Spatial Colour Gamut Mapping by Orthogonal Projection of Gradients onto Constant Hue Lines

We present a computationally efficient, artifact-free, spatial gamut mapping algorithm. The proposed algorithm offers a compromise between the colorimetrically optimal gamut clipping and an ideal spatial gamut mapping. This is achieved by the iterative nature of the method: At iteration level zero, the result is identical to gamut clipping. The more we iterate the more we approach an optimal spatial gamut mapping result. Our results show that a low number of iterations, 20-30, is sufficient to produce an output that is as good or better than that achieved in previous, computationally more expensive, methods. More importantly, we introduce a new method to calculate the gradients of a vector valued image by means of a projection operator which guarantees that the hue of the gamut mapped colour vector is identical to the original. Furthermore, the algorithm results in no visible halos in the gamut mapped image a problem which is common in previous spatial methods. Finally, the proposed algorithm is fast- Computational complexity is O(N), N being the number of pixels. Results based on a challenging small destination gamut supports our claims that it is indeed efficient.