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James Christensen, William Miller; The effects of color categorization on shadow perception. Journal of Vision 2010;10(7):446. doi: https://doi.org/10.1167/10.7.446.
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As Cavanagh and Leclerc (1989) discussed, a significant change in hue across a cast shadow boundary is unlikely under real-world conditions. Such a change would require both multiple light sources of different hues and minimal interreflections that could otherwise mute differences in hue. Despite this constraint, they found that a change in color across a shadow boundary does not prevent shape from shadow perception, as long as there are luminance differences. The present study sought to demonstrate that rather than being largely ignored by observers, the color constraint on shadows is a cue that can inhibit a shadow percept, even when another shadow cue such as a penumbra is present. Perceptually matched pairs of color patches were generated that consisted of a base color and a possible shadow color, presented as two halves of a circular stimulus image. The base color was categorized as blue but near the blue-green boundary, while the possible shadow colors were adjusted to be each equally different from the base color, either crossing into the green color category or closer to prototypical blue. Possible shadows were always less luminant than the base color. This resulted in blue/blue stimulus pairs and blue/green pairs. Observers then completed a rating task that included these pairs as well as variously achromatic pairs or equiluminant pairs. The blue/blue stimulus resulted in higher shadow ratings than the blue/green pair. We conclude that it is not merely the presence of a color difference that inhibits shadow perception, but instead a categorical difference in color that may then combine with other edge type cues, similar to cue combination theories of shape perception.
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