Selection for gamut mapping colour constancy

The requirement that the sensor responses of a camera to a,given surface re flectance be constant under changing illumination conditions has led to the development of the so called colour constancy algorithms. Given an image r ecorded under an unknown illuminant, the task for a colour constancy algori thm is to recover an estimate of the scene illuminant. One such algorithm d eveloped by D.A. Forsyth, A novel algorithm for colour constancy, Internati onal Journal of Computer Vision 5 (1) (1990) 5-36 [1] and later extended by G.D. Finlayson, Color in perspective, IEEE Transactions on Pattern Analysi s and Machine Intelligence 18(10) (1996) 1034-1038 [2] exploits the constra int that under a canonical illuminant all surface colours fall within a max imal convex set-the canonical gamut. Given a set of image colours Forsyth's algorithm recovers the set of mappings which cake these colours into the c anonical gamut. This feasible set of mappings represents all illuminants, w hich are consistent with the recorded image colours. In this article we add ress the question of how best to select a single mapping from this feasible set as an estimate of the unknown illuminant. We develop our approach in the context of Finlayson's colour-in-perspective algorithm. This algorithm performs a perspective transform on the sensor d ata to discard intensity information which, without unrealistic constraints (uniform illumination and no specularities) being placed on the world, can not be recovered accurately. Unfortunately, the feasible set of mappings re covered by this algorithm is also perspectively distorted. Here, we argue t hat this distortion must be removed prior to carving out map selection and show that this is easily achieved by inverting the perspective transform. A mean-selection criterion operating on non-perspective mapping space provid es good colour constancy for a variety of synthetic and real images. Import antly, constancy performance surpasses all other existing methods. (C) 1999 Elsevier Science B.V. All rights reserved.