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250 10. Stereopsis
Figure 10.11. Anaglyph stereo, simple paper and plastic glasses with colored filters
can give surprisingly good results. This cheap technology is ideal for use with large
audiences.
an infrared (IR) signal from an array of emitters placed conveniently in the
field of view. The projector or monitor works at twice the normal refresh
rate, typically greater than 120 Hz, and displays alternate images for left and
right eyes. The LCD shutters are configured so that when the left image is
being displayed, the left shutter is open and the right one closed, vice versa
when the right image is pr esented. Figure 10.12 illustrates the principles of
active stereo and some examples of active eyewear and emitters. Active stereo
systems have the advantage that they can be used interchangeably between
workstation monitors, for close up work, and pr ojection-screen systems, for
theater and VR suite presentation. The disadvantages are that the eyewear
requires batteries, and projector hardware capable of the high refresh rates
required tends to be prohibitively expensive, but see Chapter 4 where low-
cost options are discussed. LCD projectors cannot operate at a high enough
refresh rate to achieve acceptable results.
10.3.3 Passive Stereo
A single CRT or DLP projector operates at double refresh rate and alternates
between showing images for left and right eye. A polarizing screen is placed in
front of the projector. The screen is synchronized with the graphics adapter to
alternate between two states to match the polarizing filters in a pair of glasses
worn by the viewer. For example, the glasses may have a horizontal polarizing
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10.3. Active, Passive and Other Stereoscopic Systems 251
Figure 10.12. Active stereo single CRT–DLP projectors. Top-quality and low-cost
eyewear systems.
glass in front of the right eye and a vertical polarizing filter in the glass in
front of the left eye. Then, when the polarizing screen is vertically polarized,
the light it lets through is also vertically polarized, and will pass through the
eyewear to the left eye but be completely blocked from being seen by the right
eye. At a high enough refresh rate, greater than 100 Hz, the viewer is unaware
of any of this switching.
Unlike active stereo, this system will work equally well with two projec-
tors. One projector has a polarizing screen matching the left eye polarizing
filter and the other one’s screen matches the right eye. Since there is no need
to switch rapidly between left and right images in this case, LCD projectors
can be used. CRT projectors can also be used; despite being slightly less
bright, they allow for much higher resolutions (greater than 1024 ×768).
One last comment on passive stereopsis: horizontal and vertical polar-
ization cause a problem as the viewer tilts his head. This effect, known as
crosstalk, will increase until the tilt r eaches 90
◦
at which point the views are
completely flipped. To avoid crosstalk, circular polarization is used, clock-
wise for one eye, anticlockwise for the other. For more information on the
theory of polarization relating to light and other electromagnetic radiation,
browse through Wangsness [4] or any book discussing the basics of electro-
magnetic theory. Circular polarizing filters must use a tighter match between
glasses and polarizing screen and therefore may need to be obtained from the
same supplier. Figure 10.13 illustrates typical deployments for passive stereo
systems.
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252 10. Stereopsis
Figure 10.13. Passive stereo can be delivered with one or two projectors and a cheap
and simple pair of polarizing glasses.
10.3.4 Autostereopsis
Autostereopsis is the ultimate challenge for stereoscopic technology. Put blun-
tly, you throw away the eyewear and head-mounted displays, and the display
alone gives you the illusion of depth. One approach to autostereopsis requires
that the observer’s head be tracked and the display be adjusted to show a
different view which reflects the observer’s head movement. Two currently
successful concepts are illustrated in Figures 10.14 and 10.15, both of which
have been successfully demonstrated in practical field trials.
In the first approach, the goal is to make the left and right images appear
visible to only the left and right eyes respectively by obscuring the other image
when seen from a sweet spot in front of the screen. There are two ways in
which this can be achieved, but both ideas seek to divide the display into two
sets of interleaved vertical strips. In set 1 an image for the left eye is displayed,
Figure 10.14. Autostereoscopic displays may be built by adding either a special layer
of miniature lenses or a grating to the front of a traditional LCD monitor that causes
the left and right images to be visible at different locations in front of the monitor.
At a normal viewing distance, this corresponds to the separation of the eyes.
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10.3. Active, Passive and Other Stereoscopic Systems 253
Figure 10.15. LightSpace Technologies’ DepthCube [3] creates a volume into which
a series of layered 2D images are projected. This builds up a 3D volumetric display
that gives the viewer a sense of depth. The display is projected onto one LCD layer
in the display volume. When not acting as a translucent screen, the LCD layers are
completely transparent.
whilst in set 2 an image for the right eye is displayed. The first approach uses
a lenticular glass layer that focuses the vertical strips at two points 30 cm or
so in front of the display and approximately at the positions of the left and
right eye respectively. The second method uses the grating to obscure those
strips that are not intended to be visible from the left eye and do the same for
the right eye.
A more user-friendly system developed by LightSpace Technologies [3]
builds a layered image by back-projecting a sequence of images into a stack of
LCD shutters that can be switched from a completely transparent state into
a state that mimics a back projection screen on which the image will appear
during a 1 ms time interval. The system uses 20 LCD layers and so can
display the whole sequence of depth images at a rate of 50 Hz. A patented
antialias technique is built into the hardware so as to blend the layer image,
so that they don’t look like layered images. A DLP projector is used for the
projection source.
10.3.5 Crosstalk and Ghosting
In the context of stereopsis, crosstalk is when some of the image that should
only be visible by the left eye is seen b y the right eye, and vice versa. In active
stereo systems, it is primarily a function of the quality of the eyewear. There
are three possible sources of crosstalk.
1. Imperfect occlusion, where the polarizing filters fail to block out all
the light. Eyewear at the top end of the market is capable of deliver-
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254 10. Stereopsis
ing performance with no perceptible unwanted light leakage. Cheaper
systems still do a good job, but they may not go completely opaque.
Cheaper eyewear may not give 100% transmissivity when fully open,
and this can make the projected output look less bright.
2. Slow switching, where the opaque/clear transition may overshoot, al-
lowing some leakage from one eye’s image to the other. This should
not be a big problem, since LCD technology can now respond at rates
up to 75 Hz, which is quite acceptable for each eye.
3. CRT phosphor afterglow, which applies to screens and CRT projectors,
as well as most TVs. The light from CRTs is emitted by a phosphor
coating on the front of the CRT after it has been excited by the passing
electron beam. The light emission does not stop as soon as the electron
beam passes that spot on the screen; it decays over a short time interval.
Thus, if the eyewear switches faster than the phosphor light decays, a
little crosstalk will be evident.
In passive systems, the quality of the polarizing filters in the glasses and
the projector’s polarizing screens both play a factor. It is very hard to manu-
facture perfect polarizing filters, but unlike the active system where switched
polarization must be implemented, there will be no crosstalk during tran-
sients.
Ghosting is the term used to describe perceived crosstalk. It is a subjective
term. It varies with the brightness and color of the image and particularly
with the parallax. The larger the value of the parallax the more unpleasant
crosstalk will be.
10.3.6 Stereopsis for Multiple Users
In VR applications, there is a good chance that in the larger-scale systems, we
will want to be able to accommodate a number of individuals working with,
interacting with and viewing the displays. However, everything we have dis-
cussed so far makes an important assumption: the display is generated from a
single point of view. And the stereoscopic element will have a single parallax
too, so everyone has to accept the same depth perception. For CAD work at
individual workstations and in a movie theater, this is acceptable. In a VR
suite (e.g., a cave as described in Chapter 4, where two, three or more par-
ticipants will be enclosed in an immersive environment), unless HMDs are
in use, a screen will be needed. For monoscopic projections onto enclosing
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