Psychophysical determination of boundaries and smoothness of color gradients

Color gradients constitute an important component in the evaluation of the color quality of multicolored patterns that contain color transitions. A two-part psychophysical study was designed and employed to test the appearance of a set of hue-, chroma-, or lightness-based color gradients. The influence of several parameters on the visual determination of gradients' boundaries and perceived smoothness was tested. These parameters included gradient type, e.g., transitions based on hue, chroma, lightness or their combination, orientation, slope, and quantization step size of color transitions. The influence of these parameters on intra- and interobserver variability in responses was calculated using the standardized residual sum of squares metric. In the first part of the experiment, the perceived boundaries of color gradient stimuli as well as observer confidence ratings in performing these visual tasks were determined. In the second part, the perceived smoothness ratings of the stimuli and visual confidence ratings in assessments were examined. Four binary color transition images, i.e., brown-green, brown-tan, green-olive, and light sage-olive, were generated. Three different linear-gradient slopes were applied to each transition, and each stimulus was shown to observers, separately, in four orientations: horizontal, vertical, right diagonal, and left diagonal. Results indicate that the gradient slopes influence perceived boundaries and smoothness ratings. When determining smoothness ratings, observers were found to be more tolerant to changes in chroma and lightness than in hue.