An image is encoded to define one or more spatial regions that can be sensed by a suitably-equipped mobile device (e.g., a smart phone), but are imperceptible to humans. When such a mobile device senses one of these regions, it takes an action in response (e.g., rendering an associated tone, playing linked video, etc.). The regions may overlap in layered fashion. One form of encoding employs modification of the color content of the image at higher spatial frequencies, where human vision is not acute. In a particular embodiment, the encoding comprises altering a transform domain representation of the image by adding signal energy in a first chrominance channel, where the added signal energy falls primarily within a segmented arc region in a transform domain space.