Chroma Variations and Perceived Quality of Color Images of Natural Scenes

Transformations of natural images in the perulated with respect to luminance and chromaticity in order ceptually uniform CIELUV color space have been investito improve the display of color. Usually, some implicit gated with respect to perceptual image quality. To this assumptions are made to limit the almost infinite number end, digitized color images of four natural scenes were of possible manipulations. One of these assumptions is described on the basis of their color point distributions that only global changes need to be considered. This stems in the CIELUV color space. A new set of images was from the notion that images are coherent in the sense that created by varying the chroma value of each pixel while different parts of the same image share luminance and the lightness and hue angle were kept constant. The spectral properties due to the fact that they belong to the chroma was changed in two different ways: (1) through same object or are illuminated by the same light source. the addition or subtraction of the same amount of chroma This assumption underlies global reproduction techniques to or from the chroma value of each pixel; (2) through in which the same color operation is applied to all pixels, multiplication of the chroma value of each pixel by a irrespective of the content of the image. Another asconstant. In three experiments, subjects judged the persumption is that the optimum image equals the original ceptual quality, colorfulness, and naturalness of the im(real-life) scene. Hence, all efforts should be directed ages on a ten-point numerical category scale. The results towards finding ways of approaching this equality as indicate that colorfulness is the main perceptual attribute closely as possible. underlying image quality when chroma varies. ColorfulIn this article, a different constraint on color reproducness itself was found to depend on both the average tion is proposed, namely, the experienced naturalness of chroma and its variability. In general, the subjects prean image. This choice is motivated by the argument that ferred slightly more colorful images to the original ones. constraints should be formulated not in the physical, but The perceptual quality of the images was found to be in the psychological domain, the reason being that image closely related to the naturalness of the images. q 1997 quality judgments are based on a comparison between the John Wiley & Sons, Inc. Col Res Appl, 22, 96–110, 1997 output of the visual process and some internal reference