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