Local images of photolithographic masks are assigned to classes based on similarity of functions of circuits formed by the images, so that all of the images of a class can be corrected by correcting one of the members. Boundaries of photolithographic masks are corrected for diffusion of light by moving regions based on process light intensity and proximity of close connections. Boundaries are also corrected for shifting of photoactive material in photoresists by calculating the amount of shift based on light intensities at pattern points.