Introduction
| This is a brief explanation that we hope sorts
out some of the confusion about the many 3D display options that are
available today. We'll tell you how they work, and what the relative
tradeoffs of each technique are. Those of you that are just interested in
comparing different Liquid Crystal Shutter glasses techniques can skip to
the section at the end.
Of course, we are always happy to answer your questions personally, and point you to other leading experts in the field. Just drop us a line. |
|
How You Normally See 3D Since this oh-so-direct route to get an image to each eye had these
drawbacks, other methods were developed with the options available in the
pre-computer world. Other Tricky Methods
Normal 3D
vision depends on the fact that each of your eyes see from a slightly different
perspective. To see what I mean, hold your finger about 10cm from your nose. Now
close one eye. Open it and close the other. Notice how your finger seems to
"jump" from side to side a bit as you switch eyes. This difference is called the
parallax. Now repeat this experiment at a distance from your nose of around
20cm. Notice how the parallax is less. Look across the room at some object and
notice how the parallax for that object is much less.
Your brain uses
this information about the differing parallax to determine the distance to each
object in a scene you are viewing. It's pretty amazing how well your brain does
this, so it is kind of a shame that most communication materials are "flat" two
dimensional images.
How to Produce a
Stereoscopic (3D) Display
As you might guess, the key to getting
a 3D view is getting a different perspective to each of your eyes. Almost as
soon as photography was invented, an easy means to do this became available. The
photographer simply took a picture form two slightly different perspectives,
separated like your eyes, and then had the viewer hold the two resulting photos
in front of each eye. Holders for these photos gradually evolved and today you
have the popular Viewmaster kid's toy. Methods for taking these photos also
evolved into stereo cameras that can take both photos with a single snap (useful
for subjects in motion, if you think about it). While this technique can achieve
high quality with good equipment, it has several drawbacks:
Red and Blue Glasses
(Technically referred to as Anaglyph)
One cheap and easy method
is to print or project the left and right views as superimposed red and blue
image. Then by using red and blue cellophane lenses, each eye can "extract" its
image from the page or screen. However, it doesn't take much analysis or viewing
to realize that this technique destroys the original image colour. Not to mention
that a normal human finds viewing the world through two differently coloured
lenses for any period to be a bit disconcerting. This is a good way to get a
headache.
Polarized Glasses
A
more sophisticated technique that avoids this color distortion is to use
polarized lenses. These can still be relatively cheap paper glasses, but the
projector (or two) must be capable of projecting polarized images. Those of you
not familiar with the phenomenon of polarization should think of this as a way
of "orienting" light waves. One problem is that your glasses (and head) must
also be perfectly oriented to the projector or else the eyes get mixed images
resulting in poor quality and another headache. Even with perfect orientation or
special types of polarization (circular, for you scientists) there is usually
substantial "leaking" for any affordable types of polarizing filters. A more
serious limitation is that in order to preserve the polarization, the projectors
must display on a silvered, mirror-like, screen. It is possible to place a large
electronic polarizing plate directly over a monitor. However, the expense of
these devices is usually too large to justify use on a small device that can
only be viewed by a limited audience. Lastly, the polarizer blocks half the
light intensity, so brightness can be a problem. However, for very short term
viewing, the convenience of cheap paper glasses can be attractive. Note that the
"cheaper" IMAX theatres use this technique while the premier facilities use a
technology described below.
There are
other techniques such as lenticular prints and holograms that are used on
baseball type cards and other promotional items. Although they make interesting
novelty items, the image quality is so poor, as you have probably observed, that
we won't concern ourselves with them here.
Head Mounted Displays
The logical successor to the Viewmaster is the modern Head Mounted (or virtual reality) Display. It's basically a Viewmaster with animation. While this solves some drawbacks, it still retains the isolated viewer mode. Of course for immersive simulations, and many video games, this is a real bonus. However, one major drawback for realistic (non-video-game) applications is the very low resolution. Even though LCD screens have acceptable resolution when kept at arms-length on a laptop computer, as soon as you put anything within a few centimetres of your eyes the pixels start to look pretty large. Consequently, HMDs are best used where realism isn't a concern, such as certain classes of video games.
The military has decided that they need HMDs with photorealism for military
simulators. Since cost is no object, they take computer monitors (which have
MUCH higher resolution than LCDs) and, using extremely expensive fiber-optics,
feed the images into each eye of a HMD. This is a fantastic technology, but the
fiber optics currently push the price into the $50,000 and up price range.
