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Figure 16.8 Architecture for haptic rendering by intermediate model
This difficulty in obtaining a simulation frequency that respects the time constraints
of the simulation explains why the first virtual environments that included haptic
feedback were very rudimentary, that is to say with immobile obstacles. The simula-
tion then becomes a mere detection of obstacles and response calculation, making the
most of the environment's immobility. Several research works were conducted on this
type of environment, and they offered some specific approaches ( H-Collide method
by regular subdivision of the space and oriented bounding box hierarchy (Gregory
et al., 1999), tetrahedron grids (Held et al., 1995), sphere hierarchy (Ruspini et al.,
1997), Voxmap/PointShell method based on the discretisation of the environment and
sampling of the avatar surface (McNeely et al., 1999)). Besides, there are several
commercial haptic libraries based on this immobility principle. Certain attempts were
made to apply these methods to dynamic environments for convex polyhedral objects
(Gregory et al., 2000) or any objects (Ho et al., 1999; Kim et al., 2002), where the
number of simulated bodies was still small (about ten). This difference in frequency
requires the haptic interface to be controlled by a special loop, called control loop ,
run by the simulation computer or on a dedicated system, simultaneously with the
simulation (Shaw et al., 1992).
16.3.1 Intermediate representations
We try to guarantee a realistic and stable haptic rendering in the case of a complex
environment and/or a slow simulation. For this purpose, it is beneficial to use an inter-
mediate representation (Adachi et al., 1995), which consists of extracting a local and
sometimes simplified copy of the contact zone in a buffer. This buffer then calculates
faster. This intermediate model is sometimes called a buffer model (Balaniuk, 1999).
Using an intermediate model significantly modifies the architecture necessary for haptic
rendering (see figure 16.8): the haptic loop is complex and runs simultaneously with
the simulation loop; the information exchanges (intermediate model, position, forces,
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