Introduce cadence based VideoRendererAlgorithm.

media/filters/video_renderer_algorithm.cc

Index: media/filters/video_renderer_algorithm.cc
diff --git a/media/filters/video_renderer_algorithm.cc b/media/filters/video_renderer_algorithm.cc
new file mode 100644
index 0000000000000000000000000000000000000000..6be5db08c6d6e376cf58400c20ef851df0c93c41
--- /dev/null
+++ b/media/filters/video_renderer_algorithm.cc
@@ -0,0 +1,626 @@
+// Copyright 2015 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 "media/filters/video_renderer_algorithm.h"
+
+#include <algorithm>
+#include <limits>
+
+namespace media {
+
+// The number of frames to store for moving average calculations.  Value picked
+// after experimenting with playback of various local media and YouTube clips.
+const int kMovingAverageSamples = 25;
+
+VideoRendererAlgorithm::ReadyFrame::ReadyFrame(
+    const scoped_refptr<VideoFrame>& ready_frame)
+    : frame(ready_frame),
+      ideal_render_count(0),
+      render_count(0),
+      drop_count(0) {
+}
+
+VideoRendererAlgorithm::ReadyFrame::~ReadyFrame() {
+}
+
+bool VideoRendererAlgorithm::ReadyFrame::operator<(
+    const ReadyFrame& other) const {
+  return frame->timestamp() < other.frame->timestamp();
+}
+
+VideoRendererAlgorithm::VideoRendererAlgorithm(
+    const TimeConverterCB& time_converter_cb)
+    : cadence_estimator_(base::TimeDelta::FromSeconds(
+          kMinimumAcceptableTimeBetweenGlitchesSecs)),
+      time_converter_cb_(time_converter_cb),
+      frame_duration_calculator_(kMovingAverageSamples),
+      frame_dropping_disabled_(false) {
+  DCHECK(!time_converter_cb_.is_null());
+  Reset();
+}
+
+VideoRendererAlgorithm::~VideoRendererAlgorithm() {
+}
+
+scoped_refptr<VideoFrame> VideoRendererAlgorithm::Render(
+    base::TimeTicks deadline_min,
+    base::TimeTicks deadline_max,
+    size_t* frames_dropped) {
+  DCHECK(deadline_min < deadline_max);
+
+  if (frame_queue_.empty())
+    return nullptr;
+
+  if (frames_dropped)
+    *frames_dropped = 0;
+
+  // Once Render() is called |last_frame_index_| has meaning and should thus be
+  // preserved even if better frames come in before it due to out of order
+  // timestamps.
+  have_rendered_frames_ = true;
+
+  // Step 1: Update the current render interval for subroutines.
+  render_interval_ = deadline_max - deadline_min;
+
+  // Step 2: Figure out if any intervals have been skipped since the last call
+  // to Render().  If so, we assume the last frame provided was rendered during
+  // those intervals and adjust its render count appropriately.
+  AccountForMissedIntervals(deadline_min, deadline_max);
+  last_deadline_max_ = deadline_max;
+
+  // Step 3: Update the wall clock timestamps and frame duration estimates for
+  // all frames currently in the |frame_queue_|.
+  if (!UpdateFrameStatistics()) {
+    DVLOG(2) << "Failed to update frame statistics.";
+
+    ReadyFrame& ready_frame = frame_queue_[last_frame_index_];
+    DCHECK(ready_frame.frame);
+
+    // If duration is unknown, we don't have enough frames to make a good guess
+    // about which frame to use, so always choose the first.
+    if (average_frame_duration_ == base::TimeDelta() &&
+        !ready_frame.wall_clock_time.is_null()) {
+      ++ready_frame.render_count;
+    }
+
+    return ready_frame.frame;
+  }
+
+  DCHECK_GT(average_frame_duration_, base::TimeDelta());
+
+  base::TimeDelta selected_frame_drift;
+
+  // Step 4: Attempt to find the best frame by cadence.
+  int frame_to_render = FindBestFrameByCadence();
+  if (frame_to_render >= 0) {
+    selected_frame_drift =
+        CalculateAbsoluteDriftForFrame(deadline_min, frame_to_render);
+  }
+
+  // Step 5: If no frame could be found by cadence or the selected frame exceeds
+  // acceptable drift, try to find the best frame by coverage of the deadline.
