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Maarten Wijntjes, Katja Doerschner, Gizem Kucukoglu, Sylvia Pont; What velvet teaches us about 3D shape perception. Journal of Vision 2010;10(7):1227. https://doi.org/10.1167/10.7.1227.
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© ARVO (1962-2015); The Authors (2016-present)
Humans are able to perceive a large variety of optical material properties. The shading patterns that convey these properties are used by painters to render e.g. a luxurious dining table with golden bowls and crystal glasses. Furthermore, painters have developed techniques to render the clothing material, e.g the velvet cape of Pope Leo X by Raphael. The shading trick that is often used to convey a velvet appearance is ‘inverted Lambertian shading’. While for Lambertian shading the reflected light is highest at frontal illumination, the opposite holds for the hairy surface of velvet. Whereas it seems easy to identify velvet objects in paintings, it is unknown how these shading patterns affect the perception of shape. On the basis of a computational model we predicted that if the velvet shading is (partly) interpreted as Lambertian, the perceived 3D shape should flatten in the viewing direction. This is indeed what we found in a previous study where we used computer rendered objects. In the present study we used 3D prints of those virtual objects and applied a Lambertian (matte spray paint) and velvet (flock) layer. Photographs of these objects were used in the experiment. We found a similar flattening effect of the velvet surface material. Furthermore, we analyzed the non-linear (second order) differences in perceived shape between reflectance properties (Lambertian and velvet) and between illumination conditions. Surprisingly, we found that the non-linear differences were larger between illumination conditions. The results show that real shapes show similar perceived shape deviations as the rendered shapes. This suggests that in both cases, shape perception seems to be affected by a Lambertian prior. Furthermore, the non-linear differences can be interpreted as a stronger shape constancy for different BRDFs than illumination conditions.
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