Apparatus, method and computer program product for reducing overlay errors during a semiconductor photolithographic mask design process flow. The method obtains data representing density characteristics of a photo mask layout design; predicts stress induced displacements based on said obtained density characteristics data; and corrects the mask layout design data by specifying shift movement of individual photo mask design shapes to pre-compensate for predicted displacements. To obtain data representing density characteristics, the method merges pieces of data that are combined to make a photo mask to obtain a full reticle field data set. The merge includes a merge of data representing density characteristic driven stress effects. The density characteristics data for the merged reticle data are then computed. To predict stress-induced displacements, the method inputs said density characteristics data into a programmed model that predicts displacements as a function of density, and outputs the predicted shift data.