General hypothesis for the definition of chromatic-opponency functions are given in a black-box approach to the problem. It is supposed that the color signals in the visual color processing can be factorized into the product of the lightness times a pair of chromatic opponency functions and the whole chromatic processing consists of three independent processes: a linear transformation, a logarithmic compression, and a chromatic opponency actuation. The main chromatic opponency functions, obtainable by very general hypothesis on the symmetry and on the homogeneity degree, are supposed equal to the logarithms of tristimulus value ratios in a proper reference frame of the tristimulus space. The perceptual chromatic functions, individually with uniform scales, are a linear mixing of the main chromatic opponency functions. The Bezold–Brücke hue shift is not considered. A performance of these hypothesis is successfully realized on the OSA-UCS system, for extra macula vision, and on the chromatic discrimination ellipses, for macular vision. Unique hues are derived from the main chromatic opponency functions of spectral lights. © 2004 Wiley Periodicals, Inc. Col Res Appl, 30, 31–41, 2005; Published online in Wiley InterScience (www.interscience.wiley.com).
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