Modelling colour appearance

Stephen Westland continues the topic of human perception of colour in Chapter 4. He describes the retinal image and the effects of the surroundings on the perception and appearance of colour, especially the phenomena of colour constancy and colour contrast. The importance of white as a reference point in the field of human vision and its role in adaptation is presented. The development of colour appearance models is outhned and the relationship of these uniform colour space models to their use in the formation of digital image... 

Introduction colour specification systems and colour appearance models... 

The original CIE system of colorimetry was designed for colour specification and has only limited use as a colour-appearance model. It can be easily shown that the CIE tristimulus values that can be used effectively to specify a surface colour do not correlate well with the appearance of that surface. This chapter introduces the distinction between colour specification and colour appearance and then reviews how simple adaptive processes that pool information at different spatial positions in an image can be used as a basis for colour-appearance models. A typical colour-appearance model, CIECAM97S, is then described. Finally, some further uses of colour-appearance models are discussed. 

LUO MR and hunt rwg (1998b) Testing colour appearance models using corresponding colour and magnitude estimation data sets, Col. Res, Appl, 23, 147-153. 

The use of colour-measuring instruments allows specification and communication of colour by means of international standard terms such as CIE L a b C h°. Colour difference values can also be determined instrumentally and, if optimized colour difference equations are used, can out-perform a single trained colourist. However, the visual appreciation of colour and colour difference is still a subjective response, affected by many factors. Care must be taken to include measurement conditions in the interpretation of colour measurement data. The gold standard answer must always be that which agrees with the majority of a group of human observers. Whether this can be achieved by optimized equations based on CIE L a b colour space or by colour appearance models remains to be established. 

HUNT, R. w. G. (1987) A model of colour vision for predicting colour appearance in varions viewing conditions. Color Research and Application, 6, 297-314. 

The colonr sensitivity of the cone classes, the physical spacing of the cones within the retina, and the effects of chromatic aberration at the lens all contribute to what we call the spatiochromatic properties of the visual system. Whereas the CIE colour-matching functions describe the colour properties of the visual system reasonably adequately, if we wish to be able to analyse the colour-appearance properties of colour images or scenes then we need models that can incorporate the spatiochromatic properties of the visual system. In the next section some colour-appearance phenomena are described that demonstrate the intrinsic link between colour and spatial properties. 

It has been shown that colour appearance is a complex phenomenon and a deep understanding of the properties and function of the human visual system is required in order to understand it fully. A model of the human visual system is probably many decades from fruition, if indeed it is possible at all. Despite this, some of the properties of the visual system that have been discussed have been understood and implemented in models that can predict the quality of images, the visual difference between two images, and colour appearance of simple and complex images. An exhaustive hst of these models is not presented here. 

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