1. A method, comprising: providing a semiconductor structure including: a plurality of line substructures disposed in a region of the structure; and a plurality of types of pre-layers disposed over sub regions of the line substructures; scanning a line substructure to obtain data representing a raw line image; sectioning the data according to the sub regions; combining the sectioned data to create combined line images according to pre-layer types; and computing a measure of line roughness according to the combined line images.