An apparatus and method are provided to determine the occurrence and location of an interaction with an interface, particularly with an image of a user interface that may be projected or otherwise produced on a surface. The apparatus uses one or more single-element sensors (such as a photodiode) to sense and capture light readings of a scene, the readings corresponding to a plurality of structured light images injected within the presentation of the interface. The readings are compared to a baseline set of readings to determine the occurrence and location of an interaction event by an obstacle (i.e., a finger or a stylus) such as a touch event or a movement of the obstacle.