Bug 1018255 - Use spring physics for snap-back animation. r=kats

--HG--
extra : source : f7f4c40fc818c2342fe6c49c0351c1573f6e9f3d

b2g/app/b2g.js

@@ -960,8 +960,9 @@ pref("apz.overscroll.fling_friction", "0.02");
 pref("apz.overscroll.fling_stopped_threshold", "0.4");
 pref("apz.overscroll.clamping", "0.5");
 pref("apz.overscroll.z_effect", "0.2");
-pref("apz.overscroll.snap_back_accel", "0.003");
-pref("apz.overscroll.snap_back_init_vel", "1");
+pref("apz.overscroll.snap_back.spring_stiffness", "0.6");
+pref("apz.overscroll.snap_back.spring_friction", "0.1");
+pref("apz.overscroll.snap_back.mass", "1000");
 
 // This preference allows FirefoxOS apps (and content, I think) to force
 // the use of software (instead of hardware accelerated) 2D canvases by

gfx/layers/apz/src/AsyncPanZoomController.cpp

@@ -235,13 +235,17 @@ typedef GeckoContentController::APZStateChange APZStateChange;
  * opposite axis creates the effect of the page moving away from you along a
  * "z" axis.
  *
- * "apz.overscroll.snap_back_accel"
- * Amount of acceleration applied during the snap-back animation.
+ * "apz.overscroll.snap_back.spring_stiffness"
+ * The stiffness of the spring used in the physics model for the overscroll
+ * snap-back animation.
  *
- * "apz.overscroll.snap_back_init_vel"
- * Initial velocity of a snap-back animation along one axis.
- * Units: screen pixels per millisecond
- * requests.
+ * "apz.overscroll.snap_back.spring_damping"
+ * The friction of the spring used in the physics model for the overscroll
+ * snap-back animation.
+ *
+ * "apz.overscroll.snap_back.mass"
+ * The mass of the page in the physics model for the overscroll snap-back
+ * animation.
  *
  * "apz.pan_repaint_interval"
  * Maximum amount of time while panning before sending a viewport change. This
@@ -490,11 +494,7 @@ private:
 class OverscrollSnapBackAnimation: public AsyncPanZoomAnimation {
 public:
   OverscrollSnapBackAnimation(AsyncPanZoomController& aApzc)
-    : mApzc(aApzc)
-  {
-    mApzc.mX.StartSnapBack();
-    mApzc.mY.StartSnapBack();
-  }
+    : mApzc(aApzc) {}
 
   virtual bool Sample(FrameMetrics& aFrameMetrics,
                       const TimeDuration& aDelta) MOZ_OVERRIDE

gfx/layers/apz/src/Axis.cpp

@@ -121,21 +121,27 @@ float Axis::GetOverscroll() const {
   return mOverscroll;
 }
 
-void Axis::StartSnapBack() {
-  float initialSnapBackVelocity = gfxPrefs::APZSnapBackInitialVelocity();
-  if (mOverscroll > 0) {
-    mVelocity = -initialSnapBackVelocity;
-  } else {
-    mVelocity = initialSnapBackVelocity;
-  }
-}
-
 bool Axis::SampleSnapBack(const TimeDuration& aDelta) {
-  // Accelerate the snap-back as time goes on.
-  // Note: this method of acceleration isn't perfectly smooth, as it assumes
+  // Apply spring physics to the snap-back as time goes on.
+  // Note: this method of sampling isn't perfectly smooth, as it assumes
   // a constant velocity over 'aDelta', instead of an accelerating velocity.
   // (The way we applying friction to flings has the same issue.)
-  mVelocity *= pow(1.0f + gfxPrefs::APZSnapBackAcceleration(), float(aDelta.ToMilliseconds()));
+  // Hooke's law with damping:
+  //   F = -kx - bv
+  // where
+  //   k is a constant related to the stiffness of the spring
+  //     The larger the constant, the stiffer the spring.
+  //   x is the displacement of the end of the spring from its equilibrium
+  //     In our scenario, it's the amount of overscroll on the axis.
+  //   b is a constant that provides damping (friction)
+  //   v is the velocity of the point at the end of the spring
+  // See http://gafferongames.com/game-physics/spring-physics/
+  const float kSpringStiffness = gfxPrefs::APZOverscrollSnapBackSpringStiffness();
+  const float kSpringFriction = gfxPrefs::APZOverscrollSnapBackSpringFriction();
+  const float kMass = gfxPrefs::APZOverscrollSnapBackMass();
+  float force = -1 * kSpringStiffness * mOverscroll - kSpringFriction * mVelocity;
+  float acceleration = force / kMass;
+  mVelocity += acceleration * aDelta.ToMilliseconds();
   float screenDisplacement = mVelocity * aDelta.ToMilliseconds();
   float cssDisplacement = screenDisplacement / GetFrameMetrics().GetZoom().scale;
   if (mOverscroll > 0) {

gfx/layers/apz/src/Axis.h

@@ -98,11 +98,6 @@ public:
    */
   float GetOverscroll() const;
 
-  /**
-   * Start a snap-back animation to relieve overscroll.
-   */
-  void StartSnapBack();
-
   /**
    * Sample the snap-back animation to relieve overscroll.
    * |aDelta| is the time since the last sample.

gfx/thebes/gfxPrefs.h

@@ -126,8 +126,9 @@ private:
   DECL_GFX_PREF(Live, "apz.overscroll.fling_stopped_threshold", APZOverscrollFlingStoppedThreshold, float, 0.4f);
   DECL_GFX_PREF(Live, "apz.overscroll.clamping",               APZOverscrollClamping, float, 0.5f);
   DECL_GFX_PREF(Live, "apz.overscroll.z_effect",               APZOverscrollZEffect, float, 0.2f);
-  DECL_GFX_PREF(Once, "apz.overscroll.snap_back_accel",        APZSnapBackAcceleration, float, 0.002f);
-  DECL_GFX_PREF(Live, "apz.overscroll.snap_back_init_vel",     APZSnapBackInitialVelocity, float, 1.0f);
+  DECL_GFX_PREF(Live, "apz.overscroll.snap_back.spring_stiffness", APZOverscrollSnapBackSpringStiffness, float, 0.6f);
+  DECL_GFX_PREF(Live, "apz.overscroll.snap_back.spring_friction", APZOverscrollSnapBackSpringFriction, float, 0.1f);
+  DECL_GFX_PREF(Live, "apz.overscroll.snap_back.mass",         APZOverscrollSnapBackMass, float, 1000.0f);
   DECL_GFX_PREF(Live, "apz.pan_repaint_interval",              APZPanRepaintInterval, int32_t, 250);
   DECL_GFX_PREF(Live, "apz.subframe.enabled",                  APZSubframeEnabled, bool, false);
   DECL_GFX_PREF(Once, "apz.test.logging_enabled",              APZTestLoggingEnabled, bool, false);