1. A method comprising: receiving image data representing an image, the image data comprising a luminance component at a first spatial resolution and at least one chrominance component at a second, lower, spatial resolution; up-sampling the spatial resolution of the at least one chrominance component to form an up-sampled chrominance component; determining a value of a radius parameter for a guided image filter for filtering the image, wherein the value of the radius parameter is variable for different pixels of the image, or for different regions of pixels of the image, the radius parameter determining an extent of the guided image filter around each pixel; and filtering the up-sampled chrominance component using the guided image filter.