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Emily Cecil, Ivana Ilic, Michael Webster; An ambiguous depth-from-shading illusion. Journal of Vision 2019;19(8):112. doi: https://doi.org/10.1167/19.8.112.
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© ARVO (1962-2015); The Authors (2016-present)
We report on an image with strikingly unstable depth interpretation. The image is a digital photo that author EC took of a plaster cast of her dog’s paw print, which alternates in whether the paw impression appears to stick in or out. We sampled 20 observers to characterize differences in the perceived depth both across and within individuals. In the original image, 67% were biased to perceive outward and 33% inward depth. These dominant percepts were negatively correlated with the ratings for the same observers when the image was inverted by rotating 180 deg or inverting the contrast. This suggests that the differences in part depend on differences in the perceived direction of the lighting. In displays showing multiple upright and inverted copies of the image, observers tended to see both depths simultaneously, again consistent with a common lighting inference. However, a surprising proportion of times, some observers also experienced both orientations to have the same depth. The dominant percepts for an individual tended to be consistent across test times, while there was nevertheless large variability within a session. Reversals of depth from shading with image inversion are well known, yet the variability and instability at a single orientation is uncommon. We attribute the ambiguity to strong shading cues mixed with poor cues to the illuminant.
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