Game Development Reference
In-Depth Information
3.4 PROPRIOCEPTION
3.4.1 Introduction
The three fields related to proprioception (mechanical and internal sense of the body)
are the sensations of position in the space, body movement and forces exerted on the
muscles. The first two sensations correspond to the kinaesthetic sense.
To give the illusion of being immersed, the presentation of the virtual world sur-
rounding the subject should be subject-centred. He then perceives himself to be at the
centre of this world. For this purpose, it is important to recreate the exact position of
that subject's body in the virtual world. It is therefore necessary to use the sensors that
transfer the data regarding the position of the body and legs in the real world to the
computer controlling the virtual world. Similarly, considering the human sensitivity to
movements in the Z, X and Y axes, it is necessary to restore to the subject immersed the
sensations related to the simulated equilibration (vertical perception) and movements.
These sensations depend on relative movements, gravity and accelerations. Movement
awareness is obtained from the position of different body parts, their direction, their
rate of mobility as well as the forces of muscular contraction necessary for the resistance
to movements or to the weights of objects (Stuart, 1996). The organs contributing to
these sensations, in addition to the proprioceptive organs of the muscles, tendons and
joints (refer to the part on touch), are the organs located in the vestibule of the internal
ear and to a very limited extent, the eyes. This is why we will talk about the anatomy
and the physiology of the vestibular apparatus in this section. We will then discuss the
problem of perceptive localisation.
3.4.2 Physics of gravity and accelerations
Mobilisation of a body is expressed in acceleration given in m/s 2 or in G (1G
=
9.8m/s 2 ). Its effect on the human body depends on various factors:
Intensity - generally, the higher the intensity, the more the effects are felt, but this
should be weighted in relation to the interaction with the other factors,
Speed of application - the time taken to reach the application plateau. This factor
is important for brief accelerations (we realise it in case of Jolt, expressed in G/s),
Direction with respect to the body axes - the definition of these axes is shown in
the following figure. It is possible to differentiate between the accelerations related
to the three axes - X, Y, Z and the rotations around X, Y, Z with respect to these
axes. The vectors of acceleration or deceleration and the inertia forces are applied
on each of these axes and on each of these rotations (Figure 3.23),
Duration of application - there are two cases: Acceleration of long duration (more
than 0.1 s) and brief accelerations. The second type mainly has mechanical effects
while the first essentially disturbs the haemodynamic functions.
3.4.3 Vestibular apparatus and kinaesthetic canals
The vestibular apparatus is a series of bulges and three orthogonal semicircular canals
located in the three planes of the space. The inside of the bulges filled with a gelled-up
liquid has a layer of ciliary cells whose extensions are bent in the opposite direction
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