A method and apparatus for processing original data defining the values of pixels of an original coloured image represented in a spatial domain by two or more colour separations each colour separation defining the respective colour component content of pixels arranged in a two dimensional array extending in X and Y directions. The apparatus comprises a store 6 for storing the original data; and a frequency transform circuit 8 for performing a three dimensional transformation on the store representation to generate transformed data representing the original image in three dimensional frequency space. Two of the three dimensions represent the variation in pixel value in the X direction and the Y direction respectively and the transformed data has a single zero order frequency term.
