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descriptive level, we would now like to describe the factors which seem important in
the immersive character of a virtual environment and which have to do simultaneously
with the problem of sensory richness as well as that of interaction. Coherence
The coherence of a virtual world exists at two levels: Firstly, it has to do with tem-
porally synchronising and ensuring a spatial coherence between the different sensory
stimulations. The second level includes the first level and concerns the “response time''
of the system, between an action of the subject and one (or more) sensory feedback(s)
corresponding to this action. Also note that in case of handling of objects, the time
periods involved will for example affect the perceived movement of a virtual repre-
sentation of the hand in the virtual world, and create an incongruity with the direct
proprioceptive afferents. It is also necessary to note that the term “real time'' is mis-
leading, when we talk about an interactive system. On account of the discrete character
of the system, there necessarily exists a delay between the acquisition of a signal from
the user and its sensory restoration (at least equal to a cyclemachine, plus the time peri-
ods in the interfaces). Maintaining this time period constant is an objective in itself.
Maintaining it at a minimum value is often the purpose of looking for a compromise
with sensory richness (something which video game manufacturers have understood
for a long time). Mapping
Another aspect of virtual environments related to their immersive character is mapping
between the user and the virtual world. This is an interface problem, but above all,
it is a problem of filtering of the sensory data (afferent) and motor data (efferent)
(Sheridan, 1992). This problem is related to the action possibilities of the subject in
the virtual world. We agree with the view of Sutherland (1965), who observed that
“the ultimate device (of virtual reality) will be an environment in which the computer
can control the existence of objects (and our interaction with them). One can sit on a
chair in this environment. Handcuffs in this environment will be really coercive, and
a bullet will be fatal''. We are not sure that the latter part is desirable, but the idea is
certainly to create a sensorimotor interface corresponding to the perceptual and motor
abilities of the subject, according to the purpose of the virtual reality application.
Another aspect of mapping between the real and virtual worlds concerns the tran-
sition from one to the other. In Flatworld, real and virtual elements contribute to the
incorporation of virtual elements in the real world. For example, you can be in a room
of a house with furniture. You can see a city through the window, which is a virtual
world projected on a screen. However, here we can speak more about mixed reality
than virtual reality in the strict sense of the term (Anastassova et al., 2005). In this
regard, we can observe the works carried out at Ecole des Mines of Paris, concerning,
for example, the visit to a virtual shop or even training of driving crew members in a
railway environment (Burkhardt et al., 1999). In this type of situation, it is not possi-
ble, for example, to give the user the option of actually grabbing a virtual object, we
thus use a “habitual'' Behavioural Schema (see introduction). A “metaphor'' can thus
be used. The grabbing action can thus be rendered by a “click'' of the user. Mapping
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