Derivation of a color space for image color difference measurement

In digital image reproduction, it is often desira- ble to compute image difference of reproductions and the original images. The traditional CIE color difference for- mula, designed for simple color patches in controlled viewing conditions, is not adequate for computing image difference for spatially complex image stimuli. Zhang and Wandell (Proceedings of the SID Symposium, 1996; p 731-734) introduced the S-CIELAB model to account for complex color stimuli using spatial filtering as a prepro- cessing stage. Building on S-CIELAB, iCAM was designed to serve as both a color appearance model and also an image difference metric for complex color stimuli (IST p 33-38). These image difference models follow a similar image processing path to approximate the behavior of human observers. Generally, image pairs are first converted into device-independent coordinates such as CIE XYZ tristimu- lus values or approximate human cone responses (LMS), and then further transformed into opponent-color chan- nels approximating white-black, red-green, and yellow- blue color perceptions. Once in the opponent space, the images are filtered with approximations of human con- trast sensitivity functions (CSFs) to remove information that is invisible to the human visual system. The images are then transformed back to a color difference space such as CIELAB, and pixel-by-pixel color differences are calculated. The shape and effectiveness of the CSF spatial filters used in this type of modeling is highly dependent on the choice of opponent color space. For image differ- ence calculations, the ideal opponent color space would be both linear and orthogonal such that the linear filter- ing is correct and any spatial processing on one channel does not affect the others. This article presents a review of historical opponent color spaces and an experimental derivation of a new color space and corresponding spatial filters specifically designed for image color difference calculations. !2010 Wiley Periodicals, Inc. Col Res Appl, 35, 387-400, 2010; Published online 7 January 2010 in Wiley