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Soranzo, Alessandro; Agostini, Tiziano (2004)
Publisher: Pion
Languages: English
Types: Article
The intersection between an illumination and a reflectance edge is characterised by the\ud `ratio-invariant' property, that is the luminance ratio of the regions under different illumination\ud remains the same.\ud In a CRT experiment, we shaped two areas, one surrounding the other, and simulated\ud an illumination edge dividing them in two frames of illumination. The portion of the illumina-\ud tion edge standing on the surrounding area (labelled contextual background) was the contextual\ud edge, while the portion standing on the enclosed area (labelled mediating background) was the\ud mediating edge. On the mediating background, there were two patches, one per illumination\ud frame. Observers were asked to adjust the luminance of the patch in bright illumination to\ud equate the lightness of the other. We compared conditions in which the luminance ratio at the\ud contextual edge could be (i) equal (possible shadow), or (ii) larger (impossible shadow) than\ud that at the mediating edge. In addition, we manipulated the reflectance of the backgrounds.\ud It could be higher for the contextual than for the mediating background; or, vice versa, lower\ud for the contextual than for the mediating background. Results reveal that lightness constancy\ud significantly increases when: (i) the luminance ratio at the contextual edge is larger than that at\ud the mediating edge creating an impossible shadow, and (ii) the reflectance of the contextual\ud background is lower than that of the mediating one. We interpret our results according to the\ud albedo hypothesis, and suggest that the scission process is facilitated when the luminance ratio\ud at the contextual edge is larger than that at the mediating edge and/or the reflectance of the\ud including area is lower than that of the included one. This occurs even if the ratio-invariant\ud property is violated.
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