1. A method of distinguishing illumination boundaries from reflectance boundaries in an image of a scene, comprising: a computer executing the following steps: identifying a plurality of brightness boundaries in the image, each brightness boundary separating a high brightness side from a low brightness side, for each of three selected wavelength bands of the image and each of the brightness boundaries, determining image brightness on each side of the boundary, for each of said wavelength bands and each of the brightness boundaries, determining a brightness difference across the boundary, for each of said brightness boundaries, generating a three-component vector wherein each component is formed as a ratio of an absolute value of brightness difference across the boundary relative to brightness on the bright side of the boundary corresponding to one of the wavelength bands, normalizing said vectors, identifying a boundary as an illumination boundary if a normalized vector corresponding to said boundary forms a cluster with one or more vectors corresponding to other boundaries.