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+For drag latency WALT uses a laser that shines across the touch device and hits a detector on the
+other side. The microcontroller monitors the state of the laser detector and reports (over usb) when
+the laser beam is broken and unbroken. A finger dragged back and forth on a touchpad or touch screen
+and interrupts a laser beam. Touch events from the pad and laser events are then processed together
+to deduce the delay.
+
+A video [here](https://plus.google.com/+FrancoisBeaufort/posts/XctAif2nv4U) shows the measurement
+performed using a robotic stylus for extra accuracy and consistency, but the measurement also works
+fine with just a finger or a hand held stylus.
+
+![drag latency measurement](WALT_drag_latency.jpg)

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