+  if (frame_to_render < 0 || selected_frame_drift > max_acceptable_drift_) {
+    int second_best_by_coverage = -1;
+    const int best_by_coverage = FindBestFrameByCoverage(
+        deadline_min, deadline_max, &second_best_by_coverage);
+
+    // If the frame was previously selected based on cadence, we're only here
+    // because the drift is too large, so even if the cadence frame has the best
+    // coverage, fallback to the second best by coverage if it has better drift.
+    if (frame_to_render == best_by_coverage && second_best_by_coverage >= 0 &&
+        CalculateAbsoluteDriftForFrame(deadline_min, second_best_by_coverage) <=
+            selected_frame_drift) {
+      frame_to_render = second_best_by_coverage;
+    } else {
+      frame_to_render = best_by_coverage;
+    }
+
+    if (frame_to_render >= 0) {
+      selected_frame_drift =
+          CalculateAbsoluteDriftForFrame(deadline_min, frame_to_render);
+    }
+  }
+
+  // Step 6: If _still_ no frame could be found by coverage, try to choose the
+  // least crappy option based on the drift from the deadline. If we're here the
+  // selection is going to be bad because it means no suitable frame has any
+  // coverage of the deadline interval.
+  if (frame_to_render < 0 || selected_frame_drift > max_acceptable_drift_)
+    frame_to_render = FindBestFrameByDrift(deadline_min, &selected_frame_drift);
+
+  last_render_had_glitch_ = selected_frame_drift > max_acceptable_drift_;
+  DVLOG_IF(2, last_render_had_glitch_)
+      << "Frame drift is too far: " << selected_frame_drift.InMillisecondsF()
+      << "ms";
+
+  DCHECK_GE(frame_to_render, 0);
+
+  // Drop some debugging information if a frame had poor cadence.
+  if (cadence_estimator_.has_cadence()) {
+    const ReadyFrame& last_frame_info = frame_queue_[last_frame_index_];
+    if (static_cast<size_t>(frame_to_render) != last_frame_index_ &&
+        last_frame_info.render_count < last_frame_info.ideal_render_count) {
+      last_render_had_glitch_ = true;
+      DVLOG(2) << "Under-rendered frame " << last_frame_info.frame->timestamp()
+               << "; only " << last_frame_info.render_count
+               << " times instead of " << last_frame_info.ideal_render_count;
+    } else if (static_cast<size_t>(frame_to_render) == last_frame_index_ &&
+               last_frame_info.render_count >=
+                   last_frame_info.ideal_render_count) {
+      DVLOG(2) << "Over-rendered frame " << last_frame_info.frame->timestamp()
+               << "; rendered " << last_frame_info.render_count + 1
+               << " times instead of " << last_frame_info.ideal_render_count;
+      last_render_had_glitch_ = true;
+    }
+  }
+
+  // Step 7: Drop frames which occur prior to the frame to be rendered. If any
+  // frame has a zero render count it should be reported as dropped.
+  if (frame_to_render > 0) {
+    if (frames_dropped) {
+      for (int i = 0; i < frame_to_render; ++i) {
+        const ReadyFrame& frame = frame_queue_[i];
+        if (frame.render_count != frame.drop_count)
+          continue;
+
+        // If frame dropping is disabled, ignore the results of the algorithm
+        // and return the earliest unrendered frame.
+        if (frame_dropping_disabled_) {
+          frame_to_render = i;
+          break;
+        }
+
+        DVLOG(2) << "Dropping unrendered (or always dropped) frame "
+                 << frame.frame->timestamp()
+                 << ", wall clock: " << frame.wall_clock_time.ToInternalValue()
+                 << " (" << frame.render_count << ", " << frame.drop_count
+                 << ")";
+        ++(*frames_dropped);
+        if (!cadence_estimator_.has_cadence() || frame.ideal_render_count)
+          last_render_had_glitch_ = true;
+      }
+    }
+
+    frame_queue_.erase(frame_queue_.begin(),
+                       frame_queue_.begin() + frame_to_render);
+  }
+
+  if (last_render_had_glitch_) {
+    DVLOG(2) << "Deadline: [" << deadline_min.ToInternalValue() << ", "
+             << deadline_max.ToInternalValue()
+             << "], Interval: " << render_interval_.InMicroseconds()
+             << ", Duration: " << average_frame_duration_.InMicroseconds();
+  }
+
+  // Step 8: Congratulations, the frame selection gauntlet has been passed!
