1 files changed, 131 insertions, 0 deletions
diff --git a/gfx/layers/AxisPhysicsModel.h b/gfx/layers/AxisPhysicsModel.h
new file mode 100644
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+++ b/gfx/layers/AxisPhysicsModel.h
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+/* -*- Mode: C++; tab-width: 8; indent-tabs-mode: nil; c-basic-offset: 2 -*- */
+/* vim: set sw=2 ts=8 et 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 "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);
+  ~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();
+
+  /**
+   * 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();
+
+  /**
+   * 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