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Figure 12.23 CAT (Control Action Table) © Iparla project, LaBRI-INRIA Virtual object positioning
Positioning a virtual object amounts to a group of translations. These translations
can give priority to speed or accuracy. The possibilities offered for virtual reality in
terms of manipulation are not necessarily those that can be imagined. One of the most
significant examples continues to be positioning an object in space. The user faces more
difficulties in accurately positioning an object when he can freely move in a 3D world
than when the movement is restricted on one or several planes or axes. Hence, when
they need to be accurate, many authors decided to restrict some of these manipulations
(Hinckley et al., 1997; Gosele&Stuerzlinger, 1999; Smith et al., 1999; Lindeman et al.,
2001). Thus, it is noticed that, just like in the real world, not all situations necessarily
give rise to 3D manipulation. It is sometimes necessary to restrict the manipulation,
i.e. reduce the number of degrees of freedom.
Frees and Drew Kessler (2005) recently proposed an interesting alternative tech-
nique for improving the accuracy of manipulations. It is often difficult for a user, in the
absence of a force feedback interface, to move his arm, his hand or his fingers quickly
to an accurate position in 3D space. It can be even more difficult to keep them in one
position, or to move them in uniform direction. Frees and Kessler (2005) introduced
PRISM, an interaction technique that attempts to determine the intention of the user:
rapid movements or precise movements, and as a result adjusting the movement of the
virtual object which is controlledwith respect to the part of the user's body that controls
it. Like the Go-Go technique that changes the scale of movements of the virtual hand
in relation to the human hand for making it possible to reach distant positions, PRISM
changes the scale between the hand movements and the held object, this time to slow
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