Using the built-in camera of a mobile phone and inexpensive printed tags, users can leave and retrieve video messages in specific locations or associated to specific objects. At the heart of the project is d-touch: an open-source system for computer vision recognition of markers. D-touch's peculiarity, compared to other marker systems, is that its recognition mechanism is based on topology rather than geometry. As a consequence, the system allows considerable freedom in the visual design of the markers, so that they can be "hidden" or designed to fit the specific aesthetic requirements of different applications.