+  last_frame_index_ = 0;
+  ++frame_queue_.front().render_count;
+  DCHECK(frame_queue_.front().frame);
+  return frame_queue_.front().frame;
+}
+
+size_t VideoRendererAlgorithm::RemoveExpiredFrames(base::TimeTicks deadline) {
+  if (!UpdateFrameStatistics() || frame_queue_.size() < 2)
+    return 0;
+
+  DCHECK_GT(average_frame_duration_, base::TimeDelta());
+
+  // Update |last_deadline_max_| if it's no longer accurate.
+  if (deadline > last_deadline_max_)
+    last_deadline_max_ = deadline;
+
+  // Finds and removes all frames which are too old to be used; I.e., the end of
+  // their render interval is further than |max_acceptable_drift_| from the
+  // given |deadline|.
+  size_t frames_to_expire = 0;
+  const base::TimeTicks minimum_frame_time =
+      deadline - max_acceptable_drift_ - average_frame_duration_;
+  for (; frames_to_expire < frame_queue_.size() - 1; ++frames_to_expire) {
+    if (frame_queue_[frames_to_expire].wall_clock_time >= minimum_frame_time)
+      break;
+  }
+
+  if (!frames_to_expire)
+    return 0;
+
+  frame_queue_.erase(frame_queue_.begin(),
+                     frame_queue_.begin() + frames_to_expire);
+
+  last_frame_index_ = last_frame_index_ > frames_to_expire
+                          ? last_frame_index_ - frames_to_expire
+                          : 0;
+  return frames_to_expire;
+}
+
+void VideoRendererAlgorithm::OnLastFrameDropped() {
+  DCHECK(have_rendered_frames_);
+  DCHECK(!frame_queue_.empty());
+  // If frames were expired by RemoveExpiredFrames() this count may be zero when
+  // the OnLastFrameDropped() call comes in.
+  if (!frame_queue_[last_frame_index_].render_count)
+    return;
+
+  ++frame_queue_[last_frame_index_].drop_count;
+  DCHECK_LE(frame_queue_[last_frame_index_].drop_count,
+            frame_queue_[last_frame_index_].render_count);
+}
+
+void VideoRendererAlgorithm::Reset() {
+  last_frame_index_ = 0;
+  have_rendered_frames_ = last_render_had_glitch_ = false;
+  last_deadline_max_ = base::TimeTicks();
+  average_frame_duration_ = render_interval_ = base::TimeDelta();
+  frame_queue_.clear();
+  cadence_estimator_.Reset();
+  frame_duration_calculator_.Reset();
+
+  // Default to ATSC IS/191 recommendations for maximum acceptable drift before
+  // we have enough frames to base the maximum on frame duration.
+  max_acceptable_drift_ = base::TimeDelta::FromMilliseconds(15);
+}
+
+size_t VideoRendererAlgorithm::EffectiveFramesQueued() const {
+  if (frame_queue_.empty() || average_frame_duration_ == base::TimeDelta() ||
+      last_deadline_max_.is_null()) {
+    return frame_queue_.size();
+  }
+
+  // If we don't have cadence, subtract off any frames which are before
+  // the last rendered frame or are past their expected rendering time.
+  if (!cadence_estimator_.has_cadence()) {
+    size_t expired_frames = last_frame_index_;
+    DCHECK_LT(last_frame_index_, frame_queue_.size());
+    for (; expired_frames < frame_queue_.size(); ++expired_frames) {
+      if (frame_queue_[expired_frames].wall_clock_time.is_null() ||
+          EndTimeForFrame(expired_frames) > last_deadline_max_) {
+        break;
+      }
+    }
+    return frame_queue_.size() - expired_frames;
+  }
+
+  // Find the first usable frame to start counting from.
+  const int start_index = FindBestFrameByCadenceInternal(nullptr);
+  if (start_index < 0)
+    return 0;
+
+  size_t renderable_frame_count = 0;
+  for (size_t i = start_index; i < frame_queue_.size(); ++i) {
+    if (frame_queue_[i].render_count < frame_queue_[i].ideal_render_count)
+      ++renderable_frame_count;
+  }
+
+  return renderable_frame_count;
+}
+
+void VideoRendererAlgorithm::EnqueueFrame(
+    const scoped_refptr<VideoFrame>& frame) {
+  DCHECK(frame);
+  DCHECK(!frame->end_of_stream());
+
+  ReadyFrame ready_frame(frame);
+  auto it = frame_queue_.empty() ? frame_queue_.end()
+                                 : std::lower_bound(frame_queue_.begin(),
+                                                    frame_queue_.end(), frame);
+  DCHECK_GE(it - frame_queue_.begin(), 0);
+
+  // If a frame was inserted before the first frame, update the index. On the
+  // next call to Render() it will be dropped.
