two sets of glasses over each other, one tilted 90 degrees, one could still see through them (try that with two sets of polarizing sunglasses!). They use circular polarization. Again, to not have to install two projectors, a device in front of the projector switches between the two polarizations (apparently not using a rotating wheel, even though this should be possible since the polarization is circular instead of horizontal/vertical, in which case a rotating device would not be practical).

Here's a "How stuff works" article on 3D glasses.

Here's a good forum post on this exact subject. He points out that 3D is not new in any way - "In the 20's, sound was a gimick. In the 30's, color was a gimick, in the 50's/60's 3D and widescreens where both gimicks. Widescreens caught on, 3D didn't."

And here's an article that compares the three techniques now in existence - IMAX 3D, RealD and Dolby 3D - for the same movie, Beowulf. He goes into a bit of technical detail. (IMAX 3D uses vertical/horizontal polarization, but with two distinct projectors, currently employing actual film.)

As an absurd aside, I happened across this article about some kind of shrimp, the Mantis shrimp, that "sees" circularly polarized light. Other animals can distinguish between different linearly polarized light, which for example is nice when you have to see through a water surface (the water surface suddenly vanishes - try this with a SLR camera with a polarizing filter installed, or with your polarizing sunglasses, tilting your head to one side or the other, when standing ashore trying to see the bottom of the sea). However, these guys can distinguish any polarization from circular, through elliptic, to linear, in any phase. What benefit would this have for the shrimp? As a researcher pointed out: "Some of the animals that the [mantis shrimp] like to eat are transparent, and quite hard to see in sea-water – except that they're packed full of polarizing sugars – which makes them light up like Christmas trees as far as these shrimp are concerned". The Mantis shrimp seems like an awesome killer - they have hyperspectral vision (vision that stretches out to ultraviolet and infrared), and can as mentioned distinguish any polarization, and have, depending on species, either built-in spears or clubs, which they can employ with an acceleration of 10,400 g and which acquire speeds of 23 m/s, about the acceleration of a .22 calibre bullet. This is so fast that cavitation bubbles form, which give a second shock - so if the guy misses you with its club, you might die from the shockwave from the collapsing cavitation bubbles! Nice that I don't have to be on the lookout for such dudes every day.

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