Disclosed herein is a computer-implemented method of image simulation for a device manufacturing process, the method comprising: identifying regions of uniform optical properties from a portion or an entirety of a substrate or a patterning device, wherein optical properties are uniform within each of the regions; obtaining an image for each of the regions, wherein the image is one that would be formed from the substrate if the entirety of the substrate or the patterning device has the same uniform optical properties as that region; forming a stitched image by stitching the image for each of the regions according to locations of the regions in the portion or the entirety of the substrate of the patterning device; forming an adjusted image by applying adjustment to the stitched image for at least partially correcting for or at least partially imitating an effect of finite sizes of the regions.