"Buttery Smooth" Tab Capture.

content/browser/media/capture/video_capture_oracle_unittest.cc

 // Copyright (c) 2013 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #include "content/browser/media/capture/video_capture_oracle.h"
 
+#include <cstdlib>
+#include <utility>
+#include <vector>
+
+#include "base/logging.h"
 #include "base/strings/stringprintf.h"
 #include "base/time/time.h"
 #include "testing/gtest/include/gtest/gtest.h"
+#include "ui/gfx/geometry/rect.h"
 
 namespace content {
 namespace {
 
 void SteadyStateSampleAndAdvance(base::TimeDelta vsync,
                                  SmoothEventSampler* sampler,
                                  base::TimeTicks* t) {
   ASSERT_TRUE(sampler->AddEventAndConsiderSampling(*t));
   ASSERT_TRUE(sampler->HasUnrecordedEvent());
   sampler->RecordSample();
   ASSERT_FALSE(sampler->HasUnrecordedEvent());
   ASSERT_FALSE(sampler->IsOverdueForSamplingAt(*t));
   *t += vsync;
   ASSERT_FALSE(sampler->IsOverdueForSamplingAt(*t));
 }
 
 void SteadyStateNoSampleAndAdvance(base::TimeDelta vsync,
                                    SmoothEventSampler* sampler,
                                    base::TimeTicks* t) {
   ASSERT_FALSE(sampler->AddEventAndConsiderSampling(*t));
   ASSERT_TRUE(sampler->HasUnrecordedEvent());
   ASSERT_FALSE(sampler->IsOverdueForSamplingAt(*t));
   *t += vsync;
   ASSERT_FALSE(sampler->IsOverdueForSamplingAt(*t));
 }
 
-void TimeTicksFromString(const char* string, base::TimeTicks* t) {
+base::TimeTicks InitialTestTimeTicks() {
   base::Time time;
-  ASSERT_TRUE(base::Time::FromString(string, &time));
-  *t = base::TimeTicks::UnixEpoch() + (time - base::Time::UnixEpoch());
+  CHECK(base::Time::FromString("Sat, 23 Mar 2013 1:21:08 GMT", &time));
+  return base::TimeTicks::UnixEpoch() + (time - base::Time::UnixEpoch());
 }
 
 void TestRedundantCaptureStrategy(base::TimeDelta capture_period,
                                   int redundant_capture_goal,
                                   SmoothEventSampler* sampler,
                                   base::TimeTicks* t) {
   // Before any events have been considered, we're overdue for sampling.
   ASSERT_TRUE(sampler->IsOverdueForSamplingAt(*t));
 
   // Consider the first event.  We want to sample that.
   ASSERT_FALSE(sampler->HasUnrecordedEvent());
   ASSERT_TRUE(sampler->AddEventAndConsiderSampling(*t));
   ASSERT_TRUE(sampler->HasUnrecordedEvent());
   sampler->RecordSample();
   ASSERT_FALSE(sampler->HasUnrecordedEvent());
 
-  // After more than one capture period has passed without considering an event,
-  // we should repeatedly be overdue for sampling.  However, once the redundant
-  // capture goal is achieved, we should no longer be overdue for sampling.
-  *t += capture_period * 4;
+  // After more than 250 ms has passed without considering an event, we should
+  // repeatedly be overdue for sampling.  However, once the redundant capture
+  // goal is achieved, we should no longer be overdue for sampling.
+  *t += base::TimeDelta::FromMilliseconds(250);
   for (int i = 0; i < redundant_capture_goal; i++) {
     SCOPED_TRACE(base::StringPrintf("Iteration %d", i));
     ASSERT_FALSE(sampler->HasUnrecordedEvent());
     ASSERT_TRUE(sampler->IsOverdueForSamplingAt(*t))
         << "Should sample until redundant capture goal is hit";
     sampler->RecordSample();
     *t += capture_period;  // Timer fires once every capture period.
   }
   ASSERT_FALSE(sampler->IsOverdueForSamplingAt(*t))
       << "Should not be overdue once redundant capture goal achieved.";
 }
 
 // 60Hz sampled at 30Hz should produce 30Hz.  In addition, this test contains
 // much more comprehensive before/after/edge-case scenarios than the others.
 TEST(SmoothEventSamplerTest, Sample60HertzAt30Hertz) {
   const base::TimeDelta capture_period = base::TimeDelta::FromSeconds(1) / 30;
   const int redundant_capture_goal = 200;
   const base::TimeDelta vsync = base::TimeDelta::FromSeconds(1) / 60;
 
