filter: require minimum acceleration factor of 0.3

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@who-t.net>
Reviewed-by: Hans de Goede <hdegoede@redhat.com>

diff --git a/src/filter.c b/src/filter.c
index 6d9b9c340013bf504f92dcd27ac2c4e2876bdb79..ee4ce9e6b655e61f25629602b73f9eea5d79df14 100644 (file)
--- a/src/filter.c
+++ b/src/filter.c
@@ -385,7 +385,7 @@ 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);