Colour by Correlation: A Simple, Unifying Framework for Colour Constancy

This paper considers the problem of illuminant estimation: how, given an image of a scene, recorded under an unknown light, we can recover an estimate of that light. Obtaining such an estimate is a central part of solving the colour constancy problem that is of recovering an illuminant independent representation of the re ectances in a scene. Thus the work presented here will have applications in elds such as colour based object recognition and digital photography, where solving the colour constancy problem is important. The work in this paper di ers from much previous work in that, rather than attempting to recover a single estimate of the illuminant as many previous authors have done, we instead set out to recover a measure of the likelihood that each of a set of possible illuminants was the scene illuminant. We begin by determining which image colours can occur (and how these colours are distributed) under each of a set of possible lights. We discuss in the paper, how for a given camera, we can obtain this knowledge. We then correlate this information with the colours in a particular image to obtain a measure of the likelihood that each of the possible lights was the scene illuminant. Finally we use this likelihood information to choose a single light as an estimate of the scene illuminant. Computation is expressed and performed in a generic correlation framework which we develop in this paper. We propose a new probabilistic instantiation of this correlation framework and we show that it delivers very good colour constancy on both synthetic and real images. We show further that the proposed framework is rich enough to allow many existing algorithms to be expressed within it: the grey-world and gamut mapping algorithms are presented in this framework and we explore too the relationship of these algorithms to other probabilistic and neural network approaches to colour constancy.