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/* -*- 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 "AxisPhysicsModel.h"
namespace mozilla {
namespace layers {
/**
* The simulation is advanced forward in time with a fixed time step to ensure
* that it remains deterministic given variable framerates. To determine the
* position at any variable time, two samples are interpolated.
*
* kFixedtimestep is set to 120hz in order to ensure that every frame in a
* common 60hz refresh rate display will have at least one physics simulation
* sample. More accuracy can be obtained by reducing kFixedTimestep to smaller
* intervals, such as 240hz or 1000hz, at the cost of more CPU cycles. If
* kFixedTimestep is increased to much longer intervals, interpolation will
* become less effective at reducing temporal jitter and the simulation will
* lose accuracy.
*/
const double AxisPhysicsModel::kFixedTimestep = 1.0 / 120.0; // 120hz
/**
* Constructs an AxisPhysicsModel with initial values for state.
*
* @param aInitialPosition sets the initial position of the simulation,
* in AppUnits.
* @param aInitialVelocity sets the initial velocity of the simulation,
* in AppUnits / second.
*/
AxisPhysicsModel::AxisPhysicsModel(double aInitialPosition,
double aInitialVelocity)
: mProgress(1.0),
mPrevState(aInitialPosition, aInitialVelocity),
mNextState(aInitialPosition, aInitialVelocity) {}
AxisPhysicsModel::~AxisPhysicsModel() = default;
double AxisPhysicsModel::GetVelocity() const {
return LinearInterpolate(mPrevState.v, mNextState.v, mProgress);
}
double AxisPhysicsModel::GetPosition() const {
return LinearInterpolate(mPrevState.p, mNextState.p, mProgress);
}
void AxisPhysicsModel::SetVelocity(double aVelocity) {
mNextState.v = aVelocity;
mNextState.p = GetPosition();
mProgress = 1.0;
}
void AxisPhysicsModel::SetPosition(double aPosition) {
mNextState.v = GetVelocity();
mNextState.p = aPosition;
mProgress = 1.0;
}
void AxisPhysicsModel::Simulate(const TimeDuration& aDeltaTime) {
for (mProgress += aDeltaTime.ToSeconds() / kFixedTimestep; mProgress > 1.0;
mProgress -= 1.0) {
Integrate(kFixedTimestep);
}
}
void AxisPhysicsModel::Integrate(double aDeltaTime) {
mPrevState = mNextState;
// RK4 (Runge-Kutta method) Integration
// http://en.wikipedia.org/wiki/Runge%E2%80%93Kutta_methods
Derivative a = Evaluate(mNextState, 0.0, Derivative());
Derivative b = Evaluate(mNextState, aDeltaTime * 0.5, a);
Derivative c = Evaluate(mNextState, aDeltaTime * 0.5, b);
Derivative d = Evaluate(mNextState, aDeltaTime, c);
double dpdt = 1.0 / 6.0 * (a.dp + 2.0 * (b.dp + c.dp) + d.dp);
double dvdt = 1.0 / 6.0 * (a.dv + 2.0 * (b.dv + c.dv) + d.dv);
mNextState.p += dpdt * aDeltaTime;
mNextState.v += dvdt * aDeltaTime;
}
AxisPhysicsModel::Derivative AxisPhysicsModel::Evaluate(
const State& aInitState, double aDeltaTime, const Derivative& aDerivative) {
State state(aInitState.p + aDerivative.dp * aDeltaTime,
aInitState.v + aDerivative.dv * aDeltaTime);
return Derivative(state.v, Acceleration(state));
}
double AxisPhysicsModel::LinearInterpolate(double aV1, double aV2,
double aBlend) {
return aV1 * (1.0 - aBlend) + aV2 * aBlend;
}
} // namespace layers
} // namespace mozilla
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