1 files changed, 135 insertions, 0 deletions
diff --git a/gfx/layers/apz/src/SimpleVelocityTracker.cpp b/gfx/layers/apz/src/SimpleVelocityTracker.cpp
new file mode 100644
index 0000000000..87cae10d51
--- /dev/null
+++ b/gfx/layers/apz/src/SimpleVelocityTracker.cpp
@@ -0,0 +1,135 @@
+/* -*- 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/. */
+
+#include "SimpleVelocityTracker.h"
+
+#include "mozilla/ServoStyleConsts.h"  // for StyleComputedTimingFunction
+#include "mozilla/StaticPrefs_apz.h"
+#include "mozilla/StaticPtr.h"  // for StaticAutoPtr
+
+static mozilla::LazyLogModule sApzSvtLog("apz.simplevelocitytracker");
+#define SVT_LOG(...) MOZ_LOG(sApzSvtLog, LogLevel::Debug, (__VA_ARGS__))
+
+namespace mozilla {
+namespace layers {
+
+// When we compute the velocity we do so by taking two input events and
+// dividing the distance delta over the time delta. In some cases the time
+// delta can be really small, which can make the velocity computation very
+// volatile. To avoid this we impose a minimum time delta below which we do
+// not recompute the velocity.
+const TimeDuration MIN_VELOCITY_SAMPLE_TIME = TimeDuration::FromMilliseconds(5);
+
+extern StaticAutoPtr<StyleComputedTimingFunction> gVelocityCurveFunction;
+
+SimpleVelocityTracker::SimpleVelocityTracker(Axis* aAxis)
+    : mAxis(aAxis), mVelocitySamplePos(0) {}
+
+void SimpleVelocityTracker::StartTracking(ParentLayerCoord aPos,
+                                          TimeStamp aTimestamp) {
+  Clear();
+  mVelocitySampleTime = aTimestamp;
+  mVelocitySamplePos = aPos;
+}
+
+Maybe<float> SimpleVelocityTracker::AddPosition(ParentLayerCoord aPos,
+                                                TimeStamp aTimestamp) {
+  if (aTimestamp <= mVelocitySampleTime + MIN_VELOCITY_SAMPLE_TIME) {
+    // See also the comment on MIN_VELOCITY_SAMPLE_TIME.
+    // We don't update either mVelocitySampleTime or mVelocitySamplePos so that
+    // eventually when we do get an event with the required time delta we use
+    // the corresponding distance delta as well.
+    SVT_LOG("%p|%s skipping velocity computation for small time delta %f ms\n",
+            mAxis->OpaqueApzcPointer(), mAxis->Name(),
+            (aTimestamp - mVelocitySampleTime).ToMilliseconds());
+    return Nothing();
+  }
+
+  float newVelocity =
+      (float)(mVelocitySamplePos - aPos) /
+      (float)(aTimestamp - mVelocitySampleTime).ToMilliseconds();
+
+  newVelocity = ApplyFlingCurveToVelocity(newVelocity);
+
+  SVT_LOG("%p|%s updating velocity to %f with touch\n",
+          mAxis->OpaqueApzcPointer(), mAxis->Name(), newVelocity);
+  mVelocitySampleTime = aTimestamp;
+  mVelocitySamplePos = aPos;
+
+  AddVelocityToQueue(aTimestamp, newVelocity);
+
+  return Some(newVelocity);
+}
+
+Maybe<float> SimpleVelocityTracker::ComputeVelocity(TimeStamp aTimestamp) {
+  float velocity = 0;
+  int count = 0;
+  for (const auto& e : mVelocityQueue) {
+    TimeDuration timeDelta = (aTimestamp - e.first);
+    if (timeDelta < TimeDuration::FromMilliseconds(
+                        StaticPrefs::apz_velocity_relevance_time_ms())) {
+      count++;
+      velocity += e.second;
+    }
+  }
+  mVelocityQueue.Clear();
+  if (count > 1) {
+    velocity /= count;
+  }
+  return Some(velocity);
+}
+
+void SimpleVelocityTracker::Clear() { mVelocityQueue.Clear(); }
+
+void SimpleVelocityTracker::AddVelocityToQueue(TimeStamp aTimestamp,
+                                               float aVelocity) {
+  mVelocityQueue.AppendElement(std::make_pair(aTimestamp, aVelocity));
+  if (mVelocityQueue.Length() >
+      StaticPrefs::apz_max_velocity_queue_size_AtStartup()) {
+    mVelocityQueue.RemoveElementAt(0);
+  }
+}
+
+float SimpleVelocityTracker::ApplyFlingCurveToVelocity(float aVelocity) const {
+  float newVelocity = aVelocity;
+  if (StaticPrefs::apz_max_velocity_inches_per_ms() > 0.0f) {
+    bool velocityIsNegative = (newVelocity < 0);
+    newVelocity = fabs(newVelocity);
+
+    float maxVelocity =
+        mAxis->ToLocalVelocity(StaticPrefs::apz_max_velocity_inches_per_ms());
+    newVelocity = std::min(newVelocity, maxVelocity);
+
+    if (StaticPrefs::apz_fling_curve_threshold_inches_per_ms() > 0.0f &&
+        StaticPrefs::apz_fling_curve_threshold_inches_per_ms() <
+            StaticPrefs::apz_max_velocity_inches_per_ms()) {
+      float curveThreshold = mAxis->ToLocalVelocity(
+          StaticPrefs::apz_fling_curve_threshold_inches_per_ms());
+      if (newVelocity > curveThreshold) {
+        // here, 0 < curveThreshold < newVelocity <= maxVelocity, so we apply
+        // the curve
+        float scale = maxVelocity - curveThreshold;
+        float funcInput = (newVelocity - curveThreshold) / scale;
+        float funcOutput =
+            gVelocityCurveFunction->At(funcInput, /* aBeforeFlag = */ false);
+        float curvedVelocity = (funcOutput * scale) + curveThreshold;
+        SVT_LOG("%p|%s curving up velocity from %f to %f\n",
+                mAxis->OpaqueApzcPointer(), mAxis->Name(), newVelocity,
+                curvedVelocity);
+        newVelocity = curvedVelocity;
+      }
+    }
+
+    if (velocityIsNegative) {
+      newVelocity = -newVelocity;
+    }
+  }
+
+  return newVelocity;
+}
+
+}  // namespace layers
+}  // namespace mozilla