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/* -*- 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/. */
#ifndef mozilla_layers_AxisPhysicsModel_h
#define mozilla_layers_AxisPhysicsModel_h
#include <sys/types.h> // for int32_t
#include "mozilla/TimeStamp.h" // for TimeDuration
namespace mozilla {
namespace layers {
/**
* AxisPhysicsModel encapsulates a generic 1-dimensional physically-based motion
* model.
*
* It performs frame-rate independent interpolation and RK4 integration for
* smooth animation with stable, deterministic behavior.
* Implementations are expected to subclass and override the Acceleration()
* method.
*/
class AxisPhysicsModel {
public:
AxisPhysicsModel(double aInitialPosition, double aInitialVelocity);
virtual ~AxisPhysicsModel();
/**
* Advance the physics simulation.
* |aDelta| is the time since the last sample.
*/
void Simulate(const TimeDuration& aDeltaTime);
/**
* Gets the raw velocity of this axis at this moment.
*/
double GetVelocity() const;
/**
* Sets the raw velocity of this axis at this moment.
*/
void SetVelocity(double aVelocity);
/**
* Gets the raw position of this axis at this moment.
*/
double GetPosition() const;
/**
* Sets the raw position of this axis at this moment.
*/
void SetPosition(double aPosition);
protected:
struct State {
State(double ap, double av) : p(ap), v(av){};
double p; // Position
double v; // Velocity
};
struct Derivative {
Derivative() : dp(0.0), dv(0.0){};
Derivative(double aDp, double aDv) : dp(aDp), dv(aDv){};
double dp; // dp / delta time = Position
double dv; // dv / delta time = Velocity
};
/**
* Acceleration must be overridden and return the number of
* axis-position-units / second that should be added or removed from the
* velocity.
*/
virtual double Acceleration(const State& aState) = 0;
private:
/**
* Duration of fixed delta time step (seconds)
*/
static const double kFixedTimestep;
/**
* 0.0 - 1.0 value indicating progress between current and next simulation
* sample. Normalized to units of kFixedTimestep duration.
*/
double mProgress;
/**
* Sample of simulation state as it existed
* (1.0 - mProgress) * kFixedTimestep seconds in the past.
*/
State mPrevState;
/**
* Sample of simulation state as it will be in mProgress * kFixedTimestep
* seconds in the future.
*/
State mNextState;
/**
* Perform RK4 (Runge-Kutta method) Integration to calculate the next
* simulation sample.
*/
void Integrate(double aDeltaTime);
/**
* Apply delta velocity and position represented by aDerivative over
* aDeltaTime seconds, calculate new acceleration, and return new deltas.
*/
Derivative Evaluate(const State& aInitState, double aDeltaTime,
const Derivative& aDerivative);
/**
* Helper function for performing linear interpolation (lerp) of double's
*/
static double LinearInterpolate(double aV1, double aV2, double aBlend);
};
} // namespace layers
} // namespace mozilla
#endif
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