Add threshold property to Friction

So much easier than calculating exact counterbalancing acceleration.

Change-Id: I7cc43cc2e4d0ad39d1996d432f0e29ccf7bff554
Reviewed-by: Martin Jones <martin.jones@nokia.com>

4 files changed, 156 insertions, 13 deletions

diff --git a/src/declarative/particles/qsgfriction.cpp b/src/declarative/particles/qsgfriction.cpp
index 85a586b51c..9e32fc2c3e 100644
--- a/src/declarative/particles/qsgfriction.cpp
+++ b/src/declarative/particles/qsgfriction.cpp
@@ -54,13 +54,23 @@ QT_BEGIN_NAMESPACE
 
     A drag will be applied to moving objects which is this factor of their current velocity.
 */
+/*!
+    \qmlproperty real QtQuick.Particles2::Friction::threshold
+
+    The drag will only be applied to objects with a velocity above the threshold velocity. The
+    drag applied will bring objects down to the threshold velocity, but no further.
+
+    The default threshold is 0
+*/
 static qreal sign(qreal a)
 {
     return a >= 0 ? 1 : -1;
 }
 
+static const qreal epsilon = 0.00001;
+
 QSGFrictionAffector::QSGFrictionAffector(QQuickItem *parent) :
-    QSGParticleAffector(parent), m_factor(0.0)
+    QSGParticleAffector(parent), m_factor(0.0), m_threshold(0.0)
 {
 }
 
@@ -70,14 +80,29 @@ bool QSGFrictionAffector::affectParticle(QSGParticleData *d, qreal dt)
         return false;
     qreal curVX = d->curVX();
     qreal curVY = d->curVY();
+    if (!curVX && !curVY)
+        return false;
     qreal newVX = curVX + (curVX * m_factor * -1 * dt);
     qreal newVY = curVY + (curVY * m_factor * -1 * dt);
 
-    //Since we're modelling a continuous function, it will never pass 0.
-    if (sign(curVX) != sign(newVX))
-        newVX = 0;
-    if (sign(curVY) != sign(newVY))
-        newVY = 0;
+    if (!m_threshold) {
+        if (sign(curVX) != sign(newVX))
+            newVX = 0;
+        if (sign(curVY) != sign(newVY))
+            newVY = 0;
+    } else {
+        qreal curMag = sqrt(curVX*curVX + curVY*curVY);
+        if (curMag <= m_threshold + epsilon)
+            return false;
+        qreal newMag = sqrt(newVX*newVX + newVY*newVY);
+        if (newMag <= m_threshold + epsilon || //went past the threshold, stop there instead
+            sign(curVX) != sign(newVX) || //went so far past maybe it came out the other side!
+            sign(curVY) != sign(newVY)) {
+            qreal theta = atan2(curVY, curVX);
+            newVX = m_threshold * cos(theta);
+            newVY = m_threshold * sin(theta);
+        }
+    }
 
     d->setInstantaneousVX(newVX);
     d->setInstantaneousVY(newVY);
diff --git a/src/declarative/particles/qsgfriction_p.h b/src/declarative/particles/qsgfriction_p.h
index e2b3a0c26d..9965b147aa 100644
--- a/src/declarative/particles/qsgfriction_p.h
+++ b/src/declarative/particles/qsgfriction_p.h
@@ -54,6 +54,7 @@ class QSGFrictionAffector : public QSGParticleAffector
 {
     Q_OBJECT
     Q_PROPERTY(qreal factor READ factor WRITE setFactor NOTIFY factorChanged)
+    Q_PROPERTY(qreal threshold READ threshold WRITE setThreshold NOTIFY thresholdChanged)
 public:
     explicit QSGFrictionAffector(QQuickItem *parent = 0);
 
@@ -61,24 +62,41 @@ public:
     {
         return m_factor;
     }
+
+    qreal threshold() const
+    {
+        return m_threshold;
+    }
+
 protected:
     virtual bool affectParticle(QSGParticleData *d, qreal dt);
+
 signals:
 
     void factorChanged(qreal arg);
+    void thresholdChanged(qreal arg);
 
 public slots:
 
-void setFactor(qreal arg)
-{
-    if (m_factor != arg) {
-        m_factor = arg;
-        emit factorChanged(arg);
+    void setFactor(qreal arg)
+    {
+        if (m_factor != arg) {
+            m_factor = arg;
+            emit factorChanged(arg);
+        }
+    }
+
+    void setThreshold(qreal arg)
+    {
+        if (m_threshold != arg) {
+            m_threshold = arg;
+            emit thresholdChanged(arg);
+        }
     }
-}
 
 private:
-qreal m_factor;
+    qreal m_factor;
+    qreal m_threshold;
 };
 
