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38 lines
2.3 KiB
38 lines
2.3 KiB
## Tap latency
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* [Video demonstration of tap latency measurement](https://www.youtube.com/watch?v=1xAReF75Cts&list=PLd6Fi7WgXfcCEJg1FDqNCoQfpWo7W3J5a&index=2)
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* [Detailed instructions on how to perform the measurement in usage doc](usage/WALT_usage.md#tap-latency).
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WALT uses a “stylus” equipped with an accelerometer. The finger is imitated by a flat metal
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tip that is grounded, pretty much any rigid tip can be used as long as it triggers the touch sensor.
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When the stylus “collides” with touch screen the accelerometer senses a shock (above 3g) which is
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timestamped by the Teensy. In order to generate a similar shock when picking the stylus up from the
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screen, the conductive surface and the accelerometer are mounted on a button of a retractable pen.
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On the way up, the spring keeps the button in contact with the touch screen for the first few mm of
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motion. This allows the hand holding the main part of the pen to gain some speed to which the button is
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then abruptly accelerated generating an easily detectable shock.
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Linux [Multi Touch (MT)](https://www.kernel.org/doc/Documentation/input/multi-touch-protocol.txt)
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implementation timestamps touch events in the kernel as they arrive from the hardware. On Android
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the MT events are then exposed in Java as
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[MotionEvent](http://developer.android.com/reference/android/view/MotionEvent.html)
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and include the kernel timestamp. For tap, the relevant MotionEvent types are
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ACTION_DOWN and ACTION_UP.
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Sample measurements
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| Device | OS version | ACTION_DOWN [ms]| ACTION_UP [ms]| Kernel to Java [ms] |
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| :--- | :--- | ---: | ---: | ---: |
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| Nexus 5 | M4B30Z (6.0.1) | 26.9 | 15.9 | 3.3 |
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| Nexus 5X | NRD91P (7.0) | 25.0 | 22.5 | 2.4 |
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| Nexus 7 | LMY47Q (5.1) | 29.6 | 31.0 | 1.4 |
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| Nexus 9 | MMB29K (6.0.1) | 18.7 | 19.9 | 1.3 |
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| Pixel C | NMF26H (7.1.1) | 23.8 | 24.3 | 1.1 |
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| Pixel | NHG47K (7.1.2) | 9.6 | 28.1 | 2.0 |
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| Pixel XL | NHG47K (7.1.2) | 14.2 | 14.4 | 1.8 |
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