gfx/layers/AxisPhysicsMSDModel.h

   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 #ifndef mozilla_layers_AxisPhysicsMSDModel_h
   8 #define mozilla_layers_AxisPhysicsMSDModel_h
   9
  10 #include "AxisPhysicsModel.h"
  11
  12 namespace mozilla {
  13 namespace layers {
  14
  15 /**
  16  * AxisPhysicsMSDModel encapsulates a 1-dimensional MSD (Mass-Spring-Damper)
  17  * model.  A unit mass is assumed.
  18  */
  19 class AxisPhysicsMSDModel : public AxisPhysicsModel {
  20  public:
  21   AxisPhysicsMSDModel(double aInitialPosition, double aInitialDestination,
  22                       double aInitialVelocity, double aSpringConstant,
  23                       double aDampingRatio);
  24
  25   virtual ~AxisPhysicsMSDModel();
  26
  27   /**
  28    * Gets the raw destination of this axis at this moment.
  29    */
  30   double GetDestination() const;
  31
  32   /**
  33    * Sets the raw destination of this axis at this moment.
  34    */
  35   void SetDestination(double aDestination);
  36
  37   /**
  38    * Returns true when the position is close to the destination and the
  39    * velocity is low.
  40    */
  41   bool IsFinished(double aSmallestVisibleIncrement) const;
  42
  43  protected:
  44   double Acceleration(const State& aState) override;
  45
  46  private:
  47   /**
  48    * mDestination represents the target position and the resting position of
  49    * the simulated spring.
  50    */
  51   double mDestination;
  52
  53   /**
  54    * Greater values of mSpringConstant result in a stiffer / stronger spring.
  55    */
  56   double mSpringConstant;
  57
  58   /**
  59    * mSpringConstantSqrtTimesTwo is calculated from mSpringConstant to reduce
  60    * calculations performed in the inner loop.
  61    */
  62   double mSpringConstantSqrtXTwo;
  63
  64   /**
  65    * Damping Ratio: http://en.wikipedia.org/wiki/Damping_ratio
  66    *
  67    * When mDampingRatio < 1.0, this is an under damped system.
  68    * - Overshoots destination and oscillates with the amplitude gradually
  69    *   decreasing to zero.
  70    *
  71    * When mDampingRatio == 1.0, this is a critically damped system.
  72    * - Reaches destination as quickly as possible without oscillating.
  73    *
  74    * When mDampingRatio > 1.0, this is an over damped system.
  75    * - Reaches destination (exponentially decays) without oscillating.
  76    */
  77   double mDampingRatio;
  78 };
  79
  80 }  // namespace layers
  81 }  // namespace mozilla
  82
  83 #endif