Natural surfaces possess intrinsic chromatic texture. A banana is neither uniformly coloured nor uniformly bright, whether ripe yellow or unripe green. This feature of natural surfaces is not captured by traditional studies of colour perception, which typically employ stimuli of uniform colour and luminance. We consider the effect of surface "polychromaticity" (Beeckmans, Philosophical Psychology, 2004) on colour appearance under changing illumination. Computational models of colour constancy demonstrate (implicitly or explicitly) that the estimation of the illuminant spectral power distribution improves as the number of distinct surface reflectance samples increases. The underlying assumption of such models is that each distinct surface is uniform in reflectance. Yet, a single polychromatic surface may provide a large number of reflectance samples on its own, and thereby undergo improved colour constancy. On the other hand, chromatic texture within a surface also blocks simultaneous chromatic contrast between the surface and its background, most powerfully when the background is uniform (Hurlbert & Wolf, Prog. Brain Res., 2003). Therefore, local between-surface contrast - a strong contributor to colour constancy for artificial, homogeneous surfaces (e.g. Kraft & Brainard, Proc. Nat. Sci. Acad., 1999) - is likely to be weak for chromatically textured surfaces.
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