3D
3D. I hear it’s been all the rage.
I am, of course, referring to 3D in the sense of 3D televisions and 3D movies and such. Not real-world 3D – that’s been around for, like, forever.
Personally, I feel that 3D put a lot of added value into a movie. It just makes it feel that much more realistic, it strongly increases the sense of actually being there rather than just watching a flat image on a flat screen. Putting 3D into any random television program is unnecessary if you ask me, but 3D in movies (or video games)? Hell freakin yeah!
The problem that we have, however, is that 3D movies don’t work for everyone. Some people hardly notice the effect, or they get dizzy, or they get headaches. All these things have perfectly good reasons, which I’m going to talk about in a moment. But before we get to that, we should first discuss what the 3D thing really is, how humans perceive 3D, and how 3D video material is created.
(Note that I’ll be talking about 3D movies for the rest of this post, but the same concept applies to video games or any sort of 3D material as well.)
So, what is it, anyway?
When you’re looking at your computer monitor (like you are probably doing right now), you’re watching a flat image being displayed on a flat surface. Every point on that surface can be uniquely identified using two coordinates – one for the horizontal axis and one for the vertical axis. This much you probably already know.
When you look away from your monitor (which is probably a pretty rare occurence), you’ll notice that there is actually a rest of the world around us, too. (If you’re not used to it, I recommend to be very careful and look away only for short periods and slowly increasing the time spent looking away each time – otherwise the existence of this world may come as a bit of a shock.) This rest of the world is not projected on a flat surface. Things, as we perceive them, are in a 3-dimensional space, which means we need 3 coordinates to uniquely identify a position (from a given reference point, of course).
When you look into this mysterious ‘world’ thing, you’ll find that many of the things within it are located at some distance from yourself. You’ll also find that you are capable of estimating how far away these objects are. This is what we call depth perception. The difference between a regular movie and a 3D movie is that we somehow emulate this: rather than perceiving the movie as a flat image projected on a flat surface, you’ll be able to do this depth perception thing with the movie as well, making it a more realistic and more immersive experience.
How does depth perception in humans work?
If you have ever bothered to learn anything at all about how your body is put together, or if you have at least occasional contact with other humans (again, not likely – but just in case), you probably know that a typical human possesses two eyes, and these are the body parts we use to see things with.
Since the laws of physics prohibit two objects from occupying the same space at the same time, these eye things are set approximately horizontally at a certain distance from eachother. This has two important implications: first, you don’t look like a cyclops, and second, the image that each eye perceives (assuming they are looking in the same direction) is slightly different.
Your brain then receives an image from each eye (which actually arrives inside your brain upside-down by the way), and does two things with it. It flips the image back so it’s right side up again, and it combines the images received from each eye. The small differences between each image are used to provide a sense of depth in what you’re seeing. (If you cover one eye, stab yourself in the eye, or for another reason are only able to see with one eye, you’ll find that it gets much more difficult to estimate distances, and it’s going to be much harder to get a driver’s licence for that reason.)
How is 3D content made?
When filming a 3D movie, the main difference is the camera that is used. A traditional camera has a single lens and captures a single, flat image. A camera used for 3D filmmaking is essentially two separate cameras set next to eachother at a distance from eachother. That distance is about equal to the average distance between human eyes (which is about 7 cm).
(An alternative method is to film a slightly wider image than normal and extract both left and right sides from that. This method is usually more practical – profesional video cameras tend to be bulky and putting two next to eachother at a 7 cm center-to-center distance gets rather difficult.)
You can see where this is going (or not). If we capture two sets of images, with that same distance between them as they would have when captured by human eyes, and then feed those two images into each eye separately, the end result is about the same as when you were actually standing there and looking at it directly. You get depth perception into the scene.
How is it delivered?
When 3D content is shown, what you see is essentially two separate streams of video: one intended for the left eye, and one intended for the right eye. Both are shown at the same rate that you would have when watching a normal movie, so in a 3D video, images refresh twice as fast (at 120 Hertz, typically). This is also why a normal TV cannot show 3D content (the image for each eye would be visible at about 25 to 30 Hertz, and many people will be able to see the flickering rather than a smooth image, which would be bad). The trick is in separating the left and right images so the left image only goes into the left eye and the right image only goes into the right eye.
