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/* -*- Mode: C++; tab-width: 8; indent-tabs-mode: nil; c-basic-offset: 2 -*- */
/* 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()
{
}
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)
{
// 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
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