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+WALT uses a “stylus” equipped with an accelerometer. The finger is imitated by a flat circular metal
+tip that is grounded, pretty much any rigid tip can be used as long as it triggers the touch sensor.
+When the stylus “collides” with touch screen the accelerometer senses a shock (above 3g) which is
+timestamped by the Teensy. In order to generate a similar shock when picking the stylus up from the
+screen, the conductive surface and the accelerometer are mounted on a button of a retractable pen.
+On the way up, the spring keeps the button in contact with the touch screen for the first few mm of
+motion. This allows the hand holding the main part of the pen gain some speed to which the button is
+then abruptly accelerated generating an easily detectable shock.
+
+Linux [Multi Touch (MT)](https://www.kernel.org/doc/Documentation/input/multi-touch-protocol.txt)
+implementation timestamps touch events in the kernel as they arrive from the hardware. On Android
+the MT events are then exposed in Java as
+[MotionEvent](http://developer.android.com/reference/android/view/MotionEvent.html) 
+and include the kernel timestamp. For tap, the relevant MotionEvent types are
+ACTION_DOWN and ACTION_UP.
+

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