1 files changed, 78 insertions, 0 deletions
diff --git a/gfx/layers/AxisPhysicsMSDModel.cpp b/gfx/layers/AxisPhysicsMSDModel.cpp
new file mode 100644
index 0000000000..a506db8a05
--- /dev/null
+++ b/gfx/layers/AxisPhysicsMSDModel.cpp
@@ -0,0 +1,78 @@
+/* -*- Mode: C++; tab-width: 8; indent-tabs-mode: nil; c-basic-offset: 2 -*- */
+/* vim: set ts=8 sts=2 et sw=2 tw=80: */
+/* This Source Code Form is subject to the terms of the Mozilla Public
+ * License, v. 2.0. If a copy of the MPL was not distributed with this
+ * file, You can obtain one at http://mozilla.org/MPL/2.0/. */
+
+#include "AxisPhysicsMSDModel.h"
+#include <math.h>  // for sqrt and fabs
+
+namespace mozilla {
+namespace layers {
+
+/**
+ * Constructs an AxisPhysicsMSDModel with initial values for state.
+ *
+ * @param aInitialPosition sets the initial position of the simulated spring,
+ *        in AppUnits.
+ * @param aInitialDestination sets the resting position of the simulated spring,
+ *        in AppUnits.
+ * @param aInitialVelocity sets the initial velocity of the simulated spring,
+ *        in AppUnits / second.  Critically-damped and over-damped systems are
+ *        guaranteed not to overshoot aInitialDestination if this is set to 0;
+ *        however, it is possible to overshoot and oscillate if not set to 0 or
+ *        the system is under-damped.
+ * @param aSpringConstant sets the strength of the simulated spring.  Greater
+ *        values of mSpringConstant result in a stiffer / stronger spring.
+ * @param aDampingRatio controls the amount of dampening force and determines
+ *        if the system is under-damped, critically-damped, or over-damped.
+ */
+AxisPhysicsMSDModel::AxisPhysicsMSDModel(double aInitialPosition,
+                                         double aInitialDestination,
+                                         double aInitialVelocity,
+                                         double aSpringConstant,
+                                         double aDampingRatio)
+    : AxisPhysicsModel(aInitialPosition, aInitialVelocity),
+      mDestination(aInitialDestination),
+      mSpringConstant(aSpringConstant),
+      mSpringConstantSqrtXTwo(sqrt(mSpringConstant) * 2.0),
+      mDampingRatio(aDampingRatio) {}
+
+AxisPhysicsMSDModel::~AxisPhysicsMSDModel() = default;
+
+double AxisPhysicsMSDModel::Acceleration(const State& aState) {
+  // Simulate a Mass-Damper-Spring Model; assume a unit mass
+
+  // Hooke’s Law: http://en.wikipedia.org/wiki/Hooke%27s_law
+  double spring_force = (mDestination - aState.p) * mSpringConstant;
+  double damp_force = -aState.v * mDampingRatio * mSpringConstantSqrtXTwo;
+
+  return spring_force + damp_force;
+}
+
+double AxisPhysicsMSDModel::GetDestination() const { return mDestination; }
+
+void AxisPhysicsMSDModel::SetDestination(double aDestination) {
+  mDestination = aDestination;
+}
+
+bool AxisPhysicsMSDModel::IsFinished(double aSmallestVisibleIncrement) const {
+  // In order to satisfy the condition of reaching the destination, the distance
+  // between the simulation position and the destination must be less than
+  // aSmallestVisibleIncrement while the speed is simultaneously less than
+  // finishVelocity.  This enables an under-damped system to overshoot the
+  // destination when desired without prematurely triggering the finished state.
+  // If finishVelocity is set too low, the animation may end long after
+  // oscillation has finished, resulting in unnecessary processing.
+  // If set too high, the animation may prematurely terminate when expected
+  // to overshoot the destination in an under-damped system.
+  // aSmallestVisibleIncrement * 2 was selected through experimentation that
+  // revealed that a critically damped system will terminate within 100ms.
+  const double finishVelocity = aSmallestVisibleIncrement * 2;
+
+  return fabs(mDestination - GetPosition()) < aSmallestVisibleIncrement &&
+         fabs(GetVelocity()) <= finishVelocity;
+}
+
+}  // namespace layers
+}  // namespace mozilla