[PATCH libinput] filter: require minimum acceleration factor of 0.3

[Peter Hutterer]

For really slow motions, the previous acceleration factor would go down to
effectively zero. So the slower the mouse motion was, the more it would be
slowed down which made the mouse at low speeds almost unusable.

Cap the minimum acceleration at 0.3 which provides a predictable slow motion
for the cursor when high precision is required.

New/old acceleration functions comparison:

  ^
  |               /
  |              /
ty|    _________/
  |   / /
  |  / /
  | / /
  |/ /    <----- new minimum accel factor
  | /
  |/___________________>
      tx

i.e. the general shape is maintained, but it doesn't go to zero anymore. The
functions aren't parallel, the new shape is slightly flatter than the previous
one and they meet at the point where the functions flatten for the threshold
(tx/ty). ascii art has its limits...

https://bugzilla.redhat.com/show_bug.cgi?id=1227039

Signed-off-by: Peter Hutterer <peter.hutterer at who-t.net>
---
 src/filter.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/src/filter.c b/src/filter.c
index c54d866..ed5a184 100644
--- a/src/filter.c
+++ b/src/filter.c
@@ -354,7 +354,7 @@ pointer_accel_profile_linear(struct motion_filter *filter,
 	const double threshold = accel_filter->threshold; /* units/ms */
 	const double incline = accel_filter->incline;
 
-	s1 = min(1, speed_in * 5);
+	s1 = min(1, 0.3 + speed_in * 4);
 	s2 = 1 + (speed_in - threshold) * incline;
 
 	return min(max_accel, s2 > 1 ? s2 : s1);
-- 
2.4.1