The Structure Sensor is an attachment to an iPad, but also works with non-iPad tablets, iPhones, and can be modified to work with standalone applications. It contains two infrared LEDs, an infrared structured light projector, and an infrared camera.  The Structure projects a unique infrared pattern of dots out in front of it, and the infrared camera captures the projected pattern as it is reflected off of objects. Apparent variations in the position of the captured dots are interpreted by software to determine the relative position of a captured image.