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|
// 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 = 32;
VideoRendererAlgorithm::ReadyFrame::ReadyFrame(
const scoped_refptr<VideoFrame>& ready_frame)
: frame(ready_frame),
has_estimated_end_time(true),
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 TimeSource::WallClockTimeCB& wall_clock_time_cb)
: cadence_estimator_(base::TimeDelta::FromSeconds(
kMinimumAcceptableTimeBetweenGlitchesSecs)),
wall_clock_time_cb_(wall_clock_time_cb),
frame_duration_calculator_(kMovingAverageSamples),
frame_dropping_disabled_(false) {
DCHECK(!wall_clock_time_cb_.is_null());
Reset();
}
VideoRendererAlgorithm::~VideoRendererAlgorithm() {
}
scoped_refptr<VideoFrame> VideoRendererAlgorithm::Render(
base::TimeTicks deadline_min,
base::TimeTicks deadline_max,
size_t* frames_dropped) {
DCHECK_LT(deadline_min, deadline_max);
if (frame_queue_.empty())
return nullptr;
if (frames_dropped)
*frames_dropped = frames_dropped_during_enqueue_;
frames_dropped_during_enqueue_ = 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);
// Step 3: Update the wall clock timestamps and frame duration estimates for
// all frames currently in the |frame_queue_|.
UpdateFrameStatistics();
const bool have_known_duration = average_frame_duration_ > base::TimeDelta();
if (!(was_time_moving_ && have_known_duration)) {
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 (was_time_moving_ && !have_known_duration)
++ready_frame.render_count;
return ready_frame.frame;
}
last_deadline_max_ = deadline_max;
base::TimeDelta selected_frame_drift;
// Step 4: Attempt to find the best frame by cadence.
int cadence_overage = 0;
const int cadence_frame =
FindBestFrameByCadence(first_frame_ ? nullptr : &cadence_overage);
int frame_to_render = cadence_frame;
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);
const bool ignored_cadence_frame =
cadence_frame >= 0 && frame_to_render != cadence_frame;
if (ignored_cadence_frame) {
cadence_overage = 0;
DVLOG(2) << "Cadence frame overridden by drift: " << 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.start_time.ToInternalValue()
<< " (" << frame.render_count << ", " << frame.drop_count
<< ")";
++(*frames_dropped);
if (!cadence_estimator_.has_cadence() || frame.ideal_render_count)
last_render_had_glitch_ = true;
}
}
// Increment the frame counter for all frames removed after the last
// rendered frame.
cadence_frame_counter_ += frame_to_render - last_frame_index_;
frame_queue_.erase(frame_queue_.begin(),
frame_queue_.begin() + frame_to_render);
}
if (last_render_had_glitch_ && !first_frame_) {
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;
// If we ended up choosing a frame selected by cadence, carry over the overage
// values from the previous frame. Overage is treated as having been
// displayed and dropped for each count. If the frame wasn't selected by
// cadence, |cadence_overage| will be zero.
//
// We also don't want to start counting render counts until the first frame
// has reached its presentation time; which is considered to be when its
// start time is at most |render_interval_| / 2 before |deadline_min|.
if (!first_frame_ ||
deadline_min >= frame_queue_.front().start_time - render_interval_ / 2) {
// Ignore one frame of overage if the last call to Render() ignored the
// frame selected by cadence due to drift.
if (last_render_ignored_cadence_frame_ && cadence_overage > 0)
cadence_overage -= 1;
last_render_ignored_cadence_frame_ = ignored_cadence_frame;
frame_queue_.front().render_count += cadence_overage + 1;
frame_queue_.front().drop_count += cadence_overage;
// Once we reach a glitch in our cadence sequence, reset the base frame
// number used for defining the cadence sequence.
if (ignored_cadence_frame) {
cadence_frame_counter_ = 0;
UpdateCadenceForFrames();
}
first_frame_ = false;
}
DCHECK(frame_queue_.front().frame);
return frame_queue_.front().frame;
}
size_t VideoRendererAlgorithm::RemoveExpiredFrames(base::TimeTicks deadline) {
// Update |last_deadline_max_| if it's no longer accurate; this should always
// be done or EffectiveFramesQueued() may never expire the last frame.
if (deadline > last_deadline_max_)
last_deadline_max_ = deadline;
if (frame_queue_.size() < 2)
return 0;
UpdateFrameStatistics();
DCHECK_GT(average_frame_duration_, base::TimeDelta());
// 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|. We also always expire anything inserted before the last
// rendered frame.
size_t frames_to_expire = last_frame_index_;
const base::TimeTicks minimum_start_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].start_time >= minimum_start_time)
break;
}
if (!frames_to_expire)
return 0;
cadence_frame_counter_ += frames_to_expire - last_frame_index_;
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() {
// Since compositing is disconnected from the algorithm, the algorithm may be
// Reset() in between ticks of the compositor, so discard notifications which
// are invalid.
