Do not start scroll gesture if fingers are moving opposite directions

The driver sometimes has difficulty distinguishing scroll and zoom
gestures. This is caused by a combination of the zoom spread distance
being larger than the scroll distance and the "pointsInLine" checks being
blind to fingers moving in opposite directions. Zoom gestures that have
fingers moving in opposite directions along the same axis are especially
likely to be misinterpreted as a scroll.

To improve recognition accuracy, this commit adds a function that verifies
the two fingers are actually moving in the same direction. As long as the
motion vectors for the two fingers are more than 90 degrees apart, scrolling
will be inhibited. The allowed difference could be significantly reduced if
necessary, but this seems to do the trick for me.

Signed-off-by: Jason Gerecke <jason.gerecke@wacom.com>

1 files changed, 18 insertions, 1 deletions

diff --git a/src/wcmTouchFilter.c b/src/wcmTouchFilter.c
index 68382c8..c23b47e 100644
--- a/src/wcmTouchFilter.c
+++ b/src/wcmTouchFilter.c
@@ -189,6 +189,22 @@ static double touchDistance(WacomDeviceState ds0, WacomDeviceState ds1)
 	return distance;
 }
 
+static Bool vectorsSameDirection(WacomCommonPtr common, WacomDeviceState ds00,
+		WacomDeviceState ds01, WacomDeviceState ds10, WacomDeviceState ds11)
+{
+	float dx0 = ds01.x - ds00.x;
+	float dy0 = ds01.y - ds00.y;
+	float m0 = sqrt(dx0*dx0 + dy0*dy0);
+	float dx1 = ds11.x - ds10.x;
+	float dy1 = ds11.y - ds10.y;
+	float m1 = sqrt(dx1*dx1 + dy1*dy1);
+
+	float dot = (dx0/m0 * dx1/m1) + (dy0/m0 * dy1/m1);
+	float angle = acos(dot);
+
+	return angle < (3.141592f / 2);
+}
+
 static Bool pointsInLine(WacomCommonPtr common, WacomDeviceState ds0,
 		WacomDeviceState ds1)
 {
@@ -644,7 +660,8 @@ static void wcmFingerScroll(WacomDevicePtr priv)
 			 */
 			if (pointsInLine(common, ds[0], start[0])
 			    && pointsInLine(common, ds[1], start[1])
-			    && common->wcmGestureParameters.wcmScrollDirection)
+			    && common->wcmGestureParameters.wcmScrollDirection
+			    && vectorsSameDirection(common, ds[0], start[0], ds[1], start[1]))
 			{
 				/* left button might be down. Send it up first */
 				wcmSendButtonClick(priv, 1, 0);