+  if (static_cast<size_t>(it - frame_queue_.begin()) <= last_frame_index_ &&
+      have_rendered_frames_) {
+    ++last_frame_index_;
+  }
+
+  // The vast majority of cases should always append to the back, but in rare
+  // circumstance we get out of order timestamps, http://crbug.com/386551.
+  it = frame_queue_.insert(it, ready_frame);
+
+  // Project the current cadence calculations to include the new frame.  These
+  // may not be accurate until the next Render() call.  These updates are done
+  // to ensure EffectiveFramesQueued() returns a semi-reliable result.
+  if (cadence_estimator_.has_cadence())
+    UpdateCadenceForFrames();
+
+#ifndef NDEBUG
+  // Verify sorted order in debug mode.
+  for (size_t i = 0; i < frame_queue_.size() - 1; ++i) {
+    DCHECK(frame_queue_[i].frame->timestamp() <=
+           frame_queue_[i + 1].frame->timestamp());
+  }
+#endif
+}
+
+void VideoRendererAlgorithm::AccountForMissedIntervals(
+    base::TimeTicks deadline_min,
+    base::TimeTicks deadline_max) {
+  if (last_deadline_max_.is_null() || deadline_min <= last_deadline_max_ ||
+      !have_rendered_frames_) {
+    return;
+  }
+
+  DCHECK_GT(render_interval_, base::TimeDelta());
+  const int64 render_cycle_count =
+      (deadline_min - last_deadline_max_) / render_interval_;
+
+  // In the ideal case this value will be zero.
+  if (!render_cycle_count)
+    return;
+
+  DVLOG(2) << "Missed " << render_cycle_count << " Render() intervals.";
+
+  // Only update render count if the frame was rendered at all; it may not have
+  // been if the frame is at the head because we haven't rendered anything yet
+  // or because previous frames were removed via RemoveExpiredFrames().
+  ReadyFrame& ready_frame = frame_queue_[last_frame_index_];
+  if (!ready_frame.render_count)
+    return;
+
+  // If the frame was never really rendered since it was dropped each attempt,
+  // we need to increase the drop count as well to match the new render count.
+  // Otherwise we won't properly count the frame as dropped when it's discarded.
+  // We always update the render count so FindBestFrameByCadenceInternal() can
+  // properly account for potentially over-rendered frames.
+  if (ready_frame.render_count == ready_frame.drop_count)
+    ready_frame.drop_count += render_cycle_count;
+  ready_frame.render_count += render_cycle_count;
+}
+
+bool VideoRendererAlgorithm::UpdateFrameStatistics() {
+  // Figure out all current ready frame times at once so we minimize the drift
+  // relative to real time as the code below executes.
+  for (size_t i = 0; i < frame_queue_.size(); ++i) {
+    ReadyFrame& frame = frame_queue_[i];
+    const bool new_frame = frame.wall_clock_time.is_null();
+    frame.wall_clock_time = time_converter_cb_.Run(frame.frame->timestamp());
+
+    // If time stops or never started, exit immediately.
+    if (frame.wall_clock_time.is_null())
+      return false;
+
+    // TODO(dalecurtis): An unlucky tick of a playback rate change could cause
+    // this to skew so much that time goes backwards between calls.  Fix this by
+    // either converting all timestamps at once or with some retry logic.
+    if (i > 0) {
+      const base::TimeDelta delta =
+          frame.wall_clock_time - frame_queue_[i - 1].wall_clock_time;
+      CHECK_GT(delta, base::TimeDelta());
+      if (new_frame)
+        frame_duration_calculator_.AddSample(delta);
+    }
+  }
+
+  // Do we have enough frames to compute statistics?
+  const bool have_frame_duration = average_frame_duration_ != base::TimeDelta();
+  if (frame_queue_.size() < 2 && !have_frame_duration)
+    return false;
+
+  // Compute |average_frame_duration_|, a moving average of the last few frames;
+  // see kMovingAverageSamples for the exact number.
+  average_frame_duration_ = frame_duration_calculator_.Average();
+
+  // ITU-R BR.265 recommends a maximum acceptable drift of +/- half of the frame
+  // duration; there are other asymmetric, more lenient measures, that we're
+  // forgoing in favor of simplicity.
+  //
+  // We'll always allow at least 8.33ms of drift since literature suggests it's
+  // well below the floor of detection.