//
// If frames were expired by RemoveExpiredFrames() this count may be zero when
// the OnLastFrameDropped() call comes in.
if (!have_rendered_frames_ || frame_queue_.empty() ||
!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() {
frames_dropped_during_enqueue_ = 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();
first_frame_ = true;
cadence_frame_counter_ = 0;
last_render_ignored_cadence_frame_ = false;
was_time_moving_ = false;
// 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) {
const ReadyFrame& frame = frame_queue_[expired_frames];
if (frame.end_time.is_null() || frame.end_time > last_deadline_max_)
break;
}
return frame_queue_.size() - expired_frames;
}
// Find the first usable frame to start counting from.
const int start_index = FindBestFrameByCadence(nullptr);
if (start_index < 0)
return 0;
const base::TimeTicks minimum_start_time =
last_deadline_max_ - max_acceptable_drift_;
size_t renderable_frame_count = 0;
for (size_t i = start_index; i < frame_queue_.size(); ++i) {
const ReadyFrame& frame = frame_queue_[i];
if (frame.render_count < frame.ideal_render_count &&
(frame.end_time.is_null() || frame.end_time > minimum_start_time)) {
++renderable_frame_count;
}
}
return renderable_frame_count;
}
void VideoRendererAlgorithm::EnqueueFrame(
const scoped_refptr<VideoFrame>& frame) {
DCHECK(frame);
DCHECK(!frame->metadata()->IsTrue(VideoFrameMetadata::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);
// Drop any frames inserted before or at the last rendered frame if we've
// already rendered any frames.
const size_t new_frame_index = it - frame_queue_.begin();
if (new_frame_index <= last_frame_index_ && have_rendered_frames_) {
DVLOG(2) << "Dropping frame inserted before the last rendered frame.";
++frames_dropped_during_enqueue_;
return;
}
// Drop any frames which are less than a millisecond apart in media time (even
// those with timestamps matching an already enqueued frame), there's no way
// we can reasonably render these frames; it's effectively a 1000fps limit.
const base::TimeDelta delta =
std::min(new_frame_index < frame_queue_.size()
? frame_queue_[new_frame_index].frame->timestamp() -
frame->timestamp()
: base::TimeDelta::Max(),
new_frame_index > 0
? frame->timestamp() -
frame_queue_[new_frame_index - 1].frame->timestamp()
: base::TimeDelta::Max());
if (delta < base::TimeDelta::FromMilliseconds(1)) {
DVLOG(2) << "Dropping frame too close to an already enqueued frame: "
<< delta.InMicroseconds() << " us";
++frames_dropped_during_enqueue_;
return;
}
// 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.
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_ || !was_time_moving_) {
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 FindBestFrameByCadence() 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;
}
void VideoRendererAlgorithm::UpdateFrameStatistics() {
DCHECK(!frame_queue_.empty());
// Figure out all current ready frame times at once.
std::vector<base::TimeDelta> media_timestamps;
media_timestamps.reserve(frame_queue_.size());
for (const auto& ready_frame : frame_queue_)
media_timestamps.push_back(ready_frame.frame->timestamp());
std::vector<base::TimeTicks> wall_clock_times;
was_time_moving_ =
wall_clock_time_cb_.Run(media_timestamps, &wall_clock_times);
// Transfer the converted wall clock times into our frame queue.
DCHECK_EQ(wall_clock_times.size(), frame_queue_.size());
for (size_t i = 0; i < frame_queue_.size() - 1; ++i) {
ReadyFrame& frame = frame_queue_[i];
const bool new_sample = frame.has_estimated_end_time;
frame.start_time = wall_clock_times[i];
frame.end_time = wall_clock_times[i + 1];
frame.has_estimated_end_time = false;
if (new_sample)
frame_duration_calculator_.AddSample(frame.end_time - frame.start_time);
}
frame_queue_.back().start_time = wall_clock_times.back();
if (!frame_duration_calculator_.count())
return;
// 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();
// Update the frame end time for the last frame based on the average.
frame_queue_.back().end_time =
frame_queue_.back().start_time + average_frame_duration_;
// 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 16.66ms of drift since literature suggests it's
// well below the floor of detection and is high enough to ensure stability
// for 60fps content.
max_acceptable_drift_ = std::max(average_frame_duration_ / 2,
base::TimeDelta::FromSecondsD(1.0 / 60));
// 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;
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;
cadence_frame_counter_ = 0;
UpdateCadenceForFrames();
}
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(cadence_frame_counter_ +
(i - last_frame_index_))
: 0;
}
}
int VideoRendererAlgorithm::FindBestFrameByCadence(
int* remaining_overage) const {
DCHECK(!frame_queue_.empty());
if (!cadence_estimator_.has_cadence())
return -1;
DCHECK(!frame_queue_.empty());
DCHECK(cadence_estimator_.has_cadence());
const ReadyFrame& current_frame = frame_queue_[last_frame_index_];
if (remaining_overage) {
DCHECK_EQ(*remaining_overage, 0);
}
// If the current frame is below cadence, we should prefer it.
if (current_frame.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 (remaining_overage)
*remaining_overage = 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) {
const ReadyFrame& frame = frame_queue_[i];
// Frames which start after the deadline interval have zero coverage.
if (frame.start_time > deadline_max)
break;
// Clamp frame end times to a maximum of |deadline_max|.
const base::TimeTicks end_time = std::min(deadline_max, frame.end_time);
// Frames entirely before the deadline interval have zero coverage.
if (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 =
end_time - std::max(deadline_min, frame.start_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 {
const ReadyFrame& frame = frame_queue_[frame_index];
// If the frame lies before the deadline, compute the delta against the end
// of the frame's duration.
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
// start time.
if (frame.start_time > deadline_min)
return frame.start_time - deadline_min;
// Drift is zero for frames which overlap the deadline interval.
DCHECK_GE(deadline_min, frame.start_time);
DCHECK_GE(frame.end_time, deadline_min);
return base::TimeDelta();
}
} // namespace media
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