The presently claimed invention relates generally to creative works and determining whether two or more creative works correspond. One claim recites a method including the acts of: (a) identifying plural local extrema within a two dimensional array of data signal that corresponds to a first creative work; (b) storing information relating to the plural local extrema identified in act (a) in a computer database for future reference; (c) thereafter, identifying plural local extrema within a two dimensional array of data signal that corresponds to a second creative work; and (d) determining correspondence between information relating to the plural local extrema identified in act (c) with the stored information, to thereby determine whether the first creative work and second creative work correspond. Of course, other claims and combinations are provided as well.