Simultaneous Contrast Illusion: Koffka's Example

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Koffka's illusion is an example of how spatial configuration may significantly influence contrast perception.

A ring of an uniform gray shade is drawn on the background of two contrasting squares. It's left and right halves acquire independent identities when the pieces are slid vertically against each other:

References

E. H. Adelson, Lightness Perception and Lightness Illusions

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