Scroll offset animation curve retargeting.

cc/animation/scroll_offset_animation_curve.cc

Index: cc/animation/scroll_offset_animation_curve.cc
diff --git a/cc/animation/scroll_offset_animation_curve.cc b/cc/animation/scroll_offset_animation_curve.cc
index a7283b8b5c601aca4a6f3dfac4e269b167c17966..8211a950fe2b24382bc4cf0f7b5c276980ed28a0 100644
--- a/cc/animation/scroll_offset_animation_curve.cc
+++ b/cc/animation/scroll_offset_animation_curve.cc
@@ -15,6 +15,35 @@ const double kDurationDivisor = 60.0;
 
 namespace cc {
 
+namespace {
+
+static float MaximumDimension(gfx::Vector2dF delta) {
+  return std::max(std::abs(delta.x()), std::abs(delta.y()));
+}
+
+static base::TimeDelta DurationFromDelta(gfx::Vector2dF delta) {
+  // The duration of a scroll animation depends on the size of the scroll.
+  // The exact relationship between the size and the duration isn't specified
+  // by the CSSOM View smooth scroll spec and is instead left up to user agents
+  // to decide. The calculation performed here will very likely be further
+  // tweaked before the smooth scroll API ships.
+  return base::TimeDelta::FromMicroseconds(
+      (std::sqrt(MaximumDimension(delta)) / kDurationDivisor) *
+      base::Time::kMicrosecondsPerSecond);
+}
+
+static scoped_ptr<TimingFunction> EaseOutWithInitialVelocity(double velocity) {
+  // Based on EaseInOutTimingFunction::Create with first control point rotated.
+  const double r2 = 0.42 * 0.42;
+  const double v2 = velocity * velocity;
+  const double x1 = std::sqrt(r2 / (v2 + 1));
+  const double y1 = std::sqrt(r2 * v2 / (v2 + 1));
+  return CubicBezierTimingFunction::Create(x1, y1, 0.58, 1)
+      .PassAs<TimingFunction>();
+}
+
+}  // namespace
+
 scoped_ptr<ScrollOffsetAnimationCurve> ScrollOffsetAnimationCurve::Create(
     const gfx::Vector2dF& target_value,
     scoped_ptr<TimingFunction> timing_function) {
@@ -33,22 +62,12 @@ ScrollOffsetAnimationCurve::~ScrollOffsetAnimationCurve() {}
 void ScrollOffsetAnimationCurve::SetInitialValue(
     const gfx::Vector2dF& initial_value) {
   initial_value_ = initial_value;
-
-  // The duration of a scroll animation depends on the size of the scroll.
-  // The exact relationship between the size and the duration isn't specified
-  // by the CSSOM View smooth scroll spec and is instead left up to user agents
-  // to decide. The calculation performed here will very likely be further
-  // tweaked before the smooth scroll API ships.
-  float delta_x = std::abs(target_value_.x() - initial_value_.x());
-  float delta_y = std::abs(target_value_.y() - initial_value_.y());
-  float max_delta = std::max(delta_x, delta_y);
-  duration_ = base::TimeDelta::FromMicroseconds(
-      (std::sqrt(max_delta) / kDurationDivisor) *
-      base::Time::kMicrosecondsPerSecond);
+  total_animation_duration_ = DurationFromDelta(target_value_ - initial_value_);
 }
 
 gfx::Vector2dF ScrollOffsetAnimationCurve::GetValue(double t) const {
-  double duration = duration_.InSecondsF();
+  double duration = (total_animation_duration_ - last_retarget_).InSecondsF();
+  t -= last_retarget_.InSecondsF();
 
   if (t <= 0)
     return initial_value_;
@@ -64,7 +83,7 @@ gfx::Vector2dF ScrollOffsetAnimationCurve::GetValue(double t) const {
 }
 
 double ScrollOffsetAnimationCurve::Duration() const {
-  return duration_.InSecondsF();
+  return total_animation_duration_.InSecondsF();
 }
 
 AnimationCurve::CurveType ScrollOffsetAnimationCurve::Type() const {
@@ -77,8 +96,37 @@ scoped_ptr<AnimationCurve> ScrollOffsetAnimationCurve::Clone() const {
   scoped_ptr<ScrollOffsetAnimationCurve> curve_clone =
       Create(target_value_, timing_function.Pass());
   curve_clone->initial_value_ = initial_value_;
-  curve_clone->duration_ = duration_;
+  curve_clone->total_animation_duration_ = total_animation_duration_;
+  curve_clone->last_retarget_ = last_retarget_;
   return curve_clone.PassAs<AnimationCurve>();
 }
 
+void ScrollOffsetAnimationCurve::UpdateTarget(
+    double t,
+    const gfx::Vector2dF& new_target) {
+  gfx::Vector2dF current_position = GetValue(t);
+  gfx::Vector2dF old_delta = target_value_ - initial_value_;
+  gfx::Vector2dF new_delta = new_target - current_position;
+
+  double old_duration =
+      (total_animation_duration_ - last_retarget_).InSecondsF();
+  double new_duration = DurationFromDelta(new_delta).InSecondsF();
+
+  double old_velocity = timing_function_->Velocity(
+      (t - last_retarget_.InSecondsF()) / old_duration);
+
+  // TimingFunction::Velocity gives the slope of the curve from 0 to 1.
+  // To match the "true" velocity in px/sec we must adjust this slope for
+  // differences in duration and scroll delta between old and new curves.
+  double new_velocity =
+      old_velocity * (new_duration / old_duration) *
+      (MaximumDimension(old_delta) / MaximumDimension(new_delta));
+
+  initial_value_ = current_position;
+  target_value_ = new_target;
+  total_animation_duration_ = base::TimeDelta::FromSecondsD(t + new_duration);
+  last_retarget_ = base::TimeDelta::FromSecondsD(t);
+  timing_function_ = EaseOutWithInitialVelocity(new_velocity);
+}
+
 }  // namespace cc