3-D video in the home and in the theater

amirm

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Apr 2, 2010
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The principal behind 3-D video is very simple: perception of depth is created in the brain due to each eye seeing a different perspective of an object. The brain takes that difference and computes the respective position of each object relative to each other.

When we record an image using a single camera, we lose the differing perspective since we have a single image now and as such, the image depth perception collapses (although there are other cues the brain uses to create some limited perception of depth such as background being out of focus).

To gain back the 3-D perspective then, we need to record and playback two images as they would have been received by each eye. In case of live imagery, we can use two cameras positioned equal to average distance of human eyes. For computer generated animation, the process is simpler as the modeling software simply generates two perspective streams. Camera angle is a simple variable that can easily be varied and the scene regenerated by the computer.

The challenge then becomes how to store and display the two streams. Ideally we would do this in a backward compatible way so that someone without a 3-D display can still watch the video. Let’s examine both the storage and display requirements.

Transmission:
We have two challenges here. One is how to represent a stream in a backward compatible way. The second is how to avoid losing half of our bandwidth for the extra stream, and thereby reducing fidelity.

The solution is a partial one. We record one channel as we do today. We then take this primary stream, and find the difference between it and the second stream for the other eye. This causes anything in common to zero out leaving us with less data to transmit. For example, let’s say we have a person in front of a background of blue sky. The pixels representing the person are different in each perspective (since the angle of camera is changed). However, the blue sky remains the same. Therefore if we subtract the two images from each other, the sky pixels go away and what we have left are the pixels for the person which takes a much smaller portion of the full screen. This “additive” channel is stored alongside of the primary channel and stored on media (e.g. Blu-ray Disc) or transmitted live in the case of satellite and cable.

I mentioned the above was a partial solution to the fidelity issue. Reason is that the secondary stream while taking less space than the primary stream, still takes up space even in best case scenario of a lot of commonality between the two images. In the worst case, the entire image may be different due to angle of view, causing the system to essentially send two identical streams. While formats like Blu-ray have considerably headroom to produce high fidelity video, the impact may nevertheless be there. Live transmissions with their less efficient encoding (due to the need to run real-time), and lower channel bandwidth (less than half of Blu-ray) will likely compromise fidelity to achieve 3-D.

Put another way, people watching 2-D will be at the end of the short stick here, getting lower fidelity without any enjoyment of the 3-D system.

Now let’s move on to the display. The challenge we have here is having a single display that has to produce two images. If we attempted to output both images as is, we would see ghosting with one image offset from the other. To get a 3-D effect and eliminate ghosting, each eye must only see the image destined for it.

The solution -- at least today -- is to use polarized glasses. What is polarized glass? It is a special type of glass which only lets light through in one direction. Put another one in front of it rotated 90 degrees and you don’t get light as one would filter half the light in one dimension, and the other half gets filtered by the second one in the other dimension. Align them in the same way and the light goes through. In other words, we now have a way to selectively turn light transmission on and off.

There are two types of glasses: “active shutter” glasses and passive. The latter is just polarized glass as described above. We use vertical polarization for one eye and horizontal for the other. Active shutter glasses on the other hand, use a sandwich of two polarization glasses. One is fixed and the other can rotate to either match its direction or be opposed to it. A small electric signal tells the moving layer to switch directions, allowing us to completely block light on demand.

So far we have not accomplished anything. If you look through a polarizer in either direction, you more or less see the same image – albeit with half the light intensity gone. The trick to 3-D is to get the display system to match the eye glasses in a way that allows the image for each eye to get blocked when we want it.

How we accomplish that depends on the technology used. In the theater there is a need to use lower cost glasses which can be easily replaced if broken (or stolen!). Therefore, we use passive polarization glasses there. Each eye sees an image through either vertical or horizontally oriented polarized glass. A special device is then placed in the light path of the projector which in each frame of video either polarizes the light horizontally or vertically.

Can you guess where this is going? The projector shows the two images by alternating between the two streams fed to it. By polarizing the output of the picture one way and then the other, in sync with the projector, we are able to make sure each eye only sees the image destined for it.

Let’s assume the glass in the left eye is vertically polarized. When the projector passes its light through a vertical polarizer, the left eye sees that image just fine. The right eye has a horizontal polarizer and hence, no light goes through it. The reverse happens in the next frame with the left eye not being able to see the frame and the right eye having matching polarization.

What’s that you say? Wouldn’t we see the switch back and forth when the image goes dark every other frame? Well, we would if we did it slowly. But if we switch very quickly, then the eye doesn’t see the transitions. Same principal is in effect whereby you see a moving image in a theater, even though you are shown 24 separate stills. The brain blends the distinct frames together. In the common RealD system, the frame rate is actually 144 Hz (frames/sec) so the eye is very much fooled.

At home, the more cost effective solution is to make the glasses more expensive but make the display cheaper since the user presumably will take better care of the glasses. Here, the TV simply shows the interleaved image from each stream as we did in the theater but there is no polarization device in front of it. Instead, an infrared light (much like what is used in your remote control) or radio frequency signal is sent to the receive in the active shuttered glasses to block light on demand for each eye. When the frame for the left eye is being played, the TV tells the electronics in the active glass to polarization the right frame as to stop it from transmitting light. In the next frame, the reverse happens. Imagine putting your hand on one eye and then the other eye quickly and you would be simulating what the glasses are doing to say in sync with what is being played.

In the consumer scenario then, the only thing required of the TV is display twice as many video frames per second. Technologies such as Plasma and DLP projection are already running much faster than consumer video frame rates so they have no trouble keeping up. LCD technology however struggles as it is a slower system to start. Tricks such as insertion of black frame are used to help with this situation.

Good news is that putting aside the cost of 3-D glasses, there is essentially little increased cost for consumer displays. So people who are not interested in 3-D are not impacted cost-wise.

All is not well though. Active glasses have electronics and hence, need power in the form of batteries. Battery life which can be rather short, measured at less than 10 hours. The glasses themselves can also be costly, as of this writing going for roughly $300.

Note that with 3-D, “field of view” becomes very important. You need to have an immersive experience or the illusion will look fake. Reason is that once the eye scans past the edge of the screen, the 3-D illusion falls apart reminding you that what you are watching is not real. So where possible, you want to have larger screens and sit closer to them that you would with 2-D video. Taller aspect ratio also helps.

There is also the eye fatigue factor. Everyone will suffer some, and a few will suffer a lot. And for a small group of people, the 3-D effect will simply not be there. These are the tradeoffs we get for 3-D.
As noted, the other drawback is the loss of light due to use of polarization glass. This means even bright displays may not be bright enough to provide a vibrant image.

Another issue is loss of color balance as the polarized glass imparts a color shift. This is corrected for in the theaters (although in my viewing of Avatar, the green shift was noticeable and annoying). In the home setting this may be difficult to do as measurement through glasses may be hard. And you would need two settings: one for 2-D and one for 3-D.

So as with most things in life, there is no free lunch or perfection. But if you are fascinated by ability to finally have 3-D videos play in your home, you now have a low cost way to get it there.

More articles here: http://madronadigital.com/Library/Library.html
 

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