When going to a 3D movie in a cinema, or if you get a 3D TV at home, you’ll probably have those nice little glasses that you have to put on in order to see the 3D (without them, the image will appear fuzzy). Those glasses are the thing that separate the two image streams. The glasses basically exist in two kinds: active shutter glasses and passive shutter glasses.
Active shutter glasses are essentially two small LCD displays; a piece of glass which can either let light through or block it, controlled by an electrical current. The glasses have a sensor on front, and a transmitter is needed (which usually works with infrared, like your TV remote). The signal received by the glasses tells it which side should be blocked at what time. By quickly alternating the image that is shown on the screen, and simultaneously blacking each eye out, each eye receives a different image, and we have 3D vision.
Notable disadvantages are the high price (active shutter LCD glasses typically sell for around 100 euros, and only work with one particular brand of TV – having some friends over to watch a movie together gets very expensive very quickly!), their relatively high weight, the fact that they need a battery to work, and the fact that when the sensor is blocked or otherwise doesn’t receive the signal, the glasses won’t work anymore. Also, due to the fact that each eye will only see blackness for half of the time, the image will appear only half as bright (or less) than it is on the screen. Your television needs to have a LOT of brightness!
All of those issues are solved by passive shutter glasses. Passive shutter glasses are now used in most cinemas. They are cheap, smaller, lighter, and available in much nicer designs. They work by polarizing the light at the projector. The glasses simply contain a filter to control which image comes in at each eye. Their main disadvantage is that you’ll need a very expensive projector setup with polarizing filters. It is extremely impractical to put such capability into a television.
Technologies have also been developed that allow 3D viewing without using glasses, but these technologies have to make assumptions about the angle at which you’re viewing the screen and your distance from the screen. If either one is too far off, you’re not seeing 3D. Due to this problem it is immediately impractical for applications where more than a single person is watching.
So why does it not work for me?
There are two main reasons you, as a viewer, may experience problemens when watching a 3D movie.
First one, eye distance. 3D content is made with the assumption that your eyes are at a certain distance from eachother, and to make this work for a large audience, the average distance of about 7 cm is typically assumed. However, not everyone’s eyes are at that distance, and in order to correctly estimate distances, your brain has to do a loth of work – all of that work assumes that the eyes are at whatever distance your eyes actually are.
But, when watching a 3D movie, your brain may be fed images where the distance is off – and if the distance is off even by a little, estimating distance doesn’t work anymore as well as it should. As a result you may not see the effect at all, and you may get a headache.
Second one, focal points. Your eyes have the ability to squeeze or stretch the lens in the front in order to focus your vision to a certain point. Things located at the point you’re focussed at will appear to be sharp and other things become progressively more blurry as they are located further away from that point. This is perfectly normal, of course.
One important thing is that, in the real world, your eyes can focus on any point and that point will appear sharp to you. It just works. Always. And that is great.
However, a camera has the same limitation that the human eye has: it can only focus on one point at a time. When viewing a ‘flat’ movie, this is fine: you’re focussed on the screen and everything goes smoothly. But when shooting 3D, this means that the 3D image has one object in it that does have focus, and the rest of the image (especially the background) will be out of focus.
Now, when you look somewhere on the image which is not the point of focus when the image was made, your will perceive depth, and attempt to focus your eyes that depth. You will attempt to focus on something which is never going to be in focus. This is problematic for us. It is not a situation that would occur naturally, and the brain is just not wired to not be able to focus the eyes on something. The result is that you’ll be having a headache.
Luckily though, if (and I stress the word IF – plenty of 3D content being made today is just not what it should be) the movie is made correctly, your natural instinct will be to focus on whatever the camera was focussing on when the movie was made, and unless you wander away from that, you should be fine. However, it is still an issue that any viewer can have, regardless of eye distance.
But…
If it does work for you, you’ll have a more awesome and more immersive experience watching movies.
Also, I hope you found this post informative 
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