|The simplest optical touch screen configuration consists of two sensor modules located in adjacent corners at the top of the panel and illumination modules along the other three sides. This system can clearly identify and locate any single touch anywhere in the sensing region. This system can also do a very good job of recognizing simple two touch gestures such as zoom in / zoom out and rotate commands.
As seen in the figure, although there is a theoretical ambiguity when two objects are placed on the screen simultaneously, the relative motion of the detected positions can be interpreted unambiguously. For example, as the physical objects are separated all four potential object points will move apart from each other. Similarly, for zoom in and rotate motions the analysis is straight forward.
It is also possible in many cased to absolutely track the two individual points by relying on history of the motion (speed, direction) or the sizes of the objects particularly as the sensing is quick enough so that the two events were introduced during the same frame. Nevertheless, such tracking becomes confused when objects cross each other and can be a source of much angst to application developers. Many touch screen suppliers refer to this type of tracking incorrectly as “Dual Touch” while the correct description should be “1.7 touch” or “gesture support”.