Autostereo Displays
What the name implies is a form of stereoscopic display that requires no glasses or other aids for the viewer. This is the Holy Grail of the 3D world as it produces visions of immediate consumer acceptance. However, much like the Holy Grail, there have been innumerable false sittings. Brief reflection makes it clear that the fundamental objective of getting two different images to each eye from one source with nothing in between is pretty daunting. There are several theoretical techniques involving holograms, spinning cylinders and displays that are themselves three-dimensional; however, none of these have ventured far from the laboratory for fundamental reasons. One technique whose variants have achieved some degree of practicality involves the use of screens or louvers which are set at such an angle that your right eye can see certain pixels on a display behind it, while your left eye is blocked but can see certain other pixels. The real limitation here is that each eye must be very precisely positioned relative to the display. Most recent variants use some form of electro-optical screen instead of simple slits to allow the slits to reposition in order to accommodate your eyes being in different locations. However, this requires a means of accurately tracking eye position. This secondary problem rapidly introduces the need for expensive tracking devices. Another limitation is that the slits can only be directed in one direction, at one viewer. This makes multiple viewers practically impossible. Lastly, the physical size of these slits causes the resolutions to be fairly low.
This last technique we will look at is the first and only one to gain widespread acceptance, selling in the hundreds of thousands. However, as you will see, there are a variety of ways to implement this basic idea, with profound implications for the resulting cost and quality. Given that CRT (television tube) based technologies are very mature (over 70 years old), cheaply priced (due to sales in the hundreds of millions), and capable of extremely high resolutions, it seems obvious that this would be a good basis for any type of display technology. As mentioned above, this is what the military uses in their Head Mounted Displays. With the advent of liquid crystals came a way to harness this capability for affordable stereoscopic displays.
Almost all digital watches use liquid crystals for their displays. The relevant property is that an electronic signal can make the crystal turn from transparent to opaque. Some clever folks realized that if a pair of glasses were made with each lens being a single, large, liquid crystal, they could be used to get the required two-separate-images-from-the-same-view that is the basis of any 3D display.
This is how the technique works: one of the lenses is made opaque (say the left one) so that the viewer can only see through the right lens. At the same time, the right eye view is displayed on the monitor. Now, the situation is reversed, and the right lens is made opaque while the left view is displayed on the monitor. If this is done rapidly enough the result is that each eyes percieves a differant image from viewing the same monitor.
Differing LCD Techniques
Of course, as with any fundamental technology, there are different methods of applying the technique that address different concerns of quality, economics, and compatibility. The three currently used techniques are:
Interlaced
In the earliest days of television, when electronics were relatively limited, a technique to improve the frame rate, or number of pictures displayed per second, was to broadcast the odd lines of the image (numbered from the top) and then the even. Although all modern monitors no longer use this technique, and by default merely display each line in logical order, this still persists in commercial television displays. That's the price you pay to have your new large screen television be backward compatible with a 1959 Milton Berle broadcast. So, for stereoscopic hardware manufacturers that wish their system to be viewable on home television units, this is the technique they must then subscribe to. Consequently, the only way to synchronize the left and right images within the glasses is to synchronize the shutters with the even and odd frames. This means that each eye is now getting half of the already marginal television frame rate, resulting in horrible "flicker." This is why a television compatible display should never be used for serious stereoscopic purposes, but only short-term novelty viewing.
However, because the simplicity of the technique allows for extremely
low-cost hardware, this technique has been used for some cheap video game
oriented LCD glasses systems on computers. But, since computer display hardware
manufacturers dropped interlace mode support in the 1980's, these devices
require special 'device drivers' supplied by the LCD glasses supplier. As
supplying reliable device drivers for even a minority of the video adapter
market is beyond any one company, this has resulted in a lack of adoption of
this system for any computer modes more advanced than 1980's era DOS. Some
manufacturers have been promising Windows games for years now with no
results.
There is also a drawback in that each left or right eye view is only made up of either the odd or even lines. This results in only half of the screen being used for each image and a 50% decrease in brightness.
But, the fatal impediment to serious use is the very low frame rate. Each eye
is getting a frame at half of the normal viewing rate, and this results in
extreme amounts of annoying flicker. We discuss this important issue below.
Page Flipping
The obvious answer to this inelegant system is to simply broadcast a complete left image, followed by a complete right image. This technique is called page flipping, and can deliver excellent image quality. However, for this technique to achieve flicker free rates, the monitor must be capable of running very fast (remember, each eye only gets half the frame rate) and the computer must never fail to swap the left and right images at every frame. At a modest frame rate of only 100 frames per second (discussed below) this puts a great demand on the computer. The modern Windows computer is performing too many tasks at any give time (running the CD player, swapping memory, listening to a modem or network, or actually running some useful software) to reliably respond to every switch. The result is that the image periodically "collapses" to 2D with very annoying results. While running very CPU intensive motion-video, this can result in virtual failure on the most powerful PC. Video card manufacturers have started to build this flipping compatibility into their cards, off loading this responsibility from the computer. In future years, if this capability becomes commonplace and monitor manufacturers begin to support very high bandwidth monitors, this may become a more viable technique. At the moment, all page flipping schemes run only in DirextX video game oriented modes and don't support motion video technologies such as Windows Media or DVD playback for the reason we are about to discuss.
One last note for those concerned with motion video: High-quality video
schemes, such as Digital Video Disks, rely upon inter-frame-coherency, or the
similarity between successive frames, to save valuable space. By causing each
successive frame to be considerably different (left then right then left …) than
the one before it, this is lost completely, resulting in poor performance.
