1. A method for inspecting a patterned surface having a pattern which corresponds substantially to a set of reference data, the method comprising: (a) generating a multilevel pixel image of at least one region of the pattern, the pattern being non-repetitive within said at least one region; (b) processing the reference data corresponding to the at least one region of the pattern to identify at least two groups of pixels which are expected to generate equivalent images; and (c) comparing the at least two groups of pixels from the multilevel pixel image.