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Alexa I. Ruppertsberg, Anya Hurlbert, Marina Bloj; Sensitivity to gradients in complex scenes. Journal of Vision 2006;6(6):391. doi: 10.1167/6.6.391.
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© ARVO (1962-2015); The Authors (2016-present)
Image gradients — smooth changes in colour and luminance — may be caused by intrinsic surface reflectance properties or extrinsic illumination phenomena, including shading, shadowing and inter-reflections. In turn, image gradients may provide the visual system with information concerning these factors, such as the orientation of surfaces with respect to the light source. The colour gradients induced by mutual illumination (MI) may play a similar role to that of luminance gradients in shape-from-shading algorithms; it has been shown that 3D shape perception modulates the influence of MI on surface colour perception (Bloj, Kersten, & Hurlbert, 1999, Nature, 402, 877–879). Here, we assess human sensitivity to changes in MI-induced colour and luminance gradients that arise from changes in the light source position, within a complex natural scene.
In Experiment 1 we tested whether observers were able to discriminate between gradients due to different light source positions. We found that observers reliably detected a change in the gradient information when the light source position differed by only 4 deg from the reference scene. This sensitivity was mainly based on the luminance information in the gradient (Experiment 2 and 3). Some observers make use of the spatial distribution pattern of gradients when discriminating between them (Experiment 4). The high sensitivity to gradient differences supports the notion that MI-induced gradients contain information that can support recovery of 3D shape and scene configuration properties.
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