Seeing jelly: judging elasticity of a transparent object

Taking advantage of computer graphics technologies, recent psychophysical study on material perception has revealed how human vision estimates the mechanical property of objects, such as liquid viscosity, from image features. Here we consider how human perceive another important mechanical material property --- elasticity. We simulated scenes in which a transparent cube falling on the floor, while manipulating the elasticity of the cube. We asked observers to rate the elasticity using a 5-point scale. Human observers were quite sensitive to the change in the simulated elasticity of the cube. In comparison with the original condition, the elasticity was overestimated when only the cube contour deformation was visible, whereas underestimated when the cube contour deformation was hidden and only internal optical deformation was visible. The effects of contour and optical deformations on elasticity rating were almost the same when the observers viewed white noise fields that reproduced the optical flow fields of the cube movies. Increasing frame duration (which decreased image speed) also increased the apparent elasticity. These results suggest that human elasticity judgment is based on the pattern of image motion arising from contour and optical deformations. This scientific finding may provide a hint for computationally efficient rendering of perceptually realistic dynamic scenes.