1. An article of manufacture that includes surface features that encode information according to a code, wherein: (a) each respective surface feature consists of a bump on, or recess in, a surface of the article of manufacture; (b) each respective surface feature by itself, regardless of any state of any other surface feature, encodes a sequence of multiple bits, which sequence is determined, at least in part, by a discrete elevation of the respective surface feature; and (c) at least three different discrete elevations of surface features are significant in the code, each of the three discrete elevations, respectively, occurring in one or more of the surface features on the surface.