1. A method of processing a digitally coded image in which picture elements are each represented by a colour value, comprising, for each of a plurality of said picture elements: (a) performing a plurality of comparisons, each comparison comprising comparing a first picture element group which comprises the picture element under consideration and at least one further picture element in the vicinity thereof with a second picture element group which comprises a base picture element and at least one further picture element, the number of picture elements in the second group being the same as the number of picture elements in the first group and the position of the or each further element of the second group relative to the base picture element of the second group being the same as the position of the or a respective further element of the first group relative to the picture element under consideration, wherein each comparison determines whether the two groups match in the sense that they meet a criterion of similarity; and (b) when at least one comparison results in a match, computing a replacement colour value for the picture element under consideration, the replacement colour value being a function of the colour value for the base picture element of the or each second group for which a match was obtained.