Fix drag velocity smoothing in QScroller.

The algorithm used for smoothing the drag velocity was wrong in many ways.

Reviewed-by: Ralf Engels

3 files changed, 16 insertions, 11 deletions

diff --git a/examples/scroller/plot/settingswidget.cpp b/examples/scroller/plot/settingswidget.cpp
index af1e621..840e3fc 100644
--- a/examples/scroller/plot/settingswidget.cpp
+++ b/examples/scroller/plot/settingswidget.cpp
@@ -348,7 +348,7 @@ private:
 MetricItem items[] = {
     { METRIC(MousePressEventDelay),           1000, "ms",       qreal(0), qreal(2000), qreal(10) },
     { METRIC(DragStartDistance),              1000, "mm",       qreal(1), qreal(20), qreal(0.1) },
-    { METRIC(DragVelocitySmoothingFactor),    1,    "",         qreal(0), qreal(1), qreal(0.01) },
+    { METRIC(DragVelocitySmoothingFactor),    1,    "",         qreal(0), qreal(1), qreal(0.1) },
     { METRIC(AxisLockThreshold),              1,    "",         qreal(0), qreal(1), qreal(0.01) },
 
     { METRIC(ScrollingCurve),                 1,    "",         QEasingCurve(), 0, 0 },
diff --git a/src/gui/util/qscroller.cpp b/src/gui/util/qscroller.cpp
index b7b5903..0955f58 100644
--- a/src/gui/util/qscroller.cpp
+++ b/src/gui/util/qscroller.cpp
@@ -1046,6 +1046,9 @@ void QScrollerPrivate::setDpiFromWidget(QWidget *widget)
 */
 void QScrollerPrivate::updateVelocity(const QPointF &deltaPixelRaw, qint64 deltaTime)
 {
+    if (deltaTime <= 0)
+        return;
+
     Q_Q(QScroller);
     QPointF ppm = q->pixelPerMeter();
     const QScrollerPropertiesPrivate *sp = properties.d.data();
@@ -1057,14 +1060,12 @@ void QScrollerPrivate::updateVelocity(const QPointF &deltaPixelRaw, qint64 delta
     if (((deltaPixelRaw / qreal(deltaTime)).manhattanLength() / ((ppm.x() + ppm.y()) / 2) * 1000) > qreal(2.5))
         deltaPixel = deltaPixelRaw * qreal(2.5) * ppm / 1000 / (deltaPixelRaw / qreal(deltaTime)).manhattanLength();
 
-    qreal inversSmoothingFactor = ((qreal(1) - sp->dragVelocitySmoothingFactor) * qreal(deltaTime) / qreal(1000));
     QPointF newv = -deltaPixel / qreal(deltaTime) * qreal(1000) / ppm;
-    newv = newv * (qreal(1) - inversSmoothingFactor) + releaseVelocity * inversSmoothingFactor;
+    if (releaseVelocity != QPointF(0, 0))
+        newv = newv * sp->dragVelocitySmoothingFactor + releaseVelocity * (qreal(1) - sp->dragVelocitySmoothingFactor);
 
-    if (deltaPixel.x())
-        releaseVelocity.setX(qBound(-sp->maximumVelocity, newv.x(), sp->maximumVelocity));
-    if (deltaPixel.y())
-        releaseVelocity.setY(qBound(-sp->maximumVelocity, newv.y(), sp->maximumVelocity));
+    releaseVelocity.setX(qBound(-sp->maximumVelocity, newv.x(), sp->maximumVelocity));
+    releaseVelocity.setY(qBound(-sp->maximumVelocity, newv.y(), sp->maximumVelocity));
 
     qScrollerDebug() << "  --> new velocity:" << releaseVelocity;
 }
@@ -1569,6 +1570,9 @@ bool QScrollerPrivate::releaseWhileDragging(const QPointF &position, qint64 time
     Q_Q(QScroller);
     const QScrollerPropertiesPrivate *sp = properties.d.data();
 
+    // handleDrag updates lastPosition, lastTimestamp and velocity
+    handleDrag(position, timestamp);
+
     // check if we moved at all - this can happen if you stop a running
     // scroller with a press and release shortly afterwards
     QPointF deltaPixel = position - pressPosition;
diff --git a/src/gui/util/qscrollerproperties.cpp b/src/gui/util/qscrollerproperties.cpp
index d21f131..4a1f085 100644
--- a/src/gui/util/qscrollerproperties.cpp
+++ b/src/gui/util/qscrollerproperties.cpp
@@ -61,7 +61,7 @@ QScrollerPropertiesPrivate *QScrollerPropertiesPrivate::defaults()
 #ifdef Q_WS_MAEMO_5
         spp.mousePressEventDelay = qreal(0);
         spp.dragStartDistance = qreal(2.5 / 1000);
-        spp.dragVelocitySmoothingFactor = qreal(0.15);
+        spp.dragVelocitySmoothingFactor = qreal(10);
         spp.axisLockThreshold = qreal(0);
         spp.scrollingCurve.setType(QEasingCurve::OutQuad);
         spp.decelerationFactor = 1.0;
@@ -82,7 +82,7 @@ QScrollerPropertiesPrivate *QScrollerPropertiesPrivate::defaults()
 #else
         spp.mousePressEventDelay = qreal(0.25);
         spp.dragStartDistance = qreal(5.0 / 1000);
-        spp.dragVelocitySmoothingFactor = qreal(0.02);
+        spp.dragVelocitySmoothingFactor = qreal(0.8);
         spp.axisLockThreshold = qreal(0);
         spp.scrollingCurve.setType(QEasingCurve::OutQuad);
         spp.decelerationFactor = qreal(0.125);
@@ -342,8 +342,9 @@ void QScrollerProperties::setScrollMetric(ScrollMetric metric, const QVariant &v
     moved before the flick gesture is triggered in \c m.
 
     \value DragVelocitySmoothingFactor A value that describes how much new drag velocities are
-    included in the final scrolling velocity. This value should be in the range between \c 0 and \c
-    1. Low values meaning that the last dragging velocity is not very important.
+    included in the final scrolling velocity.  This value should be in the range between \c 0 and
+    \c 1.  The lower the value, the more smoothing will be applied to the dragging velocity.  The
+    default value is \c 0.8.
 
     \value AxisLockThreshold If greater than zero a scroll movement will be restricted to one axis
     only if the movement is inside an angle about the axis. The threshold must be in the range \c 0