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Diffstat (limited to '')
-rw-r--r-- | gfx/layers/AxisPhysicsModel.cpp | 101 |
1 files changed, 101 insertions, 0 deletions
diff --git a/gfx/layers/AxisPhysicsModel.cpp b/gfx/layers/AxisPhysicsModel.cpp new file mode 100644 index 0000000000..dc0cba3110 --- /dev/null +++ b/gfx/layers/AxisPhysicsModel.cpp @@ -0,0 +1,101 @@ +/* -*- 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 |