touchpad: normalize the touchpad resolution to 400dpi, not 10 units/mm

In an attempt to bring method into the madness, normalize the touchpad deltas
to those of a USB mouse with 400 dpi. This way the data we're dealing with in
the acceleration code is of a known quantity.

Signed-off-by: Peter Hutterer <peter.hutterer@who-t.net>
Reviewed-by: Hans de Goede <hdegoede@redhat.com>

3 files changed, 18 insertions, 8 deletions

diff --git a/src/evdev-mt-touchpad.c b/src/evdev-mt-touchpad.c
index 4f4633d7..fed39b4e 100644
--- a/src/evdev-mt-touchpad.c
+++ b/src/evdev-mt-touchpad.c
@@ -663,14 +663,17 @@ tp_init_accel(struct tp_dispatch *tp, double diagonal)
 
 	/*
 	 * Not all touchpads report the same amount of units/mm (resolution).
-	 * Normalize motion events to a resolution of 10 units/mm as base
-	 * (unaccelerated) speed. This also evens out any differences in x
-	 * and y resolution, so that a circle on the touchpad does not turn
-	 * into an elipse on the screen.
+	 * Normalize motion events to a resolution of 15.74 units/mm
+	 * (== 400 dpi) as base (unaccelerated) speed. This also evens out any
+	 * differences in x and y resolution, so that a circle on the
+	 * touchpad does not turn into an elipse on the screen.
+	 *
+	 * We pick 400dpi as thats one of the many default resolutions
+	 * for USB mice, so we end up with a similar base speed on the device.
 	 */
 	if (res_x > 1 && res_y > 1) {
-		tp->accel.x_scale_coeff = 10.0 / res_x;
-		tp->accel.y_scale_coeff = 10.0 / res_y;
+		tp->accel.x_scale_coeff = (400/25.4) / res_x;
+		tp->accel.y_scale_coeff = (400/25.4) / res_y;
 	} else {
 	/*
 	 * For touchpads where the driver does not provide resolution, fall
diff --git a/src/filter.c b/src/filter.c
index 5b0b8774..e1fb2a96 100644
--- a/src/filter.c
+++ b/src/filter.c
@@ -341,7 +341,14 @@ pointer_accel_profile_smooth_simple(struct motion_filter *filter,
 	if (accel < 1.0)
 		accel = 1.0;
 
-
+	/* We use units/ms as velocity but it has no real meaning unless all
+	   devices have the same resolution. For touchpads, we normalize to
+	   400dpi (15.75 units/mm), but the resolution on USB mice is all
+	   over the place. Though most mice these days have either 400
+	   dpi (15.75 units/mm), 800 dpi or 1000dpi, excluding gaming mice
+	   that can usually adjust it on the fly anyway and currently go up
+	   to 8200dpi.
+	  */
 	if (velocity < (threshold / 2.0))
 		return calc_penumbral_gradient(0.5 + velocity / threshold) * 2.0 - 1.0;
 
diff --git a/src/filter.h b/src/filter.h
index e670e1b2..c0b27d0d 100644
--- a/src/filter.h
+++ b/src/filter.h
@@ -26,7 +26,7 @@
 #include "config.h"
 
 struct motion_params {
-	double dx, dy;
+	double dx, dy; /* in units/ms @ 400dpi */
 };
 
 struct motion_filter;