Theoretical analyses of spectral reflectances of natural surfaces suggest that their perceived colors can be well reproduced by approximations comprising combinations of three or four spectral basis functions. The aim of the present work was to assess psychophysically the number of basis functions necessary to reproduce entire natural outdoor scenes. Hyperspectral images of 20 such scenes were each subjected to a principal component analysis and then reproduced with a variable number of basis functions. The quality of the color approximation under daylight illumination was quantified theoretically in CIELAB space and psychophysically by spatial and temporal two-alternative forced-choice measurements in which the original and the approximated images were compared on a calibrated color monitor. Although five basis functions produced on average unit error in CIELAB space, original images were visually indistinguishable from their approximations only if there were at least eight basis functions. The combination of the spectral diversity of the natural world and the observed levels of color discrimination suggest that estimates of the minimum number of basis functions necessary to reproduce natural scenes may need to be revised upward.
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