   SmoothEventSampler sampler(capture_period, true, redundant_capture_goal);
-  base::TimeTicks t;
-  TimeTicksFromString("Sat, 23 Mar 2013 1:21:08 GMT", &t);
+  base::TimeTicks t = InitialTestTimeTicks();
 
   TestRedundantCaptureStrategy(capture_period, redundant_capture_goal,
                                &sampler, &t);
 
   // Steady state, we should capture every other vsync, indefinitely.
   for (int i = 0; i < 100; i++) {
     SCOPED_TRACE(base::StringPrintf("Iteration %d", i));
     SteadyStateSampleAndAdvance(vsync, &sampler, &t);
     SteadyStateNoSampleAndAdvance(vsync, &sampler, &t);
   }
@@ skipping 18 matching lines @@
   }
 }
 
 // 50Hz sampled at 30Hz should produce a sequence where some frames are skipped.
 TEST(SmoothEventSamplerTest, Sample50HertzAt30Hertz) {
   const base::TimeDelta capture_period = base::TimeDelta::FromSeconds(1) / 30;
   const int redundant_capture_goal = 2;
   const base::TimeDelta vsync = base::TimeDelta::FromSeconds(1) / 50;
 
   SmoothEventSampler sampler(capture_period, true, redundant_capture_goal);
-  base::TimeTicks t;
-  TimeTicksFromString("Sat, 23 Mar 2013 1:21:08 GMT", &t);
+  base::TimeTicks t = InitialTestTimeTicks();
 
   TestRedundantCaptureStrategy(capture_period, redundant_capture_goal,
                                &sampler, &t);
 
   // Steady state, we should capture 1st, 2nd and 4th frames out of every five
   // frames, indefinitely.
   for (int i = 0; i < 100; i++) {
     SCOPED_TRACE(base::StringPrintf("Iteration %d", i));
     SteadyStateSampleAndAdvance(vsync, &sampler, &t);
     SteadyStateSampleAndAdvance(vsync, &sampler, &t);
@@ skipping 24 matching lines @@
   }
 }
 
 // 75Hz sampled at 30Hz should produce a sequence where some frames are skipped.
 TEST(SmoothEventSamplerTest, Sample75HertzAt30Hertz) {
   const base::TimeDelta capture_period = base::TimeDelta::FromSeconds(1) / 30;
   const int redundant_capture_goal = 32;
   const base::TimeDelta vsync = base::TimeDelta::FromSeconds(1) / 75;
 
   SmoothEventSampler sampler(capture_period, true, redundant_capture_goal);
-  base::TimeTicks t;
-  TimeTicksFromString("Sat, 23 Mar 2013 1:21:08 GMT", &t);
+  base::TimeTicks t = InitialTestTimeTicks();
 
   TestRedundantCaptureStrategy(capture_period, redundant_capture_goal,
                                &sampler, &t);
 
   // Steady state, we should capture 1st and 3rd frames out of every five
   // frames, indefinitely.
   SteadyStateSampleAndAdvance(vsync, &sampler, &t);
   SteadyStateNoSampleAndAdvance(vsync, &sampler, &t);
   for (int i = 0; i < 100; i++) {
     SCOPED_TRACE(base::StringPrintf("Iteration %d", i));
@@ skipping 28 matching lines @@
   }
 }
 
 // 30Hz sampled at 30Hz should produce 30Hz.
 TEST(SmoothEventSamplerTest, Sample30HertzAt30Hertz) {
   const base::TimeDelta capture_period = base::TimeDelta::FromSeconds(1) / 30;
   const int redundant_capture_goal = 1;
   const base::TimeDelta vsync = base::TimeDelta::FromSeconds(1) / 30;
 
   SmoothEventSampler sampler(capture_period, true, redundant_capture_goal);
-  base::TimeTicks t;
-  TimeTicksFromString("Sat, 23 Mar 2013 1:21:08 GMT", &t);
+  base::TimeTicks t = InitialTestTimeTicks();
 
   TestRedundantCaptureStrategy(capture_period, redundant_capture_goal,
                                &sampler, &t);
 
   // Steady state, we should capture every vsync, indefinitely.
   for (int i = 0; i < 200; i++) {
     SCOPED_TRACE(base::StringPrintf("Iteration %d", i));
     SteadyStateSampleAndAdvance(vsync, &sampler, &t);
   }
 
