[PATCH libinput 08/13] filter: change touchpad accel code to use mm/s

[Peter Hutterer]

That's something human brains can map to because mapping a touchpad to
equivalent units of a 1000dpi mouse requires a lot of mental acrobatics. And
I'm getting older and my physio told me acrobatics is more something for the
youngens, possibly those on my lawn listening to terrible music, etc.

The various numbers are converted either times 25.4/1000 or times 1000/25.4,
depending on the usage. Somewhere I made a mistake or a rounding error or
something, so the acceleration curve is not exactly the same, but it's close
enough that it shouldn't matter. The difference shows up in a gnuplot of the
curve but it may not even perceivable anyway. And these values will be
overhauled soon anyway, so meh.

Signed-off-by: Peter Hutterer <peter.hutterer at who-t.net>
---
 src/filter.c | 35 +++++++++++++++++++++--------------
 1 file changed, 21 insertions(+), 14 deletions(-)

diff --git a/src/filter.c b/src/filter.c
index abbfb48..f4baa99 100644
--- a/src/filter.c
+++ b/src/filter.c
@@ -49,6 +49,12 @@ v_us2ms(double units_per_us)
 	return units_per_us * 1000.0;
 }
 
+static inline double
+v_us2s(double units_per_us)
+{
+	return units_per_us * 1000000.0;
+}
+
 /* Convert speed/velocity from units/ms to units/us */
 static inline double
 v_ms2us(double units_per_ms)
@@ -129,10 +135,10 @@ filter_get_type(struct motion_filter *filter)
 #define DEFAULT_INCLINE 1.1			/* unitless factor */
 
 /* Touchpad acceleration */
-#define TOUCHPAD_DEFAULT_THRESHOLD v_ms2us(0.4)
-#define TOUCHPAD_MINIMUM_THRESHOLD v_ms2us(0.2)
+#define TOUCHPAD_DEFAULT_THRESHOLD 15.75	/* mm/s */
+#define TOUCHPAD_MINIMUM_THRESHOLD 7.87		/* mm/s */
 #define TOUCHPAD_ACCELERATION 2.0		/* unitless factor */
-#define TOUCHPAD_INCLINE 1.1			/* unitless factor */
+#define TOUCHPAD_INCLINE 0.02794		/* unitless factor */
 
 /* for the Lenovo x230 custom accel. do not touch */
 #define X230_THRESHOLD v_ms2us(0.4)		/* in units/us */
@@ -740,8 +746,8 @@ touchpad_accel_profile_linear(struct motion_filter *filter,
 	const double incline = accel_filter->incline;
 	double factor; /* unitless */
 
-	/* Normalize to 1000dpi, because the rest below relies on that */
-	speed_in = speed_in * DEFAULT_MOUSE_DPI/accel_filter->dpi;
+	/* Convert to mm/s because that's something one can understand */
+	speed_in = v_us2s(speed_in) * 25.4/accel_filter->dpi;
 
 	speed_in *= TP_MAGIC_SLOWDOWN;
 
@@ -766,21 +772,22 @@ touchpad_accel_profile_linear(struct motion_filter *filter,
 			 a is the incline of acceleration
 			 b is minimum acceleration factor
 
-	   for speeds up to 0.07 u/ms, we decelerate, down to 30% of input
-	   speed.
-		   hence 1 = a * 0.07 + 0.3
-		       0.3 = a * 0.07  => a := 10
+	   for speeds up to the lower threshold, we decelerate, down to 30%
+	   of input speed.
+		   hence 1 = a * 2.756 + 0.3
+		       0.7 = a * 2.756  => a := 0.254
 		   deceleration function is thus:
-			y = 10x + 0.3
+			y = 0.254x + 0.3
 
 	  Note:
-	  * 0.07u/ms as threshold is a result of trial-and-error and
+	  * The minimum threshold is a result of trial-and-error and
 	    has no other intrinsic meaning.
 	  * 0.3 is chosen simply because it is above the Nyquist frequency
 	    for subpixel motion within a pixel.
 	*/
-	if (v_us2ms(speed_in) < 0.07) {
-		factor = 10 * v_us2ms(speed_in) + 0.3;
+	if (speed_in < 2.756) {
+		const double incline = 0.254;
+		factor = incline * speed_in + 0.3;
 	/* up to the threshold, we keep factor 1, i.e. 1:1 movement */
 	} else if (speed_in < threshold) {
 		factor = 1;
@@ -795,7 +802,7 @@ touchpad_accel_profile_linear(struct motion_filter *filter,
 		hence 1 = ax' + 1
 			=> x' := (x - T)
 	 */
-		factor = incline * v_us2ms(speed_in - threshold) + 1;
+		factor = incline * (speed_in - threshold) + 1;
 	}
 
 	/* Cap at the maximum acceleration factor */
-- 
2.9.3