A method for configuring an illumination source of a lithographic apparatus is presented. The method includes dividing the illumination source into pixel groups, each pixel group including one or more illumination source points; selecting an illumination shape to expose a pattern, the illumination shape formed with at least one pixel group; iteratively calculating a lithographic metric as a result of a change of state of a pixel group in the illumination source, the change of the state of the pixel group creating a modified illumination shape; and adjusting the illumination shape based on the iterative results of calculations.