The present invention refers to a method and an apparatus for determining positions of a plurality of pixels to be introduced into a substrate of a photolithographic mask by use of a laser system, wherein the pixels serve to at least partly correct one or more errors of the photolithographic mask. The method comprises the steps: (a) obtaining error data associated with the one or more errors; (b) obtaining first parameters of an illumination system, the first parameters determining an illumination of the photolithographic mask of the illumination system when processing a wafer by illuminating with the illumination system using the photolithographic mask; and (c) determining the positions of the plurality of pixels based on the error data and the first parameters.