3D Video Explained

I'd like to spend a little time here explaining 3D.  You may hear about 3D as it deals with movies, and if you're up on your tech, as it relates to TV's.  What you may not know is how it all works.  To clear up some confusion and questions you may have, here's a little explanation of it all.

Principle of 3D
If you really think about it, your eyes and ears are pretty similar.  You have two of them, and your brain uses both to locate an object either giving off light or sound.  You can localize the object because your brain uses the difference of the two images (images here meaning either a sound image or a visual image) to create a 3D mapping of the environment you're in.

This is a little easier to think about in terms of sound, so we'll go with an example there first.  Say there's a noise off to your left.  You don't necessarily know how you know that it's off to your left, but you just know.  What's actually happening is that sound is going into both of your ears.  Because sound has a finite speed and isn't instantaneous, the sound gets to one ear (your left) faster than the other (your right).  Your brain processes this in terms of the delay difference between ears, and can then tell you exactly where the sound is coming from.

Now let's apply loosely the same principle to your eyes.  Your eyes work on the principle of being able to focus on an object.  If you try to focus on your nose, your eyes have to point in towards each other.  If you look at something very far away, your eyes are looking out almost parallel to each other.  Along the same lines of your ears discerning the location of a sound by the time difference between the sound getting to your ears, your eyes can discern how far away an object is by how much your eyes are pointing together when you focus on it.  Of course there are other visual clues such as juxtaposition (one object in front of another) and size relativity, but you get the point.

Now let's get on to the mind tricks.  Put your finger in front of you pointing up (like you're saying "one").  Now focus on something farther away.  Do you see two fingers?  We all know that you don't actually have two fingers, it's just your brain playing tricks on you since you aren't focusing on your finger.  With your finger still up, close your left eye.  You'll notice that your "left finger" is the one you see.  Closing your right eye instead of your left eye, you see that your "right finger" is the only one you see.  Now pretend that us audio/visual guys can do anything we want to, and we can actually somehow make both eyes see different images.  Think a pair of binoculars, but with pictures inside that you can see.  If you were to compare these two pictures side by side, everything looks the same, except for a floating finger in the middle of the picture.  It's the exact same finger, but the finger is in a slightly different position in each image, just like when you did it a second ago.  Looking into this magical pair of binoculars, what do you think you'd see?  Your brain actually wants to "fix" these images just like your finger you held up in front of you.  Because each eye only sees one finger, your brain sees it just like the finger you held up in front of you - as one actual finger.  If you were to focus on it, your brain resolves the image into one finger that actually would look like it's in front of the picture.  Using some different technologies, we can actually pull this trick off in the visual world.

Types of 3D
There are three main ways that we can actually pull this "different eye" trick off.  All three work on the same principle explained above - we want to send a different image to each eye.

Anaglyph
This is the old kind of 3D that was popularized back in the 70's (or something around there...yea, I'm young).  It used a pair of glasses with a red filter for the left eye and a cyan filter for the right eye.  Using an image specially processed with this color difference, each eye gets a different image because the filters pull out the color that the eye isn't supposed to see.  Do a Google search for "anaglyph" and you'll see plenty of examples.

Infitec
This one is really uncommon, but it's out there.  Two projectors are loaded with filters that only pass certain wavelengths of light.  Basically one projector will pass the lower half of the red spectrum, lower half of green, and lower half of blue, while the other projector passes the upper half of red, green, and blue.  Left and right eye images are sent to these separate projectors, land the glasses have the same filtering in each lens.  Because the color spectrum can't be split perfectly even across the two filters, one filter looks green and the other red.  A lot of color adjustments need to be made in the projectors because of this, and the color spectrum never really does look quite right because of the colors that are missing for each respective eye.  Nevertheless, it does look much better than anaglyph 3D.

Polarized Lenses
You may know of polarized sunglasses that reduce glare off of the road.  This is the same sort of thing.  Think of a wave on a lake or the ocean.  Looking at the wave, you see it travelling across the water.  If you were to look at a certain spot on the water, though, you'd see the water rising and falling as the waves go by.  Light acts in the same way - it travels through the air, but moves "up" and "down" through the space.  Normally light doesn't really follow the "up" and "down" rule though.  It just travels in whatever manner it wants to.  If, however, you were to put a whole bunch of really thin bars really close together pointing up and down, you could theoretically make only light travelling "up and down" go through.  This is actually a polarizing filter, and it's exactly what your polarized sunglasses do. If you've ever worn a pair, you know that some of the glare gets through.  No polarizing filter can be perfect.  In addition, if you tilt your head, you start to see a whole lot more of the glare.  This is because the filters are no longer oriented the way they're supposed to be.  Smart people have come up with ways to fix this by putting a "spin" on light coming out of a light source - either clockwise or counter-clockwise.  This is what we call circular polarization.  Because it's circular, you can tilt your head and the polarizing filter doesn't get disoriented.

Active or Shutter Glasses
This is currently the best way to do 3D.  These glasses have an LCD in each eye lens that can block light or let light through.  The display is configured to alternate between showing a "left eye" image and a "right eye" image very quickly, usually somewhere between 60 and 120 times per second.  The active glasses are synchronized to the display so that when the display is showing a right eye image, the right eye shutter is open and the left eye is closed.  Thus your right eye sees the image displayed and your left eye sees (theoretically) black.  When the display alternates to the left eye image, the glasses flip and your left eye sees the image while your right eye sees black.


The main goal of 3D technology is to do the best job of separating out the images to your left and right eyes.  The technologies I just talked about are listed in order of how well they work, with anaglyph being the worst at separating eye images, and active being the best.  Many of you have probably seen some sort of 3D show, whether it be a 3D movie or a place like Disney World.  Nowadays, theaters usually use polarized glasses because they're pretty cheap.  If people take them, no big deal.  Anaglyph glasses are usually used for magazines and commercials like the Superbowl last year because you can't control light polarization in either of these cases.  Active glasses are only used in places where the glasses can be easily kept track of because they still cost hundreds of dollars, but the cost is coming down.  Compared to a few bucks for a set of polarized glasses, it'll be a while before you see active glasses in theaters.

As a side note, here's a good example of 3D that you'd expect to see covered in news and hear about most often.  That's bad 3D.  Good 3D is much harder to come by because people want to do things as cheaply as possible most of the time.  Not only do you have to spend money on the way 3D is delivered to your eyes, but you have to pay for the content development.  I haven't seen it yet, but I've heard that Avatar was pretty good from some reliable sources.  Granted it's only using polarization technology, but it still was reportedly good.

Clear as mud?