Do the light gray triangular shapes in the diagram below have the same hue and brightness?
Believe it or not, both triangular surfaces possess the same luminance (brightness)! Despite having the same border contact with regions of higher and lower luminance, the right-hand triangle appears slightly darker, apparently at odds with an explanation in terms of color contrast. The Gestalt psychologists explain this difference by saying that one triangular shape appears to belong to the cross, while the other does not (‘belongingness’). This illusion is called the Wertheimer-Benary cross, after the Gestalt psychologists who discovered it.
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