 QT_END_NAMESPACE
diff --git a/tests/auto/particles/qsgfriction/data/threshold.qml b/tests/auto/particles/qsgfriction/data/threshold.qml
new file mode 100644
index 0000000000..27990c3e7e
--- /dev/null
+++ b/tests/auto/particles/qsgfriction/data/threshold.qml
@@ -0,0 +1,73 @@
+/****************************************************************************
+**
+** Copyright (C) 2011 Nokia Corporation and/or its subsidiary(-ies).
+** All rights reserved.
+** Contact: Nokia Corporation (qt-info@nokia.com)
+**
+** This file is part of the test suite of the Qt Toolkit.
+**
+** $QT_BEGIN_LICENSE:LGPL$
+** GNU Lesser General Public License Usage
+** This file may be used under the terms of the GNU Lesser General Public
+** License version 2.1 as published by the Free Software Foundation and
+** appearing in the file LICENSE.LGPL included in the packaging of this
+** file. Please review the following information to ensure the GNU Lesser
+** General Public License version 2.1 requirements will be met:
+** http://www.gnu.org/licenses/old-licenses/lgpl-2.1.html.
+**
+** In addition, as a special exception, Nokia gives you certain additional
+** rights. These rights are described in the Nokia Qt LGPL Exception
+** version 1.1, included in the file LGPL_EXCEPTION.txt in this package.
+**
+** GNU General Public License Usage
+** Alternatively, this file may be used under the terms of the GNU General
+** Public License version 3.0 as published by the Free Software Foundation
+** and appearing in the file LICENSE.GPL included in the packaging of this
+** file. Please review the following information to ensure the GNU General
+** Public License version 3.0 requirements will be met:
+** http://www.gnu.org/copyleft/gpl.html.
+**
+** Other Usage
+** Alternatively, this file may be used in accordance with the terms and
+** conditions contained in a signed written agreement between you and Nokia.
+**
+**
+**
+**
+**
+** $QT_END_LICENSE$
+**
+****************************************************************************/
+
+import QtQuick 2.0
+import QtQuick.Particles 2.0
+
+Rectangle {
+    color: "black"
+    width: 320
+    height: 320
+
+    ParticleSystem {
+        id: sys
+        objectName: "system"
+        anchors.fill: parent
+
+        ImageParticle {
+            groups: ["","notdefault"]
+            source: "../../shared/star.png"
+        }
+
+        Friction {
+            factor: 1000 //speed limit 50
+            threshold: 50
+        }
+
+        Emitter{
+            //0,0 position
+            speed: PointDirection{x:1000}
+            size: 32
+            emitRate: 1000
+            lifeSpan: 500
+        }
+    }
+}
diff --git a/tests/auto/particles/qsgfriction/tst_qsgfriction.cpp b/tests/auto/particles/qsgfriction/tst_qsgfriction.cpp
index ad37c3ef38..3237c92421 100644
--- a/tests/auto/particles/qsgfriction/tst_qsgfriction.cpp
+++ b/tests/auto/particles/qsgfriction/tst_qsgfriction.cpp
@@ -52,6 +52,7 @@ public:
 
 private slots:
     void test_basic();
+    void test_threshold();
 };
 
 tst_qsgfriction::tst_qsgfriction()
@@ -102,6 +103,32 @@ void tst_qsgfriction::test_basic()
     }
 }
 
+void tst_qsgfriction::test_threshold()
+{
+    QQuickView* view = createView(QCoreApplication::applicationDirPath() + "/data/threshold.qml", 600);
+    QSGParticleSystem* system = view->rootObject()->findChild<QSGParticleSystem*>("system");
+    ensureAnimTime(600, system->m_animation);
+
+    //Speed capped at 50, but it might take a frame or two to get there
+    QCOMPARE(system->groupData[0]->size(), 500);
+    foreach (QSGParticleData *d, system->groupData[0]->data) {
+        if (d->t == -1.0f)
+            continue; //Particle data unused
+        if (myFuzzyGEQ(d->t, ((qreal)system->timeInt/1000.0) - 0.1))
+            continue; //Particle data too young
+
+        QVERIFY(myFuzzyLEQ(d->vx, 50.f));
+        QCOMPARE(d->y, 0.f);
+        QCOMPARE(d->vy, 0.f);
+        QCOMPARE(d->ax, 0.f);
+        QCOMPARE(d->ay, 0.f);
+        QCOMPARE(d->lifeSpan, 0.5f);
+        QCOMPARE(d->size, 32.f);
+        QCOMPARE(d->endSize, 32.f);
+        QVERIFY(myFuzzyLEQ(d->t, ((qreal)system->timeInt/1000.0)));
+    }
+}
+
 QTEST_MAIN(tst_qsgfriction);
 
 #include "tst_qsgfriction.moc"