Abstract: A method and apparatus for modifying an image represented by digital data defining the content of one or more pluralities of cells constituting one or more color separations is described. The digital data indicates for each cell whether the respective color is "present" or "absent". The apparatus comprises scanning means (15,16) for electronically scanning along a plurality of scan lines with a mask of preselected dimensions; the respective plurality of digital data corresponding to a region including a boundary for one or more of the color separations. Programmable logic determines at each position of the mask whether the portion of the color separation within the mask includes at least one cell the content of which indicates color "present"; and modifies the digital data at another, predetermined position within the mask to change the content of the respective cell or cells of the color separation to represent color "present" or color "absent" when color "present" is detected within the mask.