Naoya Katoh

Three-dimensional gamut mapping using various color difference formulae and color spaces

Gamut mapping is a technique to transform out-of-gamut colors to the inside of the output device’s gamut. It is essential to develop effective mapping algorithms to realize ‘‘WYSIWYG’’ color reproduction. In this paper, three-dimensional gamut mapping using various color difference formulae and color spaces are considered. Visual experiments were performed to evaluate which combination of color difference formula and color space for gamut mapping was most preferred for five images. The color difference formulae used in the experiments were ?Eab * , ?Euv * , ?E94 , ?ECMC , ?EBFD , and ?Ewt . The color spaces used in the experiments were CIELAB, CIELUV, CIECAM97s, IPT, and NC-IIIC. A clipping method was used that maps all out-of-gamut colors to the surface of the gamut, and no change was made to colors inside the gamut. It was found that gamut mapping using ?E94 , ?ECMC , and ?Ewt were effective in CIELAB color space. For mapping images containing a large portion of blue colors, DEBFD and ?Euv * were found to be more effective. ?Eab * was least preferred for all images. With respect to color spaces, gamut mapping performed in the CIELUV color space was superior to any other color spaces for the blue region. We conclude that ?E94 in CIELUV and ?EBFD in CIELAB are the two most useful combinations of color difference formula and color space for gamut mapping, if we are to apply a single combination universally.

Effect of ambient light on the color appearance of softcopy images: Mixed chromatic adaptation for self-luminous displays

With the widespread use of color management systems (CMSs), users are now able to achieve device independent color across different media. However, most of the current CMSs guaran- tee the same color only if one sees color under a controlled viewing condition. If one sees color under a different viewing condition, the reproduced color does not match the original. The effect of ambient light on the appearance of the color of softcopy images is discussed in this article. In a typical office environment, a computer graphic monitor with a correlated color temperature (CCT) of 9300 K is widely used under an F6 fluorescent light of 4150 K CCT. In such a case, the human visual system is partially adapted to the CRT moni- tors white point and partially to the ambient light. A new adaptation model, S-LMS, is proposed to compensate for the mixed chromatic adaptation. Visual experiments were performed to evaluate the mixed chromatic adaptation. Experimental results indicated that hu- man visual system is 60% adapted to the monitors white point and 40% to ambient light when viewing softcopy images.

19.2: xvYCC: A New Standard for Video Systems using Extended-Gamut YCC Color Space

We have developed a new display / video camera and wide-gamut characterizing tools for a new standard for extended-gamut YCC color space called xvYCC. Video systems adapting this standard will have improved accuracy of color reproduction of real world.

Clarification of “Gamma” and the Accurate Characterization of CRT Monitors

Color images are often viewed on CRT monitors with the widespread PCs and Internet. We clarified the term “gamma1)” which represents the monitors tone characteristics, since this term is ambiguously used. Also, an accurate characterization method of CRT monitors was considered because it is essential to have color matching with other color imaging devices. We compared several characterization methods at different settings of the user-controlled contrast and brightness, and found a more accurate characterization method than the conventional methods.

Effect of user controls on CRT monitor characteristics

CRT monitors are widely used for desktop publishing (DTP) and to view images on the Internet. The color images on the computer graphic display can be printed out or displayed on other monitors through the Internet. Here, color matching between the original image on the monitor and the printed image or the image displayed on other monitors is very important. The current color management systems (CMSs) are useful for the color matching. These CMSs utilize device profiles, such as ICC profiles, in which color characteristic information is stored. These profiles are generated by device characterization. Thus, an accurate characterization of the monitor is essential for better color matching. According to the ICC specification, monitor characteristics can be described by the chromaticity and the tone reproduction curves (TRCs) of the red, green and blue channels. Although the monitor profiles on the current CMSs are based on the assumption that the monitors are maintained at the default adjustment and are viewed in a dark room, in fact the user often adjusts the settings of contrast/brightness and usually views the monitor under ambient light. In this paper, we investigated the effect of user controls on CRT monitor characteristics. We reconsidered the two characteristics of the CRT monitor: the effect of user controls on the TRCs and interaction among the channels. We compared several models of the TRC and various matrices to transform linearized RGB to CIE 1931 XYZ with different settings of the user controls. Based on these experimental results, we propose a method for more accurate monitor characterization.

Hierarchical approach to the optimal design of camera spectral sensitivities for colorimetric and spectral performance

The optimal design of spectral sensitivity functions for digital color imaging devices has been studied extensively. This paper analyzed the important requirements for designing sensor sensitivity functions. A hierarchical approach is proposed to the optimal design of camera spectral sensitivity functions by incorporating spectral fitting, colorimetric performance and noise. The approach is directly based on the filter fabrication parameters to avoid approximation deviation. A six-channel camera is designed via this approach, with the first three channels aiming at colorimetric performance and the total six channels for spectral performance.