Game Development Reference
In-Depth Information
Figure 12.21 “World in Miniature'' Technique
applying a non-linear and modifiable correspondence interactively between the move-
ments (rotations) of the user's hand and movements produced on the object. This
solution minimises “declutching'' actions (i.e. the action of releasing the object when
the limit of amovement is reached (biomechanical limit or hardware limit) for returning
to a central configuration and picking up the object again for continuing the move-
ment. This method extends the capacities of the hardware interface but calls for a
speed/accuracy compromise.
For dealing with a large number of problems, interaction techniques are not appro-
priate for all manipulation scenarios. The idea of combining several techniques was
studied. The two most significant approaches using the combination of techniques are:
aggregation of techniques: the user is provided with an explicit mechanism for
choosing the desired technique during the task;
hybrid techniques: during the task, the interfaces switch transparently between the
interaction techniques depending on the current context of the task. The idea is to
provide the user with the best-suited manipulation technique.
As a backup to aggregation techniques, a mechanism for interactive choice of the
current technique must be provided. Among the best known is the Virtual Tricorder
(Wloka & Greenfield, 1995) that is a remote control device that makes it possible to
choose from different interaction techniques.
The following two techniques are hybrid techniques that make selection of objects
beyond the user's hand reach possible and additionally make direct manipulation in
the user's hand possible. It is to be noted that the previous techniques do not permit
these advantages to be combined.
The first example combines ray casting and direct manipulation with the virtual
hand. This is the “HOMER'' technique (Hand-centered Object Manipulation Extend-
ing Ray-casting Technique) (Bowman et al., 1997). Selection is done with the help
of ray casting. At the end of the selection, the interface switches to the manipulation
mode: the virtual hand is automatically projected on the selected object with the aim of
manipulation. A coefficient K is applied to the movement of the cursor, K being equal to
the distance of the user to the virtual object divided by the hand/user distance at the time
of selection of the virtual object. This makes it possible for the user to place the object
anywhere between him and its initial position. Selection is easy and permits manipu-
lation with 6 degrees of freedom (6DoF) over a wide range of distances. However, the
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