gfx/layers/AxisPhysicsModel.cpp

   1 /* -*- Mode: C++; tab-width: 8; indent-tabs-mode: nil; c-basic-offset: 2 -*- */
   2 /* vim: set ts=8 sts=2 et sw=2 tw=80: */
   3 /* This Source Code Form is subject to the terms of the Mozilla Public
   4  * License, v. 2.0. If a copy of the MPL was not distributed with this
   5  * file, You can obtain one at http://mozilla.org/MPL/2.0/. */
   6
   7 #include "AxisPhysicsModel.h"
   8
   9 namespace mozilla {
  10 namespace layers {
  11
  12 /**
  13  * The simulation is advanced forward in time with a fixed time step to ensure
  14  * that it remains deterministic given variable framerates.  To determine the
  15  * position at any variable time, two samples are interpolated.
  16  *
  17  * kFixedtimestep is set to 120hz in order to ensure that every frame in a
  18  * common 60hz refresh rate display will have at least one physics simulation
  19  * sample.  More accuracy can be obtained by reducing kFixedTimestep to smaller
  20  * intervals, such as 240hz or 1000hz, at the cost of more CPU cycles.  If
  21  * kFixedTimestep is increased to much longer intervals, interpolation will
  22  * become less effective at reducing temporal jitter and the simulation will
  23  * lose accuracy.
  24  */
  25 const double AxisPhysicsModel::kFixedTimestep = 1.0 / 120.0;  // 120hz
  26
  27 /**
  28  * Constructs an AxisPhysicsModel with initial values for state.
  29  *
  30  * @param aInitialPosition sets the initial position of the simulation,
  31  *        in AppUnits.
  32  * @param aInitialVelocity sets the initial velocity of the simulation,
  33  *        in AppUnits / second.
  34  */
  35 AxisPhysicsModel::AxisPhysicsModel(double aInitialPosition,
  36                                    double aInitialVelocity)
  37     : mProgress(1.0),
  38       mPrevState(aInitialPosition, aInitialVelocity),
  39       mNextState(aInitialPosition, aInitialVelocity) {}
  40
  41 AxisPhysicsModel::~AxisPhysicsModel() = default;
  42
  43 double AxisPhysicsModel::GetVelocity() const {
  44   return LinearInterpolate(mPrevState.v, mNextState.v, mProgress);
  45 }
  46
  47 double AxisPhysicsModel::GetPosition() const {
  48   return LinearInterpolate(mPrevState.p, mNextState.p, mProgress);
  49 }
  50
  51 void AxisPhysicsModel::SetVelocity(double aVelocity) {
  52   mNextState.v = aVelocity;
  53   mNextState.p = GetPosition();
  54   mProgress = 1.0;
  55 }
  56
  57 void AxisPhysicsModel::SetPosition(double aPosition) {
  58   mNextState.v = GetVelocity();
  59   mNextState.p = aPosition;
  60   mProgress = 1.0;
  61 }
  62
  63 void AxisPhysicsModel::Simulate(const TimeDuration& aDeltaTime) {
  64   for (mProgress += aDeltaTime.ToSeconds() / kFixedTimestep; mProgress > 1.0;
  65        mProgress -= 1.0) {
  66     Integrate(kFixedTimestep);
  67   }
  68 }
  69
  70 void AxisPhysicsModel::Integrate(double aDeltaTime) {
  71   mPrevState = mNextState;
  72
  73   // RK4 (Runge-Kutta method) Integration
  74   // http://en.wikipedia.org/wiki/Runge%E2%80%93Kutta_methods
  75   Derivative a = Evaluate(mNextState, 0.0, Derivative());
  76   Derivative b = Evaluate(mNextState, aDeltaTime * 0.5, a);
  77   Derivative c = Evaluate(mNextState, aDeltaTime * 0.5, b);
  78   Derivative d = Evaluate(mNextState, aDeltaTime, c);
  79
  80   double dpdt = 1.0 / 6.0 * (a.dp + 2.0 * (b.dp + c.dp) + d.dp);
  81   double dvdt = 1.0 / 6.0 * (a.dv + 2.0 * (b.dv + c.dv) + d.dv);
  82
  83   mNextState.p += dpdt * aDeltaTime;
  84   mNextState.v += dvdt * aDeltaTime;
  85 }
  86
  87 AxisPhysicsModel::Derivative AxisPhysicsModel::Evaluate(
  88     const State& aInitState, double aDeltaTime, const Derivative& aDerivative) {
  89   State state(aInitState.p + aDerivative.dp * aDeltaTime,
  90               aInitState.v + aDerivative.dv * aDeltaTime);
  91
  92   return Derivative(state.v, Acceleration(state));
  93 }
  94
  95 double AxisPhysicsModel::LinearInterpolate(double aV1, double aV2,
  96                                            double aBlend) {
  97   return aV1 * (1.0 - aBlend) + aV2 * aBlend;
  98 }
  99
 100 }  // namespace layers
 101 }  // namespace mozilla