Game Development Reference
In-Depth Information
Figure 16.9 Spherical intermediate model
model's low resolution by a haptic texture giving a less rough rendering (Otaduy &
Lin, 2003). In case of these representations, it mainly involves finding the criteria
(proximity, speed, perception quality, etc.) that help selecting the level of representation
of the objects, based on which the forces are calculated.
Intermediate representation makes it possible to calculate the force updates at high
frequency, in a more or less approximate manner. However, the techniques discussed
have several limitations. Firstly, the intermediate model is local. If the operator moves
away from the position used to establish this model, the approximations appear to
be more and more erroneous. Besides, this situation arises quite frequently, especially
when the simulation is really slow. Secondly, while updating (at low frequency, which is
the frequency of the simulation) the intermediate representation, the force calculation
parameters are modified suddenly, which leads to a discontinuousness in the forces
sent. The methods generally try to restrict this discontinuity by interpolation which
however adds an additional approximation to an already approximate model of the
environment. Another, non-local, solution was suggested by Davanne et al. (2002). It
consists of providing the acceleration structures of the simulation's collision detection
as an intermediate model. A simulation doing a spatial division is assumed to be the
procedure that accelerates collision detection (refer to chapter 17). Once the mesh is
filled with the primitives of the mobile environment, i.e. during the collision detection
phase, it is provided as is to the haptic loop. This loop carries out a collision test in an
already filled mesh (which has a low calculation cost) and quickly deduces the forces
to be sent. This method manages the problems of continuity. When a new mesh is
calculated by the simulation, it is assumed that the primitives move from their position
in the oldmesh to their position in the mesh received. This movement is an interpolation
(not necessarily linear) between these two positions; the parameter for this movement
is time. In other words, this position is assumed to be reached at the assumed moment
when the next mesh needs to be generated. We thus create a pseudo-delay which is
less detrimental to the force feedback because, in any case, the new position of the
primitives is not known until the new mesh is acquired. One issue, however, remains
difficult to manage: it is difficult to predict the date of sending the next mesh. Generally,
we assume that the time interval between the two previous meshes is maintained, which
is not true when the simulation time is not constant. In practice, there can be some
occasional variations which are felt by the operator.
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