Activating a plurality of light emitters and light detectors around a screen, each emitter-detector pair corresponding to a light path crossing the screen, wherein some of the light paths are blocked when one or more objects touch the screen, providing a look-up table, listing, for each cell from a plurality of cells, those light paths that traverse that cell when no object is touching the screen, wherein the cells partition the screen, for each cell: accessing the look-up table to identify those light paths that traverse that cell, determining whether the thus-identified light paths are blocked during the activating and, if affirmative, recognizing that cell as being a touched cell, and combining adjacent touched cells into a common touch location, thereby calculating one or more touch locations wherein each touch location is a combination of one or more constituent touched cells.