  Reliable estimation of illuminant chromaticity is crucial for simulating
color constancy and for white balancing digital images. However, estimating
illuminant chromaticity from a single image is an ill-posed task, in general,
and existing solutions typically employ a variety of assumptions and
heuristics. In this paper, we present a new, physically-based, approach for
estimating illuminant chromaticity from interreflections of light between
diffuse surfaces. Our approach assumes that all of the direct illumination in
the scene has the same chromaticity, and that at least two areas where
interreflections between Lambertian surfaces occur may be detected in the
image. No further assumptions or restrictions on the illuminant chromaticty or
the shading in the scene are necessary. Our approach is based on representing
interreflections as lines in a special 2D color space, and the chromaticity of
the illuminant is estimated from the approximate intersection between two or
more such lines. Experimental results are reported on a dataset of illumination
and surface reflectance spectra, as well as on real images we captured. The
results indicate that our approach can yield state-of-the-art results when the
interreflections are significant enough to be captured by the camera.