@@ skipping 14 matching lines @@
   }
 }
 
 // 24Hz sampled at 30Hz should produce 24Hz.
 TEST(SmoothEventSamplerTest, Sample24HertzAt30Hertz) {
   const base::TimeDelta capture_period = base::TimeDelta::FromSeconds(1) / 30;
   const int redundant_capture_goal = 333;
   const base::TimeDelta vsync = base::TimeDelta::FromSeconds(1) / 24;
 
   SmoothEventSampler sampler(capture_period, true, redundant_capture_goal);
-  base::TimeTicks t;
-  TimeTicksFromString("Sat, 23 Mar 2013 1:21:08 GMT", &t);
+  base::TimeTicks t = InitialTestTimeTicks();
 
   TestRedundantCaptureStrategy(capture_period, redundant_capture_goal,
                                &sampler, &t);
 
   // Steady state, we should capture every vsync, indefinitely.
   for (int i = 0; i < 200; i++) {
     SCOPED_TRACE(base::StringPrintf("Iteration %d", i));
     SteadyStateSampleAndAdvance(vsync, &sampler, &t);
   }
 
@@ skipping 12 matching lines @@
     SCOPED_TRACE(base::StringPrintf("Iteration %d", i));
     SteadyStateSampleAndAdvance(vsync, &sampler, &t);
   }
 }
 
 TEST(SmoothEventSamplerTest, DoubleDrawAtOneTimeStillDirties) {
   const base::TimeDelta capture_period = base::TimeDelta::FromSeconds(1) / 30;
   const base::TimeDelta overdue_period = base::TimeDelta::FromSeconds(1);
 
   SmoothEventSampler sampler(capture_period, true, 1);
-  base::TimeTicks t;
-  TimeTicksFromString("Sat, 23 Mar 2013 1:21:08 GMT", &t);
+  base::TimeTicks t = InitialTestTimeTicks();
 
   ASSERT_TRUE(sampler.AddEventAndConsiderSampling(t));
   sampler.RecordSample();
   ASSERT_FALSE(sampler.IsOverdueForSamplingAt(t))
       << "Sampled last event; should not be dirty.";
   t += overdue_period;
 
   // Now simulate 2 events with the same clock value.
   ASSERT_TRUE(sampler.AddEventAndConsiderSampling(t));
   sampler.RecordSample();
   ASSERT_FALSE(sampler.AddEventAndConsiderSampling(t))
       << "Two events at same time -- expected second not to be sampled.";
   ASSERT_TRUE(sampler.IsOverdueForSamplingAt(t + overdue_period))
       << "Second event should dirty the capture state.";
   sampler.RecordSample();
   ASSERT_FALSE(sampler.IsOverdueForSamplingAt(t + overdue_period));
 }
 
 TEST(SmoothEventSamplerTest, FallbackToPollingIfUpdatesUnreliable) {
   const base::TimeDelta timer_interval = base::TimeDelta::FromSeconds(1) / 30;
 
   SmoothEventSampler should_not_poll(timer_interval, true, 1);
   SmoothEventSampler should_poll(timer_interval, false, 1);
-  base::TimeTicks t;
-  TimeTicksFromString("Sat, 23 Mar 2013 1:21:08 GMT", &t);
+  base::TimeTicks t = InitialTestTimeTicks();
 
   // Do one round of the "happy case" where an event was received and
   // RecordSample() was called by the client.
   ASSERT_TRUE(should_not_poll.AddEventAndConsiderSampling(t));
   ASSERT_TRUE(should_poll.AddEventAndConsiderSampling(t));
   should_not_poll.RecordSample();
   should_poll.RecordSample();
 
-  // One time period ahead, neither sampler says we're overdue.
-  for (int i = 0; i < 3; i++) {
+  // For the following time period, before 250 ms has elapsed, neither sampler
+  // says we're overdue.
+  const int non_overdue_intervals = static_cast<int>(
+      base::TimeDelta::FromMilliseconds(250) / timer_interval);
+  for (int i = 0; i < non_overdue_intervals; i++) {
     t += timer_interval;
     ASSERT_FALSE(should_not_poll.IsOverdueForSamplingAt(t))
         << "Sampled last event; should not be dirty.";
     ASSERT_FALSE(should_poll.IsOverdueForSamplingAt(t))
         << "Dirty interval has not elapsed yet.";
   }
 
