de H Huib Ridder

Color reproduction and the naturalness constraint

This article aims to determine the process underlying the subjective impression about the fidelity of reproduced object colors. To this end, we present the concept of the naturalness constraint and a framework for specification of naturalness judgments. We consider several research questions that are essential for this framework and discuss plausible answers supported by experiments. In general, naturalness assessment of reproduced object colors can be (1) defined as similarity to prototypical object colors, and (2) characterized by a probability density function (e.g., Gaussian). Experiments show that (3) there is a considerable amount of consistency in naturalness judgments of locally and globally processed images (although observers are slightly more tolerant of global image processing), and (4) naturalness judgments vary for different object categories; e.g., subjects are more consistent in naturalness judgments of skin, grass, and sky reproductions than shirt reproduction. We suggest that (5) naturalness of a whole picture is determined by the naturalness of the most critical object in that picture. Finally, we introduce a naturalness index predicting perceived naturalness of color reproduction of real-life scenes.

Colourfulness judgments of natural scenes

Abstract The purpose of this study was to examine the relationship between colourfulness judgments of images of natural scenes and statistical parameters of the chroma distribution over the images in the CIE 1976 L∗u∗v∗ (CIELUV) colour space. A comparative analysis of within-scenes and between-scenes colourfulness assessments was performed. Images were created by varying chroma in the CIELUV colour space while lightness and hue-angle were kept constant. Experiment 1 shows the results of a multidimensional analysis of the differences-scaling in perceived colourfulness. Experiment 2 includes a unidimensional investigation of the direct-scaling of the magnitude of perceived colourfulness. Both experiments confirm that colourfulness judgments highly correlate with a linear combination of the mean of the distribution of chroma values and its standard deviation. That was also verified in Experiment 3 with “Mondrian”-like stimuli (spatially scrambled versions of the images of natural scenes). Colourfulness judgments of natural scenes, but not “Mondrian”-like scenes, brought out systematic differences between observers. In order to interpret and quantify these differences, a model of individual strategies in colourfulness judgments is developed and discussed.