In many novel application scenarios such as smart rooms or sensing rooms visual sensors (such as cameras) need to know which visual actuators (such as displays) are visible to them. Often only parts of a display are visible from a camera. Therefore, a novel algorithm for precise visibility determination is presented. The algorithm makes the assumption that the displays are active, i.e., they can be controlled by the application. Under these conditions the algorithm determines precisely where which parts of a display are imaged by a camera.