Preserving perceptual distances in chromaticity diagrams

Chromaticity diagrams—Cartesian plots that depict the multidimensional subjective relationship between perceived colors, independent of intensity—are widely used in studies of animal color vision, as they allow direct links to be made between color discrimination and color vision–mediated behaviors. However, because these diagrams are invariant to the differences in noise levels between photoreceptor classes that underlie some models of color discrimination thresholds, the distance between colors in chromaticity space is not necessarily related to their perceptual distance. The geometric interpretation of color data from chromaticity diagrams may therefore lead to misleading conclusions. This paper describes a method of plotting visual perceptual distances derived from the noise-corrected model of Vorobyev and Osorio (Vorobyev M, Osorio D. 1998. Receptor noise as a determinant of color thresholds. Proc R Soc B. 265:351–358) such that the perceptual distance between them is exactly preserved. It also describes how both chromatic (color) and achromatic (brightness) contrasts can be visualized within the same diagram. The potential benefits of this approach over commonly used triangular and tetrahedral chromaticity diagrams are discussed with regards to the study of animal visual ecology.