Partitions of object colour space under illuminant and background changes
Smithson H., Zaidi Q.
Object colours appear to change under different illuminants and against different backgrounds yet retain sufficient constancy to enable object identification. We tested whether changes in object colour appearance are confined within colour categories. Boundaries between categories form geodesics in colour space. We examined changes in the geometry of colour geodesics as a function of illuminant and background. In order to simulate objects we used the reflectance spectra of 240 natural materials that were chosen to span colour space. To construct variegated background patterns, we divided the materials into 6 spectrally distinct sets (red-blue, blue-green, green-yellow, yellow-red, balanced, and neutral) and rendered them as random ellipses. For illuminants we used the spectra of direct sunlight and zenith skylight. For each background-illuminant pair, observers categorized each of the 240 objects as either reddish, greenish or neither in one set of trials, and as either yellowish, bluish or neither in a second set. The objects categorized as "neither" were subjected to repeated forced choices between R/G or Y/B to precisely delineate category boundaries. The results showed that the materials falling on colour boundaries show substantial overlap across the two illuminants. However, some material appearances did cross colour categories with a change in illuminant, despite prolonged adaptation to a single illuminant. Materials falling on colour boundaries also overlapped across backgrounds, but many material appearances crossed colour categories with a change in background. There were substantial changes in the shapes and locations of geodesics in chromaticity space. These changes provide tests of colour appearance models under adaptation to illuminant-background pairs.