+  max_acceptable_drift_ = std::max(average_frame_duration_ / 2,
+                                   base::TimeDelta::FromSecondsD(1.0 / 120));
+
+  // If we were called via RemoveExpiredFrames() and Render() was never called,
+  // we may not have a render interval yet.
+  if (render_interval_ == base::TimeDelta())
+    return true;
+
+  const bool cadence_changed = cadence_estimator_.UpdateCadenceEstimate(
+      render_interval_, average_frame_duration_, max_acceptable_drift_);
+
+  // No need to update cadence if there's been no change; cadence will be set
+  // as frames are added to the queue.
+  if (!cadence_changed)
+    return true;
+
+  UpdateCadenceForFrames();
+
+  // Thus far there appears to be no need for special 3:2 considerations, the
+  // smoothness scores seem to naturally fit that pattern based on maximizing
+  // frame coverage.
+  return true;
+}
+
+void VideoRendererAlgorithm::UpdateCadenceForFrames() {
+  for (size_t i = last_frame_index_; i < frame_queue_.size(); ++i) {
+    // It's always okay to adjust the ideal render count, since the cadence
+    // selection method will still count its current render count towards
+    // cadence selection.
+    frame_queue_[i].ideal_render_count =
+        cadence_estimator_.has_cadence()
+            ? cadence_estimator_.GetCadenceForFrame(i - last_frame_index_)
+            : 0;
+  }
+}
+
+int VideoRendererAlgorithm::FindBestFrameByCadence() {
+  DCHECK(!frame_queue_.empty());
+  if (!cadence_estimator_.has_cadence())
+    return -1;
+
+  int new_ideal_render_count = 0;
+  const int best_frame =
+      FindBestFrameByCadenceInternal(&new_ideal_render_count);
+  if (best_frame < 0)
+    return -1;
+
+  DCHECK_GT(new_ideal_render_count, 0);
+  frame_queue_[best_frame].ideal_render_count = new_ideal_render_count;
+  return best_frame;
+}
+
+int VideoRendererAlgorithm::FindBestFrameByCadenceInternal(
+    int* adjusted_ideal_render_count) const {
+  DCHECK(!frame_queue_.empty());
+  DCHECK(cadence_estimator_.has_cadence());
+  const ReadyFrame& current_frame = frame_queue_[last_frame_index_];
+
+  // If the current frame is below cadence, we should prefer it.
+  if (current_frame.render_count < current_frame.ideal_render_count) {
+    if (adjusted_ideal_render_count)
+      *adjusted_ideal_render_count = current_frame.ideal_render_count;
+    return last_frame_index_;
+  }
+
+  // For over-rendered frames we need to ensure we skip frames and subtract
+  // each skipped frame's ideal cadence from the over-render count until we
+  // find a frame which still has a positive ideal render count.
+  int render_count_overage = std::max(
+      0, current_frame.render_count - current_frame.ideal_render_count);
+
+  // If the current frame is on cadence or over cadence, find the next frame
+  // with a positive ideal render count.
+  for (size_t i = last_frame_index_ + 1; i < frame_queue_.size(); ++i) {
+    const ReadyFrame& frame = frame_queue_[i];
+    if (frame.ideal_render_count > render_count_overage) {
+      if (adjusted_ideal_render_count) {
+        *adjusted_ideal_render_count =
+            frame.ideal_render_count - render_count_overage;
+      }
+      return i;
+    } else {
+      // The ideal render count should always be zero or smaller than the
+      // over-render count.
+      render_count_overage -= frame.ideal_render_count;
+      DCHECK_GE(render_count_overage, 0);
+    }
+  }
+
+  // We don't have enough frames to find a better once by cadence.
+  return -1;
+}
+
+int VideoRendererAlgorithm::FindBestFrameByCoverage(
+    base::TimeTicks deadline_min,
+    base::TimeTicks deadline_max,
+    int* second_best) const {
+  DCHECK(!frame_queue_.empty());
+
+  // Find the frame which covers the most of the interval [deadline_min,
+  // deadline_max]. Frames outside of the interval are considered to have no
+  // coverage, while those which completely overlap the interval have complete
+  // coverage.
+  int best_frame_by_coverage = -1;
+  base::TimeDelta best_coverage;
+  std::vector<base::TimeDelta> coverage(frame_queue_.size(), base::TimeDelta());
+  for (size_t i = last_frame_index_; i < frame_queue_.size(); ++i) {
+    // Frames which start after the deadline interval have zero coverage.
+    if (frame_queue_[i].wall_clock_time > deadline_max)
+      break;
+
+    // Clamp frame end times to a maximum of |deadline_max|.
