1. An image labeling apparatus comprising: an input arranged to receive a first image to be labeled; a coarsening engine arranged to repeatedly reduce the resolution of the original image according to a scaling factor to form an image pyramid; a processor arranged to form an energy function using the coarsest image in the pyramid, the energy function comprising at least a pairwise function and wherein the processor is arranged to adjust parameter values of the pairwise function on the basis of the scaling factor; an optimization engine arranged to optimize the energy function to obtain a labeling of the coarsest image; the processor being arranged to project the labeling onto the next-coarsest image in the pyramid to obtain a region of unlabelled image elements; the processor being arranged to form another energy function using the current image in the pyramid, the other energy function using the current image comprising at least another pairwise function, and wherein the processor is arranged to adjust parameter values of the other pairwise function on the basis of the scaling factor; the optimization engine being further arranged to optimize the another energy function only over the region of unlabelled image elements to obtain a revised labeling; and wherein the processor and optimization engine are further arranged to repeatedly: project the revised labeling, form another energy function and optimize that energy function; to use all the images in the pyramid and so produce a labeled version of the first image.