[PATCH libinput] filter: tweak touchpad acceleration code again

[Peter Hutterer]

Reduce the magic factor a bit, thus making the pointer more (slow == precise)
on 'normal movements'. This coincidentally fixes some of the issues with the
Lenovo T450 and friends because the user's base speed must go up, thus
papering over the firmware bug that makes the pointer jump at slow movements.

Reduce the threshold speed, so acceleration kicks in a little bit earlier.
Reduce the threshold range, so at higher speeds accel kicks in earlier but not
too early.

Flatten the incline, so acceleration applies at a slower rate. This appears to
fix many of the 'overshoot' problems seen with the current code.

Rework how the accel speed affects the curve and increase the range of the
accel speed. This now covers a fairly large range from "tarpit" to "tux
racer". At all speeds bar the super-slow ones it's still relatively easy to
cross the screen with few motions.

The patch also works the touchpad magic slowdown into the factor applied.
Since this is a static number now, calculate it once at set-speed and then
store the result in the filter.

Signed-off-by: Peter Hutterer <peter.hutterer at who-t.net>
---
This is the first time I'm quite happy with the touchpad acceleration.

 src/filter.c | 37 ++++++++++++++++++++++++++-----------
 1 file changed, 26 insertions(+), 11 deletions(-)

diff --git a/src/filter.c b/src/filter.c
index 7c500f8..4bc6239 100644
--- a/src/filter.c
+++ b/src/filter.c
@@ -40,7 +40,7 @@
  * technically correct but subjectively wrong, we expect a touchpad to be a
  * lot slower than a mouse. Apply a magic factor to slow down all movements
  */
-#define TP_MAGIC_SLOWDOWN 0.37 /* unitless factor */
+#define TP_MAGIC_SLOWDOWN 0.29 /* unitless factor */
 
 /* Convert speed/velocity from units/us to units/ms */
 static inline double
@@ -135,10 +135,10 @@ filter_get_type(struct motion_filter *filter)
 #define DEFAULT_INCLINE 1.1			/* unitless factor */
 
 /* Touchpad acceleration */
-#define TOUCHPAD_DEFAULT_THRESHOLD 254		/* mm/s */
-#define TOUCHPAD_THRESHOLD_RANGE 184		/* mm/s */
-#define TOUCHPAD_ACCELERATION 9.0		/* unitless factor */
-#define TOUCHPAD_INCLINE 0.011			/* unitless factor */
+#define TOUCHPAD_DEFAULT_THRESHOLD 225		/* mm/s */
+#define TOUCHPAD_THRESHOLD_RANGE 25		/* mm/s */
+#define TOUCHPAD_ACCELERATION 3.0		/* unitless factor */
+#define TOUCHPAD_INCLINE 0.008			/* unitless factor */
 
 /* for the Lenovo x230 custom accel. do not touch */
 #define X230_THRESHOLD v_ms2us(0.4)		/* in units/us */
@@ -175,6 +175,7 @@ struct pointer_accelerator {
 	double threshold;	/* units/us */
 	double accel;		/* unitless factor */
 	double incline;		/* incline of the function */
+	double magic;		/* magic scaling factor */
 
 	int dpi;
 };
@@ -583,9 +584,16 @@ touchpad_accelerator_set_speed(struct motion_filter *filter,
 
 	/* adjust when accel kicks in */
 	accel_filter->threshold = TOUCHPAD_DEFAULT_THRESHOLD -
-		TOUCHPAD_THRESHOLD_RANGE * speed_adjustment;
+		TOUCHPAD_THRESHOLD_RANGE * (1 + speed_adjustment);
 	accel_filter->accel = TOUCHPAD_ACCELERATION;
 	accel_filter->incline = TOUCHPAD_INCLINE;
+
+	/* magic is the factor we scale everything by. For unaccelerated
+	 * motion, this is the baseline factor, for accelerated this scales
+	 * down the gain we otherwise calculate. 0.28 is a result of
+	 * trial&error, don't attach any meaning to it. */
+	accel_filter->magic = TP_MAGIC_SLOWDOWN + 0.28 * speed_adjustment;
+
 	filter->speed_adjustment = speed_adjustment;
 
 	return true;
@@ -810,7 +818,7 @@ touchpad_accel_profile_linear(struct motion_filter *filter,
 
 	  accel
 	 factor
-	   ^
+	   ^         ____
 	   |        /
 	   |  _____/
 	   | /
@@ -823,6 +831,9 @@ touchpad_accel_profile_linear(struct motion_filter *filter,
 		         x is speed_in
 			 a is the incline of acceleration
 			 b is minimum acceleration factor
+	   The flat bit is the unaccelerated 'baseline speed' where we
+	   merely slow down the input speed to provide reasonable pointer
+	   speed.
 
 	   for speeds up to the lower threshold, we decelerate, down to 30%
 	   of input speed.
@@ -836,7 +847,13 @@ touchpad_accel_profile_linear(struct motion_filter *filter,
 	    has no other intrinsic meaning.
 	  * 0.3 is chosen simply because it is above the Nyquist frequency
 	    for subpixel motion within a pixel.
-	*/
+
+	  The acceleration setting [-1.0, 1.0] affects:
+	   * the baseline speed (higher setting -> faster baseline)
+	   * the incline's angle (higher setting -> steeper incline)
+	   * the threshold when accel kicks in (higher setting -> sooner)
+	 */
+
 	if (speed_in < 7.0) {
 		factor = 0.1 * speed_in + 0.3;
 	/* up to the threshold, we keep factor 1, i.e. 1:1 movement */
@@ -860,9 +877,7 @@ touchpad_accel_profile_linear(struct motion_filter *filter,
 	factor = min(max_accel, factor);
 
 	/* Scale everything depending on the acceleration set */
-	factor *= 1 + 0.5 * filter->speed_adjustment;
-
-	return factor * TP_MAGIC_SLOWDOWN;
+	return factor * accel_filter->magic;
 }
 
 double
-- 
2.9.3