   // Next time period ahead, both samplers say we're overdue.  The non-polling
   // sampler is returning true here because it has been configured to allow one
   // redundant capture.
-  t += timer_interval;
+  t += timer_interval;  // Step past the 250 ms threshold.
   ASSERT_TRUE(should_not_poll.IsOverdueForSamplingAt(t))
       << "Sampled last event; is dirty one time only to meet redundancy goal.";
   ASSERT_TRUE(should_poll.IsOverdueForSamplingAt(t))
       << "If updates are unreliable, must fall back to polling when idle.";
   should_not_poll.RecordSample();
   should_poll.RecordSample();
 
   // Forever more, the non-polling sampler returns false while the polling one
   // returns true.
   for (int i = 0; i < 100; ++i) {
@@ skipping 13 matching lines @@
 }
 
 struct DataPoint {
   bool should_capture;
   double increment_ms;
 };
 
 void ReplayCheckingSamplerDecisions(const DataPoint* data_points,
                                     size_t num_data_points,
                                     SmoothEventSampler* sampler) {
-  base::TimeTicks t;
-  TimeTicksFromString("Sat, 23 Mar 2013 1:21:08 GMT", &t);
+  base::TimeTicks t = InitialTestTimeTicks();
   for (size_t i = 0; i < num_data_points; ++i) {
     t += base::TimeDelta::FromMicroseconds(
         static_cast<int64>(data_points[i].increment_ms * 1000));
     ASSERT_EQ(data_points[i].should_capture,
               sampler->AddEventAndConsiderSampling(t))
         << "at data_points[" << i << ']';
     if (data_points[i].should_capture)
       sampler->RecordSample();
   }
 }
@@ skipping 95 matching lines @@
     { true, 33.44 }, { false, 0 }, { true, 33.441 }, { false, 16.72 },
     { true, 16.72 }, { true, 50.16 }, { false, 0 }, { true, 16.72 },
     { true, 33.441 }, { false, 0 }, { true, 33.44 }, { false, 16.72 },
     { true, 33.44 }, { false, 0 }, { true, 16.721 }, { true, 50.161 },
     { false, 0 }, { true, 16.72 }, { true, 33.44 }, { false, 0 },
     { true, 33.441 }, { false, 16.72 }, { true, 16.72 }, { true, 50.16 }
   };
 
   SmoothEventSampler sampler(base::TimeDelta::FromSeconds(1) / 30, true, 3);
   ReplayCheckingSamplerDecisions(data_points, arraysize(data_points), &sampler);
 }

[ncarter (slow), 2014/07/26 00:57:57]

As I look through these I think we're missing a test for when the clock moves
back in time. I think SmoothEventSampler used to do handle that gracefully (by
dumping its state when monotonicity is violated).  As far as I can tell, the new
sampler will shut down in that case.

Is the idea that the use of TimeTicks() ought to guarantee monotonicity?

[miu, 2014/07/28 21:54:12]

Yes.  This code was all originally written back when base::Time was being used
instead in base::TimeTicks.  (mcasas@, IIRC, replaced the data type in response
to my migration of all of src/media to base::TimeTicks.)  So, there are still
vestiges of code that deals with clock skew which shouldn't be a problem
anymore.

I think I got all the code in video_capture_oracle.cc cleaned up, w.r.t. this
issue, but I'll look over it again, and also take a closer look at the tests.

