Game Development Reference
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be distributed over various different machines. These architectures were described
In passive stereoscopy (polarised or Infitec), the images to be sent to the left and
right eye are generally produced by different projectors. The filters, in this case, are not
placed in front of the screen, but in front of the projectors. The images displayed by the
two projectors can be calculated either by a single machine having two video outputs
or by two different machines in order to improve the performances. Once again, it is
possible to use more than two machines to calculate these two images so as to improve
the display performances. Multi-user stereoscopy
As we have discussed earlier, it is necessary to track the position of the user's head to
ensure the coherence of his point of view in a stereoscopic immersive system. If more
users take part in this simulation, they will then be observing the 3D scene from a point
of view which will belong to someone else, which will create incoherent perspectives
from their own point of view. The simplest solution in this case is to provide them
with a software mechanism to successively take the right point of view, which is fairly
easy with the new optical tools that track the observers' heads. In this solution, it is
however not always possible to allow multiple users to simultaneously observe a 3D
scene with correct perspectives. While using active stereoscopic eyeglasses, the same
old shutter principle can be used to isolate, not the left eye from the right eye, but
user 1 from user 2 (Agrawala et al., 1997). This would result in the display of the
following image sequence (random order):
user1 . eyeR
user1 . eyeL
user2 . eyeR
user2 . eyeL
etc .
The entire sequence always must be displayed at a minimum frequency of 60Hz,
which means that the frequency of images should be at least 240Hz. This naturally
means that the frequency will increase further if there are more than two users, which
will require a very high computing power and especially projectors that can attain
such frequencies. Another solution lies in using one projector per eye and putting
shutters in front of the projected light. Then the projectors can operate at normal
frequencies as the additional shutters will determine which eye of which user should see
the projection. But even if the projectors cannot work at high refresh rates, the shutters
have to work at a high frequency. To make up for this disadvantage, a derived solution
consists of using hybrid technologies: The left and right eyes are differentiated using
polarised eyeglasses, whereas the shutters are used to differentiate between different
users (Fröhlich et al., 2005). Since the shutters are no longer used to differentiate
between the eyes, the operating frequency of each projector can be divided by two.
On the whole, all these techniques have the disadvantage of considerably darkening
the images displayed because as the number of users increases, the time for which the
images are displayed to each of them reduces. Different types of projectors
One of the key elements of a system based on large screen projection is the projector
used to display the images. Three main technologies are currently available in the
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