1. A layered representation of an image, comprising: a main color layer comprising a pixel value for each pixel location of the image, wherein each main color layer pixel value specifies a pixel color deemed to be a majority contributor to the associated pixel location; a secondary color layer comprising a pixel value for each pixel location of the image, wherein each secondary color layer pixel value specifies a pixel color deemed to be a minority contributor to the associated pixel location; an alpha layer comprising a pixel value for each pixel location of the image, wherein each alpha layer pixel value specifies a proportion of the contribution of the majority contributor pixel color to the associated pixel location; and a noise layer comprising, for each pixel location of the image, a pixel value which specifies the difference in color between a pixel color defined by the majority contributor pixel color, minority contributor pixel color and proportion of the contribution of the majority contributor pixel color for the pixel location under consideration, and an observed color of that pixel location in the image.