The nonlinear color‐appearance model of Nayatani et al. is reformulated by changing the fundamentals from the Pitt to the Estévez‐Hunt‐Pointer primaries. The predictions of the reformulated model correlate with the Munsell and the NCS schemes better than those of the original. Formulas for various color‐appearance metrics are proposed, such as colorfulness, chroma, saturation, brightness (including a chromatic‐component contribution), and others. In the present model, the normalizing illuminant D65 is used in the transformation from the CIE tristimulus values to those in the fundamental‐primary system, and the normalizing illuminance is kept at 3,000 lx. The effects of changing the normalizing illuminant to D50 and the normalizing illuminance to 1,000 lx are studied. The results, however, show insignificant differences in the predictions of color appearance.
A nonlinear color‐appearance model was extended to apply to white and light‐gray background. Based on this extended model, two kinds of chromatic‐adaptation transforms were derived, which correspond to lightness‐chroma match and brightness‐colorfulness match. The chromatic‐adaptation transform for lightness‐chroma match is also an extension of the transform proposed by CIE for further testing. The differences between the two transforms were confirmed by visual observations. The usefulness of a combination of a color‐appearance model and its corresponding chromatic adaptation transform is discussed. In addition, the practical importance of brightness and colorfulness is discussed.
A model of color vision is given to predict color appearance of object colors under various illuminants and illuminance levels. The model constitutes a uniform color‐appearance space under any adapting condition, and can predict the metric quantities relating to the following color perceptions: (1) constant‐hue loci, (2) the Munsell color scheme, (3) the increase of brightness contrast of nonselective samples with increasing adapting‐illuminance level, (4) the Helson‐Judd effect, and (5) the increase of colorfulness of chromatic samples with increasing adapting‐illuminance level.
A hypothesis is proposed to predict the perceived lightness of chromatic object colors, which is well known as the Helmholtz‐Kohlrausch effect or the lightness‐reflectance ratio effect (L/Y effect). The hypothesis is, “When two chromatic object colors with different hues have the same values for each of three attributes (whiteness, blackness, and chromaticness), the two colors have the same perceived lightness, including a chromatic‐component contribution.” The hypothesis is tested by two kinds of experiments: perceivedlightness matching using the NCS Color Atlas, and chromatic tiles determined by Wyszecki. Both results strongly confirm the hypothesis. Further, contour lines of constant L/Y ratios are estimated, and are very similar to those reported previously.
The color‐appearance models developed by R. W. G. Hunt (model H) and Y. Nayatoni and his collaborators (model N) are compared. The following color perceptions are analyzed: (1) colorfulness under illuminant C for NCS samples, (2) colorfulness change by changing illuminant C to illuminant A, (3) colorfulness change by changing adaptign illuminance, (4) Helson‐Judd effect on achromatic colors under saturated chromatic illuminants, and (5) brightness and lightness. Special features of each model are made clear. In addition, a deatailed discussion is given on the mechanism introbucing the Helson‐Judd effect and on the model formulations, especially in model H.
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