 
+// A test scenario for AnimatedContentSamplerTest.
+struct Scenario {
+  base::TimeDelta vsync_interval;  // Compositor's update rate.
+  base::TimeDelta min_capture_period;  // Maximum capture rate.
+  base::TimeDelta content_period;  // Animating content frame rate.
+
+  Scenario(base::TimeDelta v, base::TimeDelta m, base::TimeDelta c)
+      : vsync_interval(v), min_capture_period(m), content_period(c) {}
+};
+
+// Value printer for Scenario.
+::std::ostream& operator<<(::std::ostream& os, const Scenario& s) {
+  return os << "{ vsync_interval=" << s.vsync_interval.InMicroseconds()
+            << ", min_capture_period=" << s.min_capture_period.InMicroseconds()
+            << ", content_period=" << s.content_period.InMicroseconds()
+            << " }";
+}
+
+class AnimatedContentSamplerTest : public ::testing::TestWithParam<Scenario> {
+ public:
+  AnimatedContentSamplerTest()
+      : count_dropped_frames_(0), count_sampled_frames_(0) {}
+
+  virtual void SetUp() OVERRIDE {
+    const base::TimeDelta since_epoch =
+        InitialTestTimeTicks() - base::TimeTicks::UnixEpoch();
+    srand(static_cast<unsigned int>(since_epoch.InMicroseconds()));
+    sampler_.reset(new AnimatedContentSampler(GetParam().min_capture_period));
+  }
+
+ protected:
+  typedef std::pair<gfx::Rect, base::TimeTicks> Event;
+
+  AnimatedContentSampler* sampler() const {
+    return sampler_.get();
+  }
+
+  std::vector<Event> GenerateEventSequence(base::TimeTicks begin,
+                                           base::TimeTicks end,
+                                           bool include_content_frame_events,
+                                           bool include_random_events) {
+    DCHECK(GetParam().content_period >= GetParam().vsync_interval);
+    base::TimeTicks next_content_time = begin - GetParam().content_period;
+    std::vector<Event> events;
+    for (base::TimeTicks compositor_time = begin; compositor_time < end;
+         compositor_time += GetParam().vsync_interval) {
+      if (include_content_frame_events && next_content_time < compositor_time) {
+        events.push_back(Event(GetContentDamageRect(), compositor_time));
+        next_content_time += GetParam().content_period;
+      } else if (include_random_events && GetRandomInRange(0, 5) == 0) {
+        events.push_back(Event(GetRandomDamageRect(), compositor_time));
+      }
+    }
+
+    return events;
+  }
+
+  void ResetFrameCounters() {
+    count_dropped_frames_ = 0;
+    count_sampled_frames_ = 0;
+  }
+
+  // Keep track what the sampler is proposing, and call RecordSample() if it
+  // proposes sampling |event|.
+  void ClientDoesWhatSamplerProposes(const Event& event) {
+    if (sampler_->next_frame_timestamp().is_null()) {
+      if (event.first == GetContentDamageRect())
+        ++count_dropped_frames_;
+    } else {
+      EXPECT_EQ(GetContentDamageRect(), event.first);
+      sampler_->RecordSample(sampler_->next_frame_timestamp());
+      ++count_sampled_frames_;
+    }
+  }
+
+  // RecordSample() is not called, but for testing, keep track of what the
+  // sampler is proposing for |event|.
+  void ClientCannotSampleFrame(const Event& event) {
+    if (sampler_->next_frame_timestamp().is_null()) {
+      if (event.first == GetContentDamageRect())
+        ++count_dropped_frames_;
+    } else {
+      EXPECT_EQ(GetContentDamageRect(), event.first);
+      ++count_sampled_frames_;
+    }
+  }
+
+  void ExpectFrameDropRatioIsCorrect() {
+    const double content_framerate =
+        1000000.0 / GetParam().content_period.InMicroseconds();
+    const double capture_framerate =
+        1000000.0 / GetParam().min_capture_period.InMicroseconds();
+    const double expected_drop_rate = std::max(
+        0.0, (content_framerate - capture_framerate) / capture_framerate);
+    const double actual_drop_rate =
+        static_cast<double>(count_dropped_frames_) / count_sampled_frames_;
+    EXPECT_NEAR(expected_drop_rate, actual_drop_rate, 0.01);
+  }
+
+  static int GetRandomInRange(int begin, int end) {
+    const int len = end - begin;
+    const int rand_offset = (len == 0) ? 0 : (rand() % (end - begin));
+    return begin + rand_offset;
+  }
+
