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10.3. Active, Passive and Other Stereoscopic Systems 249
10.3 Active, Passive and Other
Stereoscopic Systems
The alternative to HMDs as a way of presenting different views to the left and
right eye is to present them on a monitor or projection screen in the usual way
but modify or tag them in some way so that a simple device (which the person
looking at the screen uses) can unmodify them just before they enter the eyes.
Essentially, this means wearing a pair of glasses, but unlike the HMDs, these
glasses can be as simple as a few bits of cardboard and transparent plastic
1
.
10.3.1 Anaglyph Stereo Red/Green or Red/Blue
This is probably the simplest and cheapest method of achieving the stereo-
scopic effect. It was the first method to be used and achieved some popularity
in movie theaters during the first half of the twentieth century. A projector or
display device superimposes two images, one taken from a left eye view and
one from the right eye view. Each image is prefiltered to remove a different
color component. The colors filtered depend on the eyeglasses worn by the
viewer. These glasses tend to have a piece of red colored glass or plastic in
front of the left eye and a similar green or blue colored filter for the right side.
Thus, the image taken from the left eye view would h ave the green or blue
component filtered out of it so that it cannot be seen by the right eye, whilst
the image taken from the right eye viewpoint would have the red component
filtered from it so that it cannot be viewed by the left eye. The advantages of
this system are its very low cost and lack of any special display hardware. It
will work equally well on CRT and LCD monitors and on all video projec-
tors, CRT, LCD or DLP. The disadvantage is that there is no perception of
true color in the pictures or movie. Figure 10.11 illustrates a typical system.
10.3.2 Active Stereo
This system can be used with a single monitor or projector. The viewer wears
a special pair of glasses that consist of two remotely controlled LCD shut-
ters working synchronously with the projector or screen. The glasses may
be connected to the graphics adapter in the computer or they may pick up
1
This very simple idea can be dressed up in some impressive-sounding technical language:
the input signal (the pictures on the screen) is modulated onto a carrier using either time-
division multiplexing (active stereo), frequency modulation (anaglyph stereo) or phase shift
modulation (passive stereo).