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evdev: simplify hysteresis code and document it

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center + diff is the input coordinate. Simplify the code so it's clear what
we're returning. And document the function to explain what it does.

Signed-off-by: Peter Hutterer <peter.hutterer@who-t.net>
Reviewed-by: Hans de Goede <hdegoede@redhat.com>
This commit is contained in:
Peter Hutterer 2016-11-21 12:24:45 +10:00
parent c9d2d5e3de
commit 05faf7bf73
1 changed files with 35 additions and 3 deletions
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38
src/evdev.h
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@ -530,6 +530,38 @@ evdev_to_left_handed(struct evdev_device *device,
return button;
}
/**
* Apply a hysteresis filtering to the coordinate in, based on the current
* hystersis center and the margin. If 'in' is within 'margin' of center,
* return the center (and thus filter the motion). If 'in' is outside,
* return a point on the edge of the new margin. So for a point x in the
* space outside c + margin we return r:
* +---+ +---+
* | c | x → | r x
* +---+ +---+
*
* The effect of this is that initial small motions are filtered. Once we
* move into one direction we lag the real coordinates by 'margin' but any
* movement that continues into that direction will always be just outside
* margin - we get responsive movement. Once we move back into the other
* direction, the first movements are filtered again.
*
* Returning the edge rather than the point avoids cursor jumps, as the
* first reachable coordinate is the point next to the center (center + 1).
* Otherwise, the center has a dead zone of size margin around it and the
* first reachable point is the margin edge.
*
* Hysteresis is handled separately per axis (and the window is thus
* rectangular, not circular). It is unkown if that's an issue, but the
* calculation to do circular hysteresis are nontrivial, especially since
* many touchpads have uneven x/y resolutions.
*
* @param in The input coordinate
* @param center Current center of the hysteresis
* @param margin Hysteresis width (on each side)
*
* @return The new center of the hysteresis
*/
static inline int
evdev_hysteresis(int in, int center, int margin)
{
@ -537,10 +569,10 @@ evdev_hysteresis(int in, int center, int margin)
if (abs(diff) <= margin)
return center;
if (diff > margin)
return center + diff - margin;
if (diff > 0)
return in - margin;
else
return center + diff + margin;
return in + margin;
}
static inline struct libinput *