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ROBUST ESTIMATON OF ILLUMINATION COLOUR FROM DICHROMATIC PATCHES

In this paper we present a new robust approach to the problem of illumination colour estimation. We assume the dichromatic reflection model, a linear camera and interface reflection of dielectrics to be an almost constant function in the visible spectrum. Under such assumptions, observed colours corresponding to a surface of a single dielectric material should lie in a plane passing through the origin of the measurement space and including the illumination colour. Since colour measurements can arbitrarily change due to geometry, apperture, sensor gain etc., the analysis is performed in the chromatic plane. The mean and the covariance matrix of chormaticities taken from a small patch obtained by region-growing from the image are computed. A line passing through the mean with the direction of the first eigenvector of the covariance matrix is taken as the local estimate of possible illumination chromaticity. The estimates are combined by a Hough Transform-like procedure that accumulates votes in a discretised chromatic space. The framework places no constraints on the illumination spectral power distribution, surface reflectances, sensors (e.g. that they occupy some low dimensional subspace) or geometry of the scene (e.g. Mondriaan world asumption). Moreover, colour of multiple light sources may be recovered from the structure of the accumulator space. Experiments on images from a publicly available database with known illuminants show that the method is accurate and outperforms simple standard methods.