Actually, due to the complexity of combining page-flipping's tricky timing with
the demands of real-time video, it is practically impossible to have page-flipped
video anyway. Compare that with NEOTEK's existing DVD video system based on
Synch-Doubling.
Synch Doubling
While interlace mode is concerned with backward compatibility, and page flipping is concerned with cost (let the computer do all the work, and the only hardware you need is glasses), the sync-doubling technique is foremost concerned with quality and has been used by high quality medical systems for years. The technique is simple: use a normal computer with a reasonable quality display to show a normal image that has the left image on the top half of the display and the right image on the bottom. Now, between the monitor and the computer stick in some hardware that inserts an extra vertical-sync, or "new frame" signal, after the computer has displayed the top half. Have this same hardware synchronize the glasses with these image halves such that the right eye only sees the bottom half and the left the top.
By the way, those of you concerned with image quality (our customer base) may
be saying "if you cut the screen in vertical halves for the left and right
images, aren't you sacrificing vertical resolution?" An excellent question.
Before we do that, we actually put the video card into a mode that doubles the
vertical resolution. So, after we do our magic, we still have every pixel we
started with.
What we have accomplished here is to have the computer display a normal image
that happens to look like an above/below stereo picture, but after the hardware
processes it, the monitor sees an image that looks like it is page-flipped (it
sees a left image, a new screen signal, then a right and repeats) without
loading the computer at all. In addition, this extra synch signal has the effect
of doubling the refresh rate so that when it is halved for each eye we are back
where we started at a normal frame rate for each eye. A last technical , but
important point, is that although the frame rate is doubled, the bandwidth
(information content) to the monitor remains the same. This means that normal
monitors can be used to deliver flicker-free images with this technique.
As you may have guessed, this is the technique that NEOTEK has chosen for our current products. Our new motion-video products rely on this technique as it is the only one that has no compression loss. Also, as all of the video signal processing is done outside of the computer, our TriD product was working with normal Windows computers literally days after we received our first Digital Video Disk player. And, customers with existing hardware only require a software upgrade.
Frame Rates
Throughout this discussion, we have mentioned frame rates. This is important as any perceived flicker will result in eventual eye-fatigue and headache. Some of our installations are used by medical students in a library environment where multiple-hour study sessions must be comfortable. Although a lot of R&D has gone into determining the optimal frame rates for various conditions, the results can be summarized for the general case quite easily. As any computer user knows, 60Hz is generally considered the minimal frame rate for normal viewing. Simple considerations would indicate that twice that would be necessary for maintaining that same per-eye rate with LCD glasses. That isn't far from the truth:
| Frame Rate | Conditions | Result |
| 30-100Hz | Any | Flicker/Headache
Used by cheap video game systems for short term novelty purpose |
| 100Hz | Ideal room lighting | Most users perceive slight flicker |
| 120Hz | Normal room lighting | Most users perceive no flicker |
| 140+Hz | Worst case lighting | No flicker |
As you might guess from this chart, our customers always operate at 120+ Hz.
Some installations in harsh lighting conditions are operated at 140Hz, and all
systems display zero flicker. Anything less will inevitably lead to eye fatigue
and headaches. Always compare any electronic system you are investigating
against these minimal standards.
Upon first glance, it seems that by conquering the "flicker" problem, flawless image quality is ours. However ,there are still important issues to be addressed. We'll describe two of very briefly. This is where dedicated image processing algorithms are necessary.
Persistence Ghosting
Most monitors were not designed for stereo viewing, and consequently, the glow time, or persistence, of the screen phosphors is not always equalized for images that alternate 120+ times per second. This can result in "ghosts" where one eye sees the persistence of an image left from the previous eye. For economic reasons, this is usually worst on one particular phosphor color. By muting this color component at high parallax and high contrast areas, this problem can be eliminated.
Focal Plane Disparity
If you are viewing an object that appears to be hovering a foot in front of the monitor, are your eyes focused at that location, or on the screen where the image is actually displayed? The answer is that they will want to focus on the object and should be focused on the screen. This would cause eye fatigue but for our algorithm that selectively sharpens and de-focuses image elements based upon parallax.
These are just two examples of how important software is to a quality system.
Any electronic system that doesn't incorporate some serious digital image
processing at some stage will not be able to resolve these problems, as well as
even more subtle ones we haven't gone into here.
In conclusion, we hope you, the consumer, now understand why Neotek has chosen a sync-doubling LCD based system with proprietary software algorithms for our photorealistic quality systems. When we started, all of these techniques, and more, were available to us. We went with sync-doubling because it was the only high quality technique available for PCs, and remains so. Our decision was vindicated yet again with the recent, painless, roll-out of our Digital Video Disk system. Fortunately for both you and us, electronic trends have enabled us to greatly reduce the hardware cost. For serious quality 3D displays, there is no alternative.
Copyright© 2006 Serrata Pty. Limited All rights
reserved.