<NME> ScreenLatency.md
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<MSG> Add README about screen latency

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+## Screen latency
+
+WALT can detect when screen changes color between white and black using a photodiode.
+
+TODO: photo of WALT's bottom side
+
+Because the screen refreshes at predefined intervals (typically 60 Hz), most drawing commands do not take effect immediately but are processes during the next frame render event. See [Choreographer.FrameCallback](https://developer.android.com/reference/android/view/Choreographer.FrameCallback.html). An in-depth explanation about Android graphics can be found in [this video](https://www.youtube.com/watch?v=Q8m9sHdyXnE).
+
+WALT Android app uses View.setBackgroundColor to change the color and reports the time from the beginning of the next frame rendering until the screen changes physically.
+
+The physical transition between colors is not instantaneous and may look very different depending on the direction - from black to white or white to black. Moreover some sections of the screen may update several milliseconds later than others. These details vary considerably between screen technologies and  device models.
+
+TODO: Example plot of brightness rise and fall
+
+Sample measurements:
+
+| Device       | OS version     | Blk->white [ms]| White->blk [ms]| Average [ms] |
+| :---         | :---           |           ---: |           ---: |         ---: |
+| Nexus 5      | M4B30Z (6.0.1) |           53.5 |           62.6 |         58.1 |
+| Nexus 5X     | NRD91P (7.0)   |           60.7 |           75.4 |         68.1 |
+| Nexus 7      | LMY47Q (5.1)   |           47.3 |           50.2 |         48.8 |
+| Nexus 9      | MMB29K (6.0.1) |           49.0 |           53.8 |         51.4 |
+
+All measurements were taken with the device set to max brightness and WALT positioned in the middle of the screen.
+
+Sample output on Nexus 7:
+```
+-------------------------------
+Median screen response latencies (N=100):
+Black to white: 47.3 ms (N=50)
+White to black: 50.2 ms (N=50)
+Average: 48.8 ms
+-------------------------------
+```

Add README about screen latency