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Vivian Paulun, Jan Jaap van Assen, Roland Fleming; Visual cues to stiffness of elastic objects . Journal of Vision 2016;16(12):637. doi: 10.1167/16.12.637.
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© 2017 Association for Research in Vision and Ophthalmology.
Non-rigid materials such as rubber or jelly respond to external forces by bending, bulging and wobbling in distinctive ways. These characteristic motions and shape changes could provide visual cues to the material's stiffness. However, there is an inherent ambiguity between the amount of force applied and the physical parameters of the object, as both factors contribute to the resulting deformations. We simulated two sets of scenes in which we varied both the softness of a cube, and the depth that a rigid cylinder was pushed into the cube (i.e. simulating an external force). In one scene, the cylinder was pushed into the cube from above and the simulation stopped when the perturbation reached its maximum depth. This allows us to isolate shape-related cues to stiffness. In the second scene, the cylinder was pushed in from behind the cube (invisible to the observer) and the animation only showed what happened after the cylinder was quickly removed. This emphasises motion-related cues to stiffness. Observers rated the apparent stiffness of the cubes. In the first scene, ratings were almost completely dominated by the magnitude of the perturbations (rather than the intrinsic material properties of the cube). This is consistent with observers assuming the cylinder acted with constant force, and attributing differences in deformation to differences in softness. Thus, although not invariant across different interactions, the depth of the shape deformation was used as a visual cue to softness. If, on the other hand, motion was the dominant feature in a scene, softness perception depended less on the shape deformation but strongly on the amplitude and frequency of the object's motion. An analysis of the geometries of the simulated cubes revealed several cues related to curvatures, and frequency and damping of oscillations, which correspond well with the perceptual ratings of stiffness.
Meeting abstract presented at VSS 2016
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