Because the perceived weight of objects may be affected by various nonweight properties, such as their size and the density of their surface material, relative weight is sometimes misperceived (the size-weight illusion and the material-weight illusion, respectively). A widely accepted explanation for weight illusions is provided by the so-called expectation model, according to which the perceived weight stems from the contrast between the actual and expected weights. In the present study, we varied both the surface material and the size of stimuli, while keeping constant their physical weights. In Experiment 1, the participants lifted the stimuli by grasping them on opposite sides, whereas in Experiment 2 they lifted them by using a string that was attached to their top surface. We used a variant of the random conjoint measurement paradigm to obtain subjective interval scales of the contributions of surface material and size to the expected and the perceived weight of the stimuli. Inconsistently with the predictions from the expectation model, we found, in both experiments, that the surface material contributed more than the size to the expected weight, whereas the size contributed more than the surface material to the perceived weight. The results support the hypothesis that perceived weight may depend on implicit, rather than explicit, weight expectations.

Animation budget constraints during the development of a game often call for the use of a limited set of generic motions. Editing operations are thus generally required to animate virtual characters with a sufficient level of variety. Evaluating the perceptual plausibility of edited animations can therefore contribute greatly towards producing visually plausible animations. In this article, we study observers' sensitivity to manipulations of overarm and underarm biological throwing animations. In the first experiment, we modified the release velocity of the ball while leaving the motion of the virtual thrower and the angle of release of the ball unchanged. In the second experiment, we evaluated the possibility of further modifying throwing animations by simultaneously editing the motion of the thrower and the release velocity of the ball, using dynamic time warping. In both experiments, we found that participants perceived shortened underarm throws to be particularly unnatural. We also found that modifying the thrower's motion in addition to modifying the release velocity of the ball does not significantly improve the perceptual plausibility of edited throwing animations. In the third experiment, we modified the angle of release of the ball while leaving the magnitude of release velocity and the motion of the thrower unchanged, and found that this editing operation is efficient for improving the perceptual plausibility of shortened underarm throws. Finally, in Experiment 4, we replaced the virtual human thrower with a mechanical throwing device (a ramp) and found the opposite pattern of sensitivity to modifications of the release velocity, indicating that biological and physical throws are subject to different perceptual rules. Our results provide valuable guidelines for developers of games and virtual reality applications by specifying thresholds for the perceptual plausibility of throwing manipulations while also providing several interesting insights for researchers in visual perception of biological motion.

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Horizontal collisions have long been used as a tool for exploring people's intuitive understanding of elementary physical laws. Here, we explored intuitive understanding of the relationship between the kinematic patterns of collisions and the elasticity of the colliding objects. In Experiment 1A, we manipulated the simulated materials of two virtually colliding spheres and asked the participants to judge whether the simulated collisions appeared Bnatural^or Bunnatural.^We did the same in Experiments 1B and 2, but asked the participants to adjust the velocities until the collisions appeared to be Bperfectly natural.^In Experiment 3, we removed pictorial cues to the materials of the colliding spheres and asked the participants to rate the bounciness of the materials, in view of the kinematics of simulated collisions. Overall, the results showed that observers intuitively understood that collisions between more elastic objects subtend a higher coefficient of restitution than collisions between objects with lesser elasticity. The results also highlighted some discrepancies between the intuitive and Newtonian physics of collisions. Observers were somewhat insensitive to violations of the principle of energy conservation, and their responses were influenced by irrelevant kinematic features of the collisions, such as the collision type and precollision velocity. We discuss our experimental results in relation to salient theoretical perspectives on intuitive physics.

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