+  static gfx::Rect GetRandomDamageRect() {
+    return gfx::Rect(0, 0, GetRandomInRange(1, 600), GetRandomInRange(1, 600));
+  }
+
+  static gfx::Rect GetContentDamageRect() {
+    // This must be distinct from anything GetRandomDamageRect() could return.
+    return gfx::Rect(0, 0, 1280, 720);
+  }
+
+ private:
+  scoped_ptr<AnimatedContentSampler> sampler_;
+
+  // These counters only include the frames with the desired content.
+  int count_dropped_frames_;
+  int count_sampled_frames_;
+};
+
+// Tests that the implementation locks in/out of frames containing stable
+// animated content, whether or not random events are also simultaneously
+// present.
+TEST_P(AnimatedContentSamplerTest, LocksIntoMajorityAnimatedContent) {
+  // |begin| refers to the start of an event sequence in terms of the
+  // Compositor's clock.
+  base::TimeTicks begin = InitialTestTimeTicks();
+
+  // Provide three minutes of random events and expect no lock-in.
+  EXPECT_TRUE(sampler()->next_frame_timestamp().is_null());
+  base::TimeTicks end = begin + base::TimeDelta::FromMinutes(3);
+  std::vector<Event> events = GenerateEventSequence(begin, end, false, true);
+  for (std::vector<Event>::const_iterator i = events.begin(); i != events.end();
+       ++i) {
+    EXPECT_FALSE(sampler()->ConsiderPresentationEvent(i->first, i->second));
+    EXPECT_TRUE(sampler()->next_frame_timestamp().is_null());
+    sampler()->RecordSample(i->second);
+  }
+  begin = end;
+
+  // Provide content frame events with some random events mixed-in, and expect
+  // the sampler to lock-in once 1000 ms has elapsed, and also to remain in a
+  // continuous lock-in 1250 ms after that.
+  end = begin + base::TimeDelta::FromSeconds(10);
+  events = GenerateEventSequence(begin, end, true, true);
+  bool is_locked_in = false;
+  ResetFrameCounters();
+  for (std::vector<Event>::const_iterator i = events.begin(); i != events.end();
+       ++i) {
+    const base::TimeDelta elapsed = i->second - events.begin()->second;
+    if (elapsed < base::TimeDelta::FromMilliseconds(1000)) {
+      EXPECT_FALSE(sampler()->ConsiderPresentationEvent(i->first, i->second));
+      sampler()->RecordSample(i->second);
+    } else {
+      if (sampler()->ConsiderPresentationEvent(i->first, i->second)) {
+        is_locked_in = true;
+        ClientDoesWhatSamplerProposes(*i);
+      } else {
+        if (elapsed > base::TimeDelta::FromMilliseconds(1250))
+          EXPECT_FALSE(is_locked_in);
+        EXPECT_TRUE(sampler()->next_frame_timestamp().is_null());
+        sampler()->RecordSample(i->second);
+      }
+    }
+  }
+  EXPECT_TRUE(is_locked_in);
+  ExpectFrameDropRatioIsCorrect();
+  begin = end;
+
+  // Continue providing content frame events without random events mixed-in and
+  // expect the lock-in to hold.
+  end = begin + base::TimeDelta::FromSeconds(30);
+  events = GenerateEventSequence(begin, end, true, false);
+  ResetFrameCounters();
+  for (std::vector<Event>::const_iterator i = events.begin(); i != events.end();
+       ++i) {
+    EXPECT_TRUE(sampler()->ConsiderPresentationEvent(i->first, i->second));
+    ClientDoesWhatSamplerProposes(*i);
+  }
+  ExpectFrameDropRatioIsCorrect();
+  begin = end;
+
+  // Continue providing content frame events and expect the lock-in to hold.
+  // RecordSample() is only sometimes called, which simulates the capture
+  // pipeline experiencing back pressure.
+  end = begin + base::TimeDelta::FromSeconds(30);
+  events = GenerateEventSequence(begin, end, true, false);
+  ResetFrameCounters();
+  for (std::vector<Event>::const_iterator i = events.begin(); i != events.end();
+       ++i) {
+    EXPECT_TRUE(sampler()->ConsiderPresentationEvent(i->first, i->second));
+    if (GetRandomInRange(0, 2) == 0)
+      ClientCannotSampleFrame(*i);
+    else
+      ClientDoesWhatSamplerProposes(*i);
+  }
+  ExpectFrameDropRatioIsCorrect();
+  begin = end;
+
+  // Provide a half-second of random events only, and expect the lock-in to be
+  // broken.
+  end = begin + base::TimeDelta::FromMilliseconds(500);
+  events = GenerateEventSequence(begin, end, false, true);
+  is_locked_in = true;
+  for (std::vector<Event>::const_iterator i = events.begin(); i != events.end();
