An optical trackpad is an input device based on an optical sensor, which detects the displacement of a finger that is moving on top of it. The sensor is used typically in smartphones, where it replaces the D-pad, and in ultra-portable or ultra-mobile PCs, where it replaces touchpads, pointing sticks or trackballs as pointing device. The main advantages over a D-pad are: * It can track movements in 360 degrees and with varying speeds. * It uses space efficiently, without the need for small buttons that are difficult to press.