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/* -*- Mode: C++; tab-width: 8; indent-tabs-mode: nil; c-basic-offset: 2 -*- */
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/* vim: set ts=8 sts=2 et sw=2 tw=80: */
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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 "AxisPhysicsModel.h"
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namespace mozilla {
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namespace layers {
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/**
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* The simulation is advanced forward in time with a fixed time step to ensure
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* that it remains deterministic given variable framerates. To determine the
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* position at any variable time, two samples are interpolated.
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*
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* kFixedtimestep is set to 120hz in order to ensure that every frame in a
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* common 60hz refresh rate display will have at least one physics simulation
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* sample. More accuracy can be obtained by reducing kFixedTimestep to smaller
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* intervals, such as 240hz or 1000hz, at the cost of more CPU cycles. If
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* kFixedTimestep is increased to much longer intervals, interpolation will
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* become less effective at reducing temporal jitter and the simulation will
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* lose accuracy.
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*/
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const double AxisPhysicsModel::kFixedTimestep = 1.0 / 120.0; // 120hz
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/**
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* Constructs an AxisPhysicsModel with initial values for state.
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*
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* @param aInitialPosition sets the initial position of the simulation,
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* in AppUnits.
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* @param aInitialVelocity sets the initial velocity of the simulation,
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* in AppUnits / second.
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*/
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AxisPhysicsModel::AxisPhysicsModel(double aInitialPosition,
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double aInitialVelocity)
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: mProgress(1.0),
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mPrevState(aInitialPosition, aInitialVelocity),
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mNextState(aInitialPosition, aInitialVelocity) {}
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AxisPhysicsModel::~AxisPhysicsModel() = default;
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double AxisPhysicsModel::GetVelocity() const {
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return LinearInterpolate(mPrevState.v, mNextState.v, mProgress);
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}
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double AxisPhysicsModel::GetPosition() const {
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return LinearInterpolate(mPrevState.p, mNextState.p, mProgress);
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}
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void AxisPhysicsModel::SetVelocity(double aVelocity) {
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mNextState.v = aVelocity;
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mNextState.p = GetPosition();
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mProgress = 1.0;
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}
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void AxisPhysicsModel::SetPosition(double aPosition) {
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mNextState.v = GetVelocity();
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mNextState.p = aPosition;
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mProgress = 1.0;
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}
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void AxisPhysicsModel::Simulate(const TimeDuration& aDeltaTime) {
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for (mProgress += aDeltaTime.ToSeconds() / kFixedTimestep; mProgress > 1.0;
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mProgress -= 1.0) {
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Integrate(kFixedTimestep);
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}
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}
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void AxisPhysicsModel::Integrate(double aDeltaTime) {
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mPrevState = mNextState;
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// RK4 (Runge-Kutta method) Integration
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Derivative a = Evaluate(mNextState, 0.0, Derivative());
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Derivative b = Evaluate(mNextState, aDeltaTime * 0.5, a);
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Derivative c = Evaluate(mNextState, aDeltaTime * 0.5, b);
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Derivative d = Evaluate(mNextState, aDeltaTime, c);
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double dpdt = 1.0 / 6.0 * (a.dp + 2.0 * (b.dp + c.dp) + d.dp);
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double dvdt = 1.0 / 6.0 * (a.dv + 2.0 * (b.dv + c.dv) + d.dv);
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mNextState.p += dpdt * aDeltaTime;
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mNextState.v += dvdt * aDeltaTime;
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}
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AxisPhysicsModel::Derivative AxisPhysicsModel::Evaluate(
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const State& aInitState, double aDeltaTime, const Derivative& aDerivative) {
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State state(aInitState.p + aDerivative.dp * aDeltaTime,
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aInitState.v + aDerivative.dv * aDeltaTime);
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return Derivative(state.v, Acceleration(state));
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}
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double AxisPhysicsModel::LinearInterpolate(double aV1, double aV2,
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double aBlend) {
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return aV1 * (1.0 - aBlend) + aV2 * aBlend;
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}
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} // namespace layers
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} // namespace mozilla