Game Development Reference
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Figure 12.22 “Voodoo dolls'' Technique
movement of objects beyond their initial distance continues to be difficult. In addi-
tion, the variability of the scaling factor, depending on the distance to an object at the
moment of selection (the further the object is at the time of selection, the larger is the
coefficient), may disturb the user and needs an adaptation time for each distance.
Another solution that makes selection of distant objects and direct manipulation
possible combines the selection by the “image plane'' technique and WIM. With this
solution, called “World scale Grab'' (Mine et al., 1997), once the object is selected,
the technique scales down the whole virtual environment around the user's virtual
viewpoint so that objects are brought within the user's reach and are manipulated. The
scaling coefficient is such that the visual size of objects remains unchanged (closing-in
of objects compensated by reduction in size). The moment the user releases the object,
the scene comes back to its normal size.
Another example of the hybrid solution is the “Voodoo Dolls'' technique intro-
duced by Pierce et al. (1999). This technique combines selection by “image plane'' and
direct manipulation in “WIM'' (Figure 12.22).
Dedicated equipment can also accomplish selection and manipulation tasks, like
CAT (Control Action Table) of Hachet et al. (2003) (Figure 12.23). CAT is a six degrees
of freedom (6DoF) device with a circular movable tabletop on the top of a prop. CAT
uses isotonic information for orientation of the tabletop and isometric information for
the forces applied, at the same time (see 7 for more details)
All techniques proposed have a certain number of advantages but flaws as well.
This raises the problem of selection of technique to be used. Poupyrev et al. (1997)
propose an experimental evaluation of three techniques (Go-go, virtual hand, ray
casting). They conclude that none of these techniques is “better'' since each is adapted
to a specific manipulation situation. Similarly, other techniques explained here are
adapted to special conditions. Combination of techniques is a possible answer to the
emergence of new restrictions in virtual environments. Selection and manipulation in
virtual environments however remains an open problem.
12.5.3 Accuracy
We have seen that many techniques make manipulation of objects in a virtual environ-
ment possible. These techniques create a new difficulty with respect to 2D workstation
environments, position accuracy and in the direction of movements carried out.
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