1. A method comprising, in combination: (a) a first set of one or more light sources illuminating an external surface of an object with coherent light during a first time period; (b) a second set of one or more light sources illuminating the external surface with incoherent light during a second time period, such that the first time period and the second time period do not overlap; (c) one or more cameras capturing images of the external surface (i) during the first time period while the external surface is illuminated with the coherent light, and (ii) during the second time period while the external surface is illuminated with the incoherent light; and (d) one or more computers (i) taking, as input, data indicative of or derived from the images, (ii) determining a classification of the external surface, and (iii) based on the classification, outputting signals to control an input/output (I/O) device, such that what content is displayed by the I/O depends, a least in part, on the classification; wherein (1) features are embedded in a transparent layer, which layer is illuminated by the coherent light during the first time period and by the incoherent light during the second time period, (2) the features are invisible to an unaided human eye and have material properties that are different than the transparent layer, and (3) the content that is displayed by the I/O device depends on data encoded by the features.