Evaluating Observers’ Sensitivity to Errors in Human and Physical Throws
Michele Vicovaro1∗
2
Ludovic Hoyet2†
Luigi Burigana1‡
Carol O’Sullivan2§
1
Department of General Psychology, University of Padua
Graphics, Vision and Visualisation Group, Trinity College Dublin
Figure 1: Left: Captured biological human throw. Right: Corresponding ramp throw.
1
Introduction
the ball recorded in the captured throw was manipulated according
to the following design: 2 modified Component (horizontal, vertical) × 2 Sign of the manipulation (decrease, increase) × 3 Magnitude of the manipulation (15%, 30%, 45%) × 6 repetitions. The
participants had to indicate whether the trajectory of the ball was
correct or incorrect compared with the force exerted by the virtual
character (Human block) or compared to the motion during the descent (Ramp block). The ball disappeared before making contact
with the ground.
Understanding whether observers are sensitive to physical distortions in dynamic events is important in order to develop plausible
simulations while saving time on details that observers cannot perceive [Barzel et al. 1996]. In this experiment, we evaluated observers’ sensitivity to physical distortions in virtual throwing animations. As previous results showed a greater sensitivity to physical distortions on virtual human characters rather than on simple
objects [Reitsma et al. 2008], we studied the effect of using both a
biological human throw and a simple physical simulation (free fall
of a ball on a ramp). The results show that observers’ sensitivity to
errors depends on the animacy of the thrower.
2
We performed a four-way, mixed, repeated measure ANOVA on
the percentage of incorrect responses. Here we discuss the most
interesting results for the comparison between human and physical throws. We found that observers are more sensitive to decreases than to increases in release velocity (horizontal and vertical
components) in the Human block, whereas the opposite is true for
the Ramp block. The sensitivity to increases is similar in the two
blocks, whereas observers’ sensitivity to decreases is greater in the
Human block than in the Ramp block.
Experiment
The motion of a projectile can be divided into two phases: the motion before it is released (preparatory motion) and the motion after the release (ballistic motion). The parabolic trajectory followed
by a projectile after it is released depends on the horizontal and
vertical components of its release velocity. In this experiment we
manipulated separately horizontal and vertical components of the
release velocity, while leaving unchanged the preparatory motion.
We evaluated observers’ sensitivity to these physical mismatches.
3
Conclusion
Observers are more sensitive to decreases in release velocity when
presented with biological human throws rather than with simple
physical simulations. Reitsma et al.[2008] found that observers
tend to overestimate the effect of gravity on ballistic motion. This
may explain the low sensitivity to decreases in velocity in the Ramp
block. Results for the Human block show that the biological motion is not affected by this bias, but by the opposite one. The fact
that in everyday life we do not use underarm throws aiming at a
short throwing distance may have added ‘semantic’ information to
the biological motion, causing short throws to be perceived as incorrect. Our results are important for they show that animations
representing captured human throws need a higher level of physical
correctness than animations representing a simple physical throw.
Human block. We motion captured the full body movement of an
actor who threw a standard tennis ball at a distance of 5m with an
underarm throw (Fig 1 Left). We also captured the trajectory of the
ball using four markers placed on it. The recorded body motion was
then mapped onto a skeleton, where joint angles were computed and
used to drive a virtual character whose morphology matched that of
the actor.
Ramp block. We built a ramp model which was designed to match
the release velocity and the release position of the ball in the human
throw (Fig 1 Right). The starting position and descending motion
of the ball on the ramp were computed by physically simulating its
free-fall on the surface under the effect of gravity (ignoring friction).
References
BARZEL , R., H UGHES , J. F., AND W OOD , D. N. 1996. Plausible
motion simulation for computer graphics animation. In Proc. of
Eurographics Workshop on Comp. Animation and Simulation.
Thirty participants took part in this experiment, 15 in the Human
block, and 15 in the Ramp block. The original release velocity of
R EITSMA , P. S. A., A NDREWS , J., AND P OLLARD , N. S. 2008.
Effect of character animacy and preparatory motion on perceptual magnitude of errors in ballistic motion. Comput. Graph.
Forum 27, 2, 201–210.
∗ e-mail:[email protected]
† e-mail:[email protected]
‡ e-mail:[email protected]
§ e-mail:[email protected]
Copyright is held by the author / owner(s).
SAP 2012, Los Angeles, CA, August 3 – 4, 2012.
ISBN 978-1-4503-1431-2/12/0008 $15.00
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