Shape-from-shading perception with temporally modulated shadings T Sato, K Hosokawa |
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Department of Psychology, University of Tokyo, Japan
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| Shape from shading is computationally ambiguous if lighting direction is not known. Thus, the visual system assumes that the light is coming from above. What happens if the shading is temporally modulated? The object should change its 3D-shape (convex/concave) if the light-from-above hypothesis is sturdy, but the movement of light source should be experienced if rigidity constraint is stronger. To answer this question we examined the 3D shape perception for an egg-crate stimulus with temporally modulated shading (0.5 to 8.0Hz) having either vertical or horizontal shading-gradient. For vertically graded stimuli, it was found that shape-change was dominant at 0.5 and 1 Hz but it disappeared very quickly and was almost never perceived at 2Hz. Light-source-movement almost never occurred at 0.5Hz, but it became predominant at 2Hz. It then decreased and was replaced by simple flicker beyond 3-4Hz. For horizontally graded stimuli, the shape-change almost never occurred at any temporal frequency. Light-source-movement was dominant at low temporal frequencies, and it was replace by simple flicker beyond 3Hz. These results revealed an interesting relationship between the two constraints. Rigidity generally functions at low temporal frequencies, but it is overridden by light-from-above hypothesis at frequencies below 2Hz when the stimulus was vertically graded. |
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