+       ++i) {
+    if (sampler()->ConsiderPresentationEvent(i->first, i->second)) {
+      EXPECT_TRUE(is_locked_in);
+      ClientDoesWhatSamplerProposes(*i);
+    } else {
+      is_locked_in = false;
+      EXPECT_TRUE(sampler()->next_frame_timestamp().is_null());
+      sampler()->RecordSample(i->second);
+    }
+  }
+  EXPECT_FALSE(is_locked_in);
+  begin = end;
+
+  // Now, go back to providing content frame events, and expect the sampler to
+  // lock-in once again.
+  end = begin + base::TimeDelta::FromSeconds(10);
+  events = GenerateEventSequence(begin, end, true, false);
+  for (std::vector<Event>::const_iterator i = events.begin(); i != events.end();
+       ++i) {
+    const base::TimeDelta elapsed = i->second - events.begin()->second;
+    if (elapsed < base::TimeDelta::FromMilliseconds(1000)) {
+      EXPECT_FALSE(sampler()->ConsiderPresentationEvent(i->first, i->second));
+      sampler()->RecordSample(i->second);
+    } else {
+      if (sampler()->ConsiderPresentationEvent(i->first, i->second)) {
+        is_locked_in = true;
+        ClientDoesWhatSamplerProposes(*i);
+      } else {
+        if (elapsed > base::TimeDelta::FromMilliseconds(1250))
+          EXPECT_FALSE(is_locked_in);
+        EXPECT_TRUE(sampler()->next_frame_timestamp().is_null());
+        sampler()->RecordSample(i->second);
+      }
+    }
+  }
+  EXPECT_TRUE(is_locked_in);
+  begin = end;
+}
+
+// Tests that the frame timestamps are smooth; meaning, that when run through a
+// simulated compositor, each frame is held displayed for the right number of
+// v-sync intervals.
+TEST_P(AnimatedContentSamplerTest, FrameTimestampsAreSmooth) {
+  // Generate 30 seconds of animated content events, run the events through
+  // AnimatedContentSampler, and record all frame timestamps being proposed
+  // once lock-in is continuous.
+  base::TimeTicks begin = InitialTestTimeTicks();
+  std::vector<Event> events = GenerateEventSequence(
+      begin,
+      begin + base::TimeDelta::FromSeconds(30),
+      true,
+      false);
+  typedef std::vector<base::TimeTicks> Timestamps;
+  Timestamps frame_timestamps;
+  for (std::vector<Event>::const_iterator i = events.begin(); i != events.end();
+       ++i) {
+    if (sampler()->ConsiderPresentationEvent(i->first, i->second)) {
+      if (!sampler()->next_frame_timestamp().is_null()) {
+        frame_timestamps.push_back(sampler()->next_frame_timestamp());
+        sampler()->RecordSample(sampler()->next_frame_timestamp());
+      }
+    } else {
+      frame_timestamps.clear();  // Reset until continuous lock-in.
+    }
+  }
+  ASSERT_LE(2u, frame_timestamps.size());
+
+  // Iterate through the |frame_timestamps|, building a histogram counting the
+  // number of times each frame was displayed k times.  For example, 10 frames
+  // of 30 Hz content on a 60 Hz v-sync interval should result in
+  // display_counts[2] == 10.  Quit early if any one frame was obviously
+  // repeated too many times.
+  const int64 max_expected_repeats_per_frame = 1 +
+      std::max(GetParam().min_capture_period, GetParam().content_period) /
+          GetParam().vsync_interval;
+  std::vector<size_t> display_counts(max_expected_repeats_per_frame + 1, 0);
+  base::TimeTicks last_present_time = frame_timestamps.front();
+  for (Timestamps::const_iterator i = frame_timestamps.begin() + 1;
+       i != frame_timestamps.end(); ++i) {
+    const size_t num_vsync_intervals = static_cast<size_t>(
+        (*i - last_present_time) / GetParam().vsync_interval);
+    ASSERT_LT(0u, num_vsync_intervals);
+    ASSERT_GT(display_counts.size(), num_vsync_intervals);  // Quit early.
+    ++display_counts[num_vsync_intervals];
+    last_present_time += num_vsync_intervals * GetParam().vsync_interval;
+  }
+
+  // Analyze the histogram for an expected result pattern.  If the frame
+  // timestamps are smooth, there should only be one or two buckets with
+  // non-zero counts and they should be next to each other.  Because the clock
+  // precision for the event_times provided to the sampler is very granular
+  // (i.e., the vsync_interval), it's okay if other buckets have a tiny "stray"
+  // count in this test.
