1. A method of applying tone mapping to a block of a plurality of lines of pixel values, the method comprising, in each of a plurality of iterations: receiving a current line of pixel values of the block; retrieving stored information relating to pixel values of at least one previous line of the block which have been processed in at least one previous iteration; using the retrieved information to: (i) determine updated information which is updated in dependence on the pixel values of the current line, and (ii) determine one or more tone mapping relationships for use in mapping the pixel values of the current line to tone-mapped pixel values; mapping the pixel values of the current line to tone-mapped pixel values using the determined one or more tone mapping relationships; and storing the updated information for use in processing pixel values of a subsequent line of pixel values of the block in a subsequent iteration.