AxisPhysicsMSDModel.cpp (3691B)
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