Learning to Use Illumination Gradients as an Unambiguous Cue to Three Dimensional Shape

Abstract

The luminance and colour gradients across an image are the result of complex interactions between object shape, material and illumination. Using such variations to infer object shape or surface colour is therefore a difficult problem for the visual system. We know that changes to the shape of an object can affect its perceived colour, and that shading gradients confer a sense of shape. Here we investigate if the visual system is able to effectively utilise these gradients as a cue to shape perception, even when additional cues are not available. We tested shape perception of a folded card object that contained illumination gradients in the form of shading and more subtle effects such as inter-reflections. Our results suggest that observers are able to use the gradients to make consistent shape judgements. In order to do this, observers must be given the opportunity to learn suitable assumptions about the lighting and scene. Using a variety of different training conditions, we demonstrate that learning can occur quickly and requires only coarse information. We also establish that learning does not deliver a trivial mapping between gradient and shape; rather learning leads to the acquisition of assumptions about lighting and scene parameters that subsequently allow for gradients to be used as a shape cue. The perceived shape is shown to be consistent for convex and concave versions of the object that exhibit very different shading, and also similar to that delivered by outline, a largely unrelated cue to shape. Overall our results indicate that, although gradients are less reliable than some other cues, the relationship between gradients and shape can be quickly assessed and the gradients therefore used effectively as a visual shape cue.

Short-Term Memory Affects Color Perception in Context

“…Our conceptual framework makes the following assumptions based on previous literature: 1) observers are estimating (explicitly or implicitly) both surface reflectance and illumination [28] , [36] , [37] ; on each trial, they pick the stimulus whose inferred reflectance is relatively more “bluish” 2) the background color dominates the observers’ illumination estimate based on its spatial extent [28] ; 3) observers have available priors for reflectance and illumination [28] , [36] ; 4) both priors can be updated based on reflectance and illumination estimates in a block of trials [29] , [30] , [39] – [41] . In the following, we interpret our results in this framework and show that the framework qualitatively accounts for the patterns of biases across experimental conditions.…”

Can you see what you feel? Color and folding properties affect visual–tactile material discrimination of fabrics

“…Corduroy has tiny fibers that have distinctive reflective properties (there are strong color gradients from shaded areas to bright areas), but this information is reduced when color information is removed. One possibility is that the inter-reflectance between the folds in the color images could be characteristic of material properties and thus provide information about the lighting geometry (Fleming, Holtmann-Rice, & Bulthoff, 2011;Harding, Harris, & Bloj, 2012;Ruppertsberg, Bloj, & Hurlbert, 2008). To further understand the effects of color on the perception of complex materials, one can use computer-rendered stimuli to systematically measure material perception by isolating surface reflection, textures, and lighting parameters.…”

How Is Perception Tractable?

“…A vivid illustration of this joint inference can be found in the bistable chromatic Mach card (Bloj et al 1999, Harding et al 2012. In this e ect, a folded card with two colored sides is shown to participants.…”