Game Development Reference

In-Depth Information

etc.) synchronise these two loops. A multiprocessor hardware architecture is essential

in this type of approaches. There are several strategies to “copy'' the environment. It is

possible to extract a local force field, a local geometry from the environment or even

try, if possible, to reproduce this environment at a lower scale:

•

Approaches by force field

On the basis of a given position of the device, we canmark out a force field, i.e. associate

a force to every position taken by the interface around the initial position The first

approach consists of calculating the Jacobian of the force

∂F/∂

J
=

x
(Cavusoglu &

Tendisk, 2000). Any difference

x
(
t
) with respect to the initial position creates a force

F
(
t
)

=
F
0
+
J
×

x
(
t
). It is also possible to use the temporal coherence by doing a

linear extrapolation in the course of time, on the basis of the last two forces
F
n
and

F
n
−
1
calculated by the simulation at the instants
t
n
and
t
n
−
1
:

t

−

t
n

F
(
t
)

=
F
n
+

(
F
n
−
F
n
−
1
)

t
n
−

t
n
−
1

It is also possible to use spatial coherence by inspecting how the forces have changed

with respect to the last two positions and by extrapolating for the current position

(Picinbono & Lombardo, 1999). If
P
n
−
1
and
P
n
are the two last positions taken into

account by the simulation and
P
(
t
) is the current measured position, then this position

is projected on
P
(
t
) on axis (
P
n
−
1
P
n
). Then the force returned is:

P
(
t
)

P
n

−

F
(
t
)

=
F
n

(
F
n

−
F
n
−
1
)

+

P
n

−

P
n
−
1

In general, the interpolations calculated on a spatial base are more stable than the

purely temporal interpolations. The calculations involved in these intermediate meth-

ods are moderate and the simulation does not provide additional information (except

in the case of Jacobian which needs to be calculated).

•

Geometric approaches

A simple model of the local geometry of the zone in which the avatar is located

is extracted from the environment. Adachi et al. (1995) use a plane tangent to the

point of collision, whereas Mark et al. (1996) suggest using several planes. Balaniuk

(1999) describes a general approach that makes it possible to approximate the local

geometry by a parameterised surface, and applies this formula in the case of a plane

and a sphere (see figure 16.9). These techniques being restricted to a convex geom-

etry, Mendoza and Laugier (2000) suggest recovering (using the graphics hardware

(Lombardo et al., 1999)) the triangle closest to the avatar as well as the adjacent tri-

angles from the environment. It is then sufficient to calculate the collisions with this

sub-model.

•

Multiresolution approaches

For deformable objects, certain models suggest creating a local model from a level of

lower resolution of a mechanical model (Astley &Hayward, 1997), or by reducing and

linearising a model (Cavusoglu & Tendisk, 2000). It is possible to counterbalance the

Search Nedrilad ::

Custom Search