+    const base::TimeTicks frame_end_time =
+        std::min(deadline_max, EndTimeForFrame(i));
+
+    // Frames entirely before the deadline interval have zero coverage.
+    if (frame_end_time < deadline_min)
+      continue;
+
+    // If we're here, the current frame overlaps the deadline in some way; so
+    // compute the duration of the interval which is covered.
+    const base::TimeDelta duration =
+        frame_end_time -
+        std::max(deadline_min, frame_queue_[i].wall_clock_time);
+
+    coverage[i] = duration;
+    if (coverage[i] > best_coverage) {
+      best_frame_by_coverage = i;
+      best_coverage = coverage[i];
+    }
+  }
+
+  // Find the second best frame by coverage; done by zeroing the coverage for
+  // the previous best and recomputing the maximum.
+  *second_best = -1;
+  if (best_frame_by_coverage >= 0) {
+    coverage[best_frame_by_coverage] = base::TimeDelta();
+    auto it = std::max_element(coverage.begin(), coverage.end());
+    if (*it > base::TimeDelta())
+      *second_best = it - coverage.begin();
+  }
+
+  // If two frames have coverage within half a millisecond, prefer the earliest
+  // frame as having the best coverage.  Value chosen via experimentation to
+  // ensure proper coverage calculation for 24fps in 60Hz where +/- 100us of
+  // jitter is present within the |render_interval_|. At 60Hz this works out to
+  // an allowed jitter of 3%.
+  const base::TimeDelta kAllowableJitter =
+      base::TimeDelta::FromMicroseconds(500);
+  if (*second_best >= 0 && best_frame_by_coverage > *second_best &&
+      (best_coverage - coverage[*second_best]).magnitude() <=
+          kAllowableJitter) {
+    std::swap(best_frame_by_coverage, *second_best);
+  }
+
+  // TODO(dalecurtis): We may want to make a better decision about what to do
+  // when multiple frames have equivalent coverage over an interval.  Jitter in
+  // the render interval may result in irregular frame selection which may be
+  // visible to a viewer.
+  //
+  // 23.974fps and 24fps in 60Hz are the most common susceptible rates, so
+  // extensive tests have been added to ensure these cases work properly.
+
+  return best_frame_by_coverage;
+}
+
+int VideoRendererAlgorithm::FindBestFrameByDrift(
+    base::TimeTicks deadline_min,
+    base::TimeDelta* selected_frame_drift) const {
+  DCHECK(!frame_queue_.empty());
+
+  int best_frame_by_drift = -1;
+  *selected_frame_drift = base::TimeDelta::Max();
+
+  for (size_t i = last_frame_index_; i < frame_queue_.size(); ++i) {
+    const base::TimeDelta drift =
+        CalculateAbsoluteDriftForFrame(deadline_min, i);
+    // We use <= here to prefer the latest frame with minimum drift.
+    if (drift <= *selected_frame_drift) {
+      *selected_frame_drift = drift;
+      best_frame_by_drift = i;
+    }
+  }
+
+  return best_frame_by_drift;
+}
+
+base::TimeDelta VideoRendererAlgorithm::CalculateAbsoluteDriftForFrame(
+    base::TimeTicks deadline_min,
+    int frame_index) const {
+  // If the frame lies before the deadline, compute the delta against the end
+  // of the frame's duration.
+  const base::TimeTicks frame_end_time = EndTimeForFrame(frame_index);
+  if (frame_end_time < deadline_min)
+    return deadline_min - frame_end_time;
+
+  // If the frame lies after the deadline, compute the delta against the frame's
+  // wall clock time.
+  const ReadyFrame& frame = frame_queue_[frame_index];
+  if (frame.wall_clock_time > deadline_min)
+    return frame.wall_clock_time - deadline_min;
+
+  // Drift is zero for frames which overlap the deadline interval.
+  DCHECK_GE(deadline_min, frame.wall_clock_time);
+  DCHECK_GE(frame_end_time, deadline_min);
+  return base::TimeDelta();
+}
+
+base::TimeTicks VideoRendererAlgorithm::EndTimeForFrame(
+    size_t frame_index) const {
+  DCHECK_LT(frame_index, frame_queue_.size());
+  DCHECK_GT(average_frame_duration_, base::TimeDelta());
+  return frame_index + 1 < frame_queue_.size()
+             ? frame_queue_[frame_index + 1].wall_clock_time
+             : frame_queue_[frame_index].wall_clock_time +
+                   average_frame_duration_;
+}
+
+}  // namespace media