gfx/layers/AxisPhysicsMSDModel.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 "AxisPhysicsMSDModel.h"
   8 #include <math.h>  // for sqrt and fabs
   9
  10 namespace mozilla {
  11 namespace layers {
  12
  13 /**
  14  * Constructs an AxisPhysicsMSDModel with initial values for state.
  15  *
  16  * @param aInitialPosition sets the initial position of the simulated spring,
  17  *        in AppUnits.
  18  * @param aInitialDestination sets the resting position of the simulated spring,
  19  *        in AppUnits.
  20  * @param aInitialVelocity sets the initial velocity of the simulated spring,
  21  *        in AppUnits / second.  Critically-damped and over-damped systems are
  22  *        guaranteed not to overshoot aInitialDestination if this is set to 0;
  23  *        however, it is possible to overshoot and oscillate if not set to 0 or
  24  *        the system is under-damped.
  25  * @param aSpringConstant sets the strength of the simulated spring.  Greater
  26  *        values of mSpringConstant result in a stiffer / stronger spring.
  27  * @param aDampingRatio controls the amount of dampening force and determines
  28  *        if the system is under-damped, critically-damped, or over-damped.
  29  */
  30 AxisPhysicsMSDModel::AxisPhysicsMSDModel(double aInitialPosition,
  31                                          double aInitialDestination,
  32                                          double aInitialVelocity,
  33                                          double aSpringConstant,
  34                                          double aDampingRatio)
  35     : AxisPhysicsModel(aInitialPosition, aInitialVelocity),
  36       mDestination(aInitialDestination),
  37       mSpringConstant(aSpringConstant),
  38       mSpringConstantSqrtXTwo(sqrt(mSpringConstant) * 2.0),
  39       mDampingRatio(aDampingRatio) {}
  40
  41 AxisPhysicsMSDModel::~AxisPhysicsMSDModel() = default;
  42
  43 double AxisPhysicsMSDModel::Acceleration(const State& aState) {
  44   // Simulate a Mass-Damper-Spring Model; assume a unit mass
  45
  46   // Hooke’s Law: http://en.wikipedia.org/wiki/Hooke%27s_law
  47   double spring_force = (mDestination - aState.p) * mSpringConstant;
  48   double damp_force = -aState.v * mDampingRatio * mSpringConstantSqrtXTwo;
  49
  50   return spring_force + damp_force;
  51 }
  52
  53 double AxisPhysicsMSDModel::GetDestination() const { return mDestination; }
  54
  55 void AxisPhysicsMSDModel::SetDestination(double aDestination) {
  56   mDestination = aDestination;
  57 }
  58
  59 bool AxisPhysicsMSDModel::IsFinished(double aSmallestVisibleIncrement) const {
  60   // In order to satisfy the condition of reaching the destination, the distance
  61   // between the simulation position and the destination must be less than
  62   // aSmallestVisibleIncrement while the speed is simultaneously less than
  63   // finishVelocity.  This enables an under-damped system to overshoot the
  64   // destination when desired without prematurely triggering the finished state.
  65   // If finishVelocity is set too low, the animation may end long after
  66   // oscillation has finished, resulting in unnecessary processing.
  67   // If set too high, the animation may prematurely terminate when expected
  68   // to overshoot the destination in an under-damped system.
  69   // aSmallestVisibleIncrement * 2 was selected through experimentation that
  70   // revealed that a critically damped system will terminate within 100ms.
  71   const double finishVelocity = aSmallestVisibleIncrement * 2;
  72
  73   return fabs(mDestination - GetPosition()) < aSmallestVisibleIncrement &&
  74          fabs(GetVelocity()) <= finishVelocity;
  75 }
  76
  77 }  // namespace layers
  78 }  // namespace mozilla