Visual anomalies with stereoscopic 3D display technologies

We all know what stereoscopic 3D should look and behave like … we see it every day.  Unfortunately, this isn't how stereoscopic 3D behaves in the home today.  Here's a list of ways where the idea doesn't quite match up with reality.

Color distortion

Glasses distort the color of the screen.  It's as simple as that.
In the case of active shutter displays, the glasses used to shutter each eye have a slight yellowish or greenish tint to them.  It’s an unavoidable side effect of looking through a liquid crystal lens.  In the case of polarized glasses, the distortion is not as pronounced and may be more color neutral, but it's easy enough to look through a pair of glasses and observe a difference in hue and contrast without the glasses on.
HMD and auto-stereoscopic display technologies don't suffer from color distortion directly as they have separate pixels directly allocated for each eye.

Significant loss of brightness and contrast

Active shutter displays refresh about 120 times a second.  That means that, even in an ideal scenario, each eye only gets 50% of the light.
However, we live in an analog world … and despite what you may initially think, the fact is that the switching doesn’t happen instantaneously.  What actually happens is that it takes time for the displays to transition from the left eye image to the right eye image and back again.  If each shutter were really open 50% of the time, a good portion of your time would be spent seeing the screen transitioning between frames … which would completely ruin the 3D effect.
In practice, what I see is that shutters are open about 25% of the time.  Keep in mind the glasses themselves don’t perfectly pass light through either.  So you’re probably talking about 20% of the original light getting through to each eye or less.
All that time you spent calibrating your TV to be THX spec?  Totally out the window with 3D glasses on. 

Top/bottom stereo extinction problems (or ghosting)

This is a specific problem with active shutter displays and an important variation on the loss of brightness issue. Most TV’s do not refresh every portion of the screen at exactly the same time.  Instead, they typically refresh each row of pixels from top to bottom.  Again, the shutter for a particular eye should not be open until the image finishes refreshing on the TV … otherwise all that eye will see is an incorrect blend of the previous and current image.
So not only do you have the problem of waiting for the screen to transition its pixels from the left to the right eye image and vice versa, the screen isn't even trying to change all the pixels at the same time!
In practice, this is a very big problem for LCD’s.  The screens can barely switch between each eye's image in time in order to provide an adequately long shutter time for each eye.  So the manufacturers play loose with the shutter timing in order to let enough light through to each eye.  Unfortunately, this means in cases where the screen doesn't switch in time (usually in high contrast scenarios such as dark building against blue or white sky), bits of the previous and next image are leaking through.
Another interesting aspect of this is that the leaking through of the opposite eye's image tends to happen more at the top of the screen.  Why is this?  Again, all refreshing happens from top to bottom … and the majority of the change in a pixel while it is changing happens at the very start.  So even playing slightly loose with the shutter timings can have a bad effect on the image near the top of the screen.

DLP projectors seem to have no top/bottom stereo extinction problems.  Presumably the design of the technology allows the images to flip nearly instantaneously. Plasma’s are also less susceptible to due to the fast duty cycle/switching times of the technology.

If you want to see this problem in action, I took a high speed 600 fps video of some 3D ready monitors.  The video shows these things in a much more clear way than I can describe by writing it.  Here's the YouTube link.

Fast motion stuttering

This is another active shutter specific problem.  The sequential display of left and right eye images in active shutter systems has additional implications for fast moving objects.  While the correct images may be getting to each eye, they are not arriving at the same time.  Is this a problem?  Yes, it definitely is.  The human brain is sensitive to this incongruency … and 120Hz is not a fast enough of a refresh rate for your brain to be unaware of the difference.  In short, 3D scenes with heavy horizontal panning will appear very stuttery and possibly disorienting.  This is not a problem for other 3D display technologies which display the left and right images at the same time.

As noted above, it may be possible for the the stuttering to be remedied by even faster switching times such as 240Hz, but the state of the art isn't caught up yet.

Overdrive ghosting

Another active shutter specific problem.  With current technology, most displays are just at the limit of being able to switch back and forth at 120Hz.  To achieve fast switching times, manufacturers use much higher voltage differentials than normal when a pixel transition begins.  This allows the display to get the pixel to the correct color more quickly.

Unfortunately, this is an imperfect process and has tradeoffs.  Over or undershooting the value results in the pixel not quite arriving at its intended value before having to switch back.  The end effect is weirdly colored ghosts of the opposite eye image leaking into the target eye.

Expensive shutter glasses

Each pair of active shutter glasses costs around $100 to $200.  For most people, that’s not an insignificant expense.  I would guess these prices will drop like a commodity over time … there's really no good reason for these things to cost that much.  But, for now, expect financial pain if you’re going to have more than one viewer at a time.

Glasses need recharging

Active shutter glasses last a long time if fully charged … around 40 hours.  Charging only takes a couple of hours.  But nevertheless, with the dearth of 3D content out there, it's pretty likely that your glasses will go unused for long periods of time right now.  So when you do finally decide to watch something, you may have to deal with some delayed gratification issues.
Off-angle/Off-level viewing issues
All stereoscopic 3D technologies, with the exception of HMD's, require the viewer to be sitting level with the TV and reasonably directly in front of it so that the left and right eye images are displayed properly spaced and oriented to each eye.  This means you can't lie on the couch and watch in 3D, and you also can't stand off in the corner in the kitchen and watch the TV in 3D while it's tucked away in the other corne
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Polarized displays and glasses have the additional problem of each lens actually letting through light from the other eye's image if you are not sitting perfectly horizontally.  Real3D glasses in the theater work around this by using circular polarization, which is not sensitive to the rotation of your head in the viewing scenario.  But since the entire stereoscopic 3D viewing process relies on your head being level and well placed anyway, this is of minor consequence. 

Conclusion

As you can see, stereoscopic 3D is the home is not just fire and forget … and it does not "just work".  In particular, active shutter displays have a host of problems not present in other display approaches … but they make up for it by being the cheapest way to go.  At least now you know all the ways things can go wrong!  Don't let this document scare you off completely though … I think the 3D viewing experience is very immersive and enjoyable in many contexts.  You just need to know what you're getting.
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