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Impossible shadows and lightness constancy.

Alessandro Soranzo1, Tiziano Agostini

  • 1Department of Psychology, University of Trieste, via S Anastasio 12, I 34134 Trieste, Italy. soranzo@psico.univ.trieste.it

Perception
|February 8, 2005
PubMed
Summary
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Lightness constancy improves when an "impossible shadow" is present and the surrounding background reflectance is lower. This challenges the ratio-invariant property, aiding visual scene segmentation.

Area of Science:

  • Visual perception
  • Computational neuroscience
  • Image processing

Background:

  • The 'ratio-invariant' property describes how luminance ratios remain constant across illumination changes at an edge.
  • Understanding how the visual system interprets illumination and reflectance is crucial for visual scene perception.

Purpose of the Study:

  • To investigate how variations in illumination and reflectance edges affect lightness constancy.
  • To determine if violating the ratio-invariant property influences visual scene segmentation.

Main Methods:

  • A CRT experiment simulated illumination edges with varying luminance ratios and background reflectances.
  • Observers adjusted patch luminance to equate perceived lightness under different illumination conditions.
  • Conditions included 'possible' and 'impossible' shadows, manipulating edge luminance ratios and background reflectances.

Related Experiment Videos

Main Results:

  • Lightness constancy significantly increased when an 'impossible shadow' was created (contextual edge ratio > mediating edge ratio).
  • Lightness constancy also increased when the contextual background reflectance was lower than the mediating background reflectance.
  • These effects occurred even when the ratio-invariant property was violated.

Conclusions:

  • Visual scene segmentation is facilitated by specific configurations of illumination and reflectance edges.
  • The albedo hypothesis provides a framework for understanding these perceptual effects.
  • Perceptual judgments of lightness and scene structure can be influenced by violations of established optical invariants.