In one embodiment, a sidewalk-facing display window of a retail store is treated to scatter (or absorb) a portion of incident light, in a narrow wavelength spectral band. A machine-readable pattern, which encodes an identifier, is projected onto the treated window from outside the store. The reflection (absorption) of that projected pattern, within the narrow spectral band, escapes notice of shoppers on the sidewalk. Yet if a shopper captures imagery of a product displayed on the other side of the display window, using a mobile device camera, the pattern is captured with the imagery, and can be analyzed to decode the identifier. The mobile device can use this decoded identifier to access online information about the displayed product, for presentation to the shopper on the mobile device display. The technology is particularly suited for wearable computing devices, and more generally enables glass windows to subliminally convey digital information to image sensors conveyed by passing shoppers. A great variety of other features and arrangements are also detailed.