Defining Consistent Color Appearance for Print Images

摘要

Perceptually, consistency of color appearance (CCA) can be defined as the degree of visual consistency or shared visual appearance that a set of images possesses in the presence of visual differences. The CCA varies according to printing conditions and can be affected by paper substrates, inks, environments, printing devices and printing parameters, such as, color balance and color gamut. ISO 15339-2 [1] specifies the CRPC1-CRPC7 reference printing conditions for different CMYK devices to maintain (a) same hue angle of CMYRGB solids, (b) same high-light-to-midtone tonality, and (c) same neutral gray balance as defined by CGATS TR015 [2], and exhibit good CCA despite substantial differences in color gamut and substrate color. Although the assumption of consistency of color appearance among these reference printing conditions was not experimentally tested, it was based on expertise and printing practice accumulated in printing industry. Motivated by the approach used for the CRPCs specifications, a previous study [3] demonstrated that alterations of tonality and gray balance diminished the CCA for the set of images compared to the changes in color gamut only. In the present paper, we focused on further quantifying the CCA for print images by 1) selectively changing gamut volume, tonality and gray balance in a sequence of colorimetrically measurable steps and 2) conducting psychometric evaluations to derive perceptual CCA scale. The results show that the CCA of the image set with changes due to gamut variation only, appears to be higher compared to the CCA for the types of changes that involve tonality and gray balance. Device-based 95th percentile AEoo values for adjacent datasets were shown to correlate with the consistency of color appearance in the present experiment. We also observed a discrepancy between experts and novices when judging CCA. Additional experiments are needed to evaluate the effects of pictorial scene on CCA.