Color context effects. (A) Color contrast. The two spectrally identical central targets (see Insets above) appear different when placed in different chromatic contexts. (B) Color constancy. Two spectrally different central targets appear more similar when embedded in different chromatic contexts. (C) Color assimilation (spectrally identical targets). Spectrally identical targets (the small squares) appear different in the context of the greenish lines on the left vs. the reddish lines on the right. (D) Color assimilation (spectrally different targets). Iteration of the stimulus elements can also make spectrally different targets appear more similar. Note that the contexts in A and B make the color appearance of the targets shift away from that of the contexts; the contexts in C and D, however, shift the color appearance of the targets toward that of the contexts.

The mean of the probability distributions derived from the database [i.e., P(C_t |C_c, N_t)] in an example where C_c = [0.32, 0.36] (a greenish context whose chromaticity is indicated by the square symbol), compared to an example where C_c = [0.42, 0.38] (a reddish context whose chromaticity is indicated by the triangle); N_t varied from 1 to 25 in both cases, increasing in the direction indicated by the arrows. The mean values in the greenish and reddish chromatic contexts (circles and asterisks, respectively) are superimposed on a blow-up of the corresponding region of the 1931 CIE chromaticity diagram. See text for further explanation.

Sampling of the organization of spectral returns in natural images. (A) Representative image taken with a calibrated hyperspectral camera by using 31 optical interference filters to span the visible light spectrum (400-700 nm) at 10-nm intervals (4). The image resolution was 256 × 256, with each pixel subtending ≈0.056 × 0.056°. To remove the influence of the chromatic properties of the illuminants under which the hyperspectral images were taken, the radiance spectrum of each pixel was divided by the radiance spectrum of a reference gray calibration card that had been placed in each scene. (B1) Blow-up of one of the boxed regions in A. (B2) Each such region was analyzed by first assessing the spectral characteristics of a patch near the center of the region, representing an element of the potential target (white outline). If the spectral characteristics of this patch were relatively uniform (see Methods), it was accepted as a target element and the rest of the region was evaluated for spectrally similar areas (in this example, the five other squares); the set of such spectrally similar areas was then defined as the target. (B3) The remaining area of the scene (i.e., all of the region except the blacked-out target set) was defined as the context. (C1) Example of another region of the image in A, similarly analyzed, in which the target comprises a greater number of elements. (C2) As before, the target elements were identified by their spectral similarity to a central reference patch (white outline). (C3) The context defined by exclusion, as in B3.

Average chromaticity difference between target and context (|C_c-C_t|) as a function of the number of the iterated elements in the sampled targets (N_t).