Texte intégral / Full text (pdf, 20 MiB) - Infoscience - EPFL

5.4. Automatic Motion Adaptation
The main difference with the method proposed in the previous chapter is that this method
works online. We have integrated the gaze motion adaptation directly into the crowd simulation
animation loop. We thus do not have any a priori knowledge of the duration of an
interest point. We therefore do not have a set of pre-determined gaze constraints as in the
offline method. This does not have much impact on the spatial resolution but it does on
the temporal resolution. The computation of the overall displacement map to be applied to
the current character posture remains the same as for the offline method. We thus do not
discuss this part of the method in this chapter. However, the computation of the amount of
rotation to be applied to each joint at each frame needs to be adapted in order to meet online
requirements. These modifications are discussed in the following section.
5.4.1 Temporal Resolution
The main problem of not having any a priori knowledge of the constraint durations is that we
can not filter out the constraints which last under the minimum gaze duration. We therefore
have to deal with this on the fly in order to avoid very small, saccadic movements when a
point of interest lasts only a fraction of a second.
Whenever a new point of interest is selected, we define the amount of time the character
should take to perform the gaze motion, depending on the angle of rotation which has to
be done; the larger the angle, the longer it takes to perform the motion. We then simply
interpolate at each time step to determine the amount of total rotation which has to be done
by each joint.
However, the problem which can arise is that the interest point can change or exit the field
of view before the gaze motion is finished, i.e. before the joints have attained the final gaze
posture. The fact that the motion is not finished is not actually a big problem. However, if
this is the case, the gaze duration will necessarily be very small. This induces very unrealistic
behaviors where characters perform very small saccadic movements. In order to counter this,
we have defined a minimum gaze duration of half a second. If the interest point is deactivated
before attaining this minimum duration, we artificially maintain the interest at the previous
point of interest. Actually, we maintain the previous character posture until the minimum
gaze duration threshold is attained.
Another difference in the gaze simulation loop between the offline and online methods
is that, in the online method, we adapt each character at the current frame, whereas in the
offline method, we adapt each character’s complete animation before going on to the next
one. We thus have to keep track of each character’s previous posture in the online method.
Moreover, we also need to keep track of the starting posture when initiating gaze deactivation
or a change in interest point. Indeed, at each time step, we start from the character’s original
walking posture and not from the adapted posture at the previous time step.
Finally, the remainder of the temporal resolution stays the same as in the offline method.
More specifically, the computation of the different time values for eyes, head, and spine
to satisfy the gaze constraints and the desynchronization between these three sets of joints,
remains the same.
69