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evdev: document the change-of-directions issue with the hysteresis

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Signed-off-by: Peter Hutterer <peter.hutterer@who-t.net>
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Peter Hutterer 2017-10-26 13:13:14 +10:00
parent 021865232d
commit e66cf8def1
1 changed files with 13 additions and 0 deletions
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src/evdev.h
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@ -610,6 +610,19 @@ evdev_to_left_handed(struct evdev_device *device,
* calculation to do circular hysteresis are nontrivial, especially since
* many touchpads have uneven x/y resolutions.
*
* Given coordinates, 0, 1, 2, ... this is what we return for a margin of 3
* and a center of 0:
*
* Input: 1 2 3 4 5 6 5 4 3 2 1 0 -1
* Coord: 0 0 0 1 2 3 3 3 3 3 3 3 2
* Center: 0 0 0 1 2 3 3 3 3 3 3 3 2
*
* Problem: viewed from a stationary finger that starts moving, the
* hysteresis margin is M in both directions. Once we start moving
* continuously though, the margin is 0 in the movement direction and 2*M to
* change direction. That makes the finger less responsive to directional
* changes than to the original movement.
*
* @param in The input coordinate
* @param center Current center of the hysteresis
* @param margin Hysteresis width (on each side)