1. A method of adjusting the contrast of an input image formed of pixels in which each pixel has an input brightness level to produce an output image in which at least some of the pixels have an output brightness level that is different from their input brightness level, the method comprising: obtaining a blurred image corresponding to the input image, the brightness level of at least some of the pixels in the input image being varied to provide the blurred image; obtaining a ring likelihood for pixels in the input image, the ring likelihood providing a measure of the likelihood that said pixels are non-edge pixels in the neighbourhood of an edge pixel; producing the output image as a sum of the brightness levels in the input image and the blurred image in dependence on the ring likelihood, wherein the blurred image is obtained by filtering the brightness levels of the input image with at least one recursive spatially-adaptive edge-preserving filter, wherein the filter has recursion coefficients that are adaptive to edge information in the input image, wherein the ring likelihood for a pixel is a product of (i) a likelihood that said pixel is a non-edge pixel and (ii) a likelihood of a strongest edge in the neighbourhood of said pixel, wherein said likelihoods are obtained from said recursion coefficients.