+  size_t highest_count = 0;
+  size_t second_highest_count = 0;
+  for (size_t repeats = 1; repeats < display_counts.size(); ++repeats) {
+    DVLOG(1) << "display_counts[" << repeats << "] is "
+             << display_counts[repeats];
+    if (display_counts[repeats] >= highest_count) {
+      second_highest_count = highest_count;
+      highest_count = display_counts[repeats];
+    } else if (display_counts[repeats] > second_highest_count) {
+      second_highest_count = display_counts[repeats];
+    }
+  }
+  size_t stray_count_remaining =
+      (frame_timestamps.size() - 1) - (highest_count + second_highest_count);
+  // Expect no more than 0.5% of frames fall outside the two main buckets.
+  EXPECT_GT(frame_timestamps.size() * 5 / 1000, stray_count_remaining);
+  for (size_t repeats = 1; repeats < display_counts.size() - 1; ++repeats) {
+    if (display_counts[repeats] == highest_count) {
+      EXPECT_EQ(second_highest_count, display_counts[repeats + 1]);
+      ++repeats;
+    } else if (display_counts[repeats] == second_highest_count) {
+      EXPECT_EQ(highest_count, display_counts[repeats + 1]);
+      ++repeats;
+    } else {
+      EXPECT_GE(stray_count_remaining, display_counts[repeats]);
+      stray_count_remaining -= display_counts[repeats];
+    }
+  }
+}
+
+base::TimeDelta FpsAsPeriod(int frame_rate) {
+  return base::TimeDelta::FromSeconds(1) / frame_rate;
+}
+
+INSTANTIATE_TEST_CASE_P(
+    ,
+    AnimatedContentSamplerTest,
+    ::testing::Values(
+         // Typical frame rate content: Compositor runs at 60 Hz, capture at 30
+         // Hz, and content video animates at 30, 25, or 24 Hz.
+         Scenario(FpsAsPeriod(60), FpsAsPeriod(30), FpsAsPeriod(30)),
+         Scenario(FpsAsPeriod(60), FpsAsPeriod(30), FpsAsPeriod(25)),
+         Scenario(FpsAsPeriod(60), FpsAsPeriod(30), FpsAsPeriod(24)),
+
+         // High frame rate content that leverages the Compositor's
+         // capabilities, but capture is still at 30 Hz.
+         Scenario(FpsAsPeriod(60), FpsAsPeriod(30), FpsAsPeriod(60)),
+         Scenario(FpsAsPeriod(60), FpsAsPeriod(30), FpsAsPeriod(50)),
+         Scenario(FpsAsPeriod(60), FpsAsPeriod(30), FpsAsPeriod(48)),
+
+         // High frame rate content that leverages the Compositor's
+         // capabilities, and capture is also a buttery 60 Hz.
+         Scenario(FpsAsPeriod(60), FpsAsPeriod(60), FpsAsPeriod(60)),
+         Scenario(FpsAsPeriod(60), FpsAsPeriod(60), FpsAsPeriod(50)),
+         Scenario(FpsAsPeriod(60), FpsAsPeriod(60), FpsAsPeriod(48)),
+
+         // On some platforms, the Compositor runs at 50 Hz.
+         Scenario(FpsAsPeriod(50), FpsAsPeriod(30), FpsAsPeriod(30)),
+         Scenario(FpsAsPeriod(50), FpsAsPeriod(30), FpsAsPeriod(25)),
+         Scenario(FpsAsPeriod(50), FpsAsPeriod(30), FpsAsPeriod(24)),
+         Scenario(FpsAsPeriod(50), FpsAsPeriod(30), FpsAsPeriod(50)),
+         Scenario(FpsAsPeriod(50), FpsAsPeriod(30), FpsAsPeriod(48)),
+
+         // Stable, but non-standard content frame rates.
+         Scenario(FpsAsPeriod(60), FpsAsPeriod(30), FpsAsPeriod(16)),
+         Scenario(FpsAsPeriod(60), FpsAsPeriod(30), FpsAsPeriod(20)),
+         Scenario(FpsAsPeriod(60), FpsAsPeriod(30), FpsAsPeriod(23)),
+         Scenario(FpsAsPeriod(60), FpsAsPeriod(30), FpsAsPeriod(26)),
+         Scenario(FpsAsPeriod(60), FpsAsPeriod(30), FpsAsPeriod(27)),
+         Scenario(FpsAsPeriod(60), FpsAsPeriod(30), FpsAsPeriod(28)),
+         Scenario(FpsAsPeriod(60), FpsAsPeriod(30), FpsAsPeriod(29)),
+         Scenario(FpsAsPeriod(60), FpsAsPeriod(30), FpsAsPeriod(31)),
+         Scenario(FpsAsPeriod(60), FpsAsPeriod(30), FpsAsPeriod(32)),
+         Scenario(FpsAsPeriod(60), FpsAsPeriod(30), FpsAsPeriod(33))));
+

[ncarter (slow), 2014/07/26 00:57:58]

Given how the oracle in practice drives both samplers, and the samplers could
possibly interact with each other, should there also be tests that exercise the
fused-together samplers?

For that, you might consider capturing traces in the manner of the data_points[]
type test.

[miu, 2014/07/28 21:54:12]

Good point.  I'll work on adding this.

[miu, 2014/07/31 01:25:32]

Done.

 }  // namespace
 }  // namespace content