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1 Introduction This paper is about a phenomenon that combines a delicate beauty with a profound significance for understanding visual perception. The neon-like glow of a color that escapes the boundaries of a real figure and fills the surrounding area until it is halted by the boundaries of an illusory figure has an ethereal quality unlike any other brightness and color effect. It also has deep implications for the way in which our visual system uses seemingly incomplete stimuli to generate meaningful percepts, segregate objects from their backgrounds, and provide them with color and depth. As it so often happens in science, the effect was discovered and then independently rediscovered in the span of a few years. In 1971, Dario Varin of the University of Milan published a monograph on'chromatic contrast and diffusion'whose front cover is reproduced in figure 1. A circular transparent veil can be seen to extend over …