Game Development Reference
In-Depth Information
a mesh is more effective than a hierarchical division into octrees and becomes useful
only for a sufficient number of objects (some dozens) compared to the approaches based
on inter-object distances or separator planes. He also shows that k -DOP hierarchies
can be more effective than OBB hierarchies. However, this study does not consider all
the criteria operating in the choice of a collision detection strategy, which remains a
random choice even today. In addition, several acceleration methods remain unsuit-
able for deformable bodies and new approaches are necessary (Teschner et al., 2004).
However, the problem of acceleration is crucial because it is linked to the manage-
ment of calculation time. Since the calculation time is not limited and the complexity
is not constant, the programmer using a collision detection never knows how much
calculation time it will require. This lacuna is very detrimental to certain applications,
where time management is critical. He can at the most design some routines so as to
try mastering this time either by reducing the accuracy or by using a stochastic but not
guaranteed detection or by defining collision in probabilistic terms. Certain research
projects are exploring hardware acceleration, but these projects are still in their initial
stages.
17.3 PROCESSINGTHE COLLISION
The step of collision detection is crucial in graphic animation of virtual environments.
Once the collision is specified and the space of contacts is determined, the animation
models take over from there. A simple solution is to select and impose a position of
objects which guarantees that there will be no collision. However, this solution can
sometimes create discontinuity in positions and in any case makes the object movement
look non-realistic.
Physical approaches provide a more realistic collision resolution. It is however
necessary to choose a suitable detection method to be able to use various response cal-
culation methods. Since it is necessary to measure interpenetration, it is clear that the
possible collision detection algorithms can be none other than those based on intersec-
tion detection and quantification. However, these algorithms should be able to provide
interpenetration measurement used to calculate penalty. A volume measurement can
be used for calculation, but the penalty is generally based on an assessment of depth
and direction of interpenetration. However, the response calculated by this method is
sometimes very different than the expected one. In fact, an interpenetration vector cal-
culated only on the basis of a geometrical approach, i.e. without considering the paths,
provides a direction of the repulsive forces that is not consistent with the movement
(see figure 17.14).
A contact detection is necessary for other response calculations such as approaches
by constraints or by impulsions. The moment of contact can be determined by a suit-
able temporal detection (discrete or continuous). As a result, the 4D collision detection
algorithms prove to be the most suitable. In case of constraints, it is even desirable to
be able to integrate the constraints well in advance so that the acceleration of objects
can be corrected to prevent interpenetration. In this case, the most appropriate colli-
sion detection algorithms are in fact the distance calculating algorithms. In fact, the
constraint is then built based on the primitives closest to each of the objects on the
basis of a certain predefined threshold.
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