The Tricolore Effect: Illusory
Surface Colour Perception
The flag on the left appears to be the French tricolore, whereas the one on the right appears to be the Italian tricolore. In fact, the flags are identical.
This effect is related to the lightness illusion discovered by Barton Anderson and Jonathon Winawer . In estimating the reflectance of surfaces, the visual system attempts to take into account the effects of illumination and other factors such as the presence of fog or shadows. Segmentation of the scene into layers may be a part of this process. We interpret the leftmost panel of the "French" flag as a blue rectangle behind a green fog, whereas for the "Italian" flag it appears as a green panel seen through blue haze. Context determines whether we see le tricolore or il tricolore.
 Anderson, B.L. and Winawer, J., Image segmentation and lightness perception, Nature 434(7029): 79-83 (2005).
Note: This effect was described in 2006 by Michael J. Lyons (ATR) and
Kazunori Morikawa (Osaka University), and submitted as an entry to the
2008 "Illusion of the Year" contest in Naples, Florida.
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