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..b4461d85523bfa84d67174b44ccf283d3bd6ec9e |
--- /dev/null |
+++ b/media/filters/video_renderer_algorithm.cc |
@@ -0,0 +1,603 @@ |
+// 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 { |
+ |
+enum FrameSelector { NONE, CADENCE, COVERAGE, DRIFT }; |
+ |
+VideoRendererAlgorithm::ReadyFrame::ReadyFrame( |
+ const scoped_refptr<VideoFrame>& ready_frame) |
+ : frame(ready_frame), |
+ media_timestamp(ready_frame->timestamp()), |
+ ideal_render_count(0), |
+ render_count(0) { |
+} |
+ |
+VideoRendererAlgorithm::ReadyFrame::~ReadyFrame() { |
+} |
+ |
+bool VideoRendererAlgorithm::ReadyFrame::operator<( |
+ const ReadyFrame& other) const { |
+ return media_timestamp < other.media_timestamp; |
+} |
+ |
+VideoRendererAlgorithm::VideoRendererAlgorithm( |
+ const TimeConverterCB& time_converter_cb) |
+ : time_converter_cb_(time_converter_cb) { |
+ Reset(); |
+} |
+ |
+VideoRendererAlgorithm::~VideoRendererAlgorithm() { |
+} |
+ |
+scoped_refptr<VideoFrame> VideoRendererAlgorithm::Render( |
+ base::TimeTicks deadline_min, |
+ base::TimeTicks deadline_max, |
+ int* frames_dropped) { |
+ DCHECK(deadline_min < deadline_max); |
+ |
+ if (frame_queue_.empty()) |
+ return nullptr; |
+ |
+ // 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_|. |
+ if (!UpdateFrameStatistics()) { |
+ DVLOG(2) << "Failed to update frame statistics."; |
+ DCHECK(frame_queue_[last_frame_index_].frame); |
+ return frame_queue_[last_frame_index_].frame; |
+ } |
+ |
+ FrameSelector frame_selector = NONE; |
xhwang
2015/04/17 06:18:32
This is set but not used for anything?
DaleCurtis
2015/04/18 01:29:21
Was used for debug logs, removed.
|
+ base::TimeDelta selected_frame_drift; |
+ |
+ // Step 4: Attempt to find the best frame by cadence. |
+ int frame_to_render = -1; |
+ if (ideal_cadence_) { |
+ frame_selector = CADENCE; |
+ frame_to_render = FindBestFrameByCadence(); |
+ |
+ if (frame_to_render >= 0) { |
+ selected_frame_drift = |
+ CalculateDriftForFrame(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_) { |
+ frame_selector = COVERAGE; |
+ 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 && |
+ CalculateDriftForFrame(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 = |
+ CalculateDriftForFrame(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_selector = DRIFT; |
+ frame_to_render = FindBestFrameByDrift(deadline_min); |
+ selected_frame_drift = |
+ CalculateDriftForFrame(deadline_min, frame_to_render); |
+ } |
+ |
+ last_render_had_glitch_ = selected_frame_drift > max_acceptable_drift_; |
+ if (last_render_had_glitch_) { |
+ DVLOG(2) << "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 (ideal_cadence_) { |
+ const ReadyFrame& last_frame_info = frame_queue_[last_frame_index_]; |
+ if (frame_to_render != last_frame_index_ && |
+ last_frame_info.render_count < last_frame_info.ideal_render_count) { |
+ last_render_had_glitch_ = true; |
+ DVLOG(2) << "Underdisplayed frame " << last_frame_info.media_timestamp |
+ << "; only " << last_frame_info.render_count |
+ << " times instead of " << last_frame_info.ideal_render_count; |
+ } else if (frame_to_render == last_frame_index_ && |
+ last_frame_info.render_count >= |
+ last_frame_info.ideal_render_count) { |
+ DVLOG(2) << "Overdisplayed frame " << last_frame_info.media_timestamp |
+ << "; displayed " << 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) { |
+ if (!frame_queue_[i].render_count) { |
+ DVLOG(2) << "Dropping undisplayed frame " |
+ << frame_queue_[i].media_timestamp; |
+ ++(*frames_dropped); |
+ if (!fractional_cadence_) |
+ last_render_had_glitch_ = true; |
+ } |
+ } |
+ } |
+ |
+ frame_queue_.erase(frame_queue_.begin(), |
+ frame_queue_.begin() + frame_to_render); |
+ } |
+ |
+ // Step 8: Congratulations, the frame selection guantlet has been passed! |
+ last_frame_index_ = 0; |
+ ++frame_queue_.front().render_count; |
+ DCHECK(frame_queue_.front().frame); |
+ return frame_queue_.front().frame; |
+} |
+ |
+int VideoRendererAlgorithm::RemoveExpiredFrames(base::TimeTicks deadline_min) { |
+ if (!UpdateFrameStatistics() || frame_duration_ == base::TimeDelta()) |
+ return 0; |
+ |
+ DCHECK_GE(frame_queue_.size(), 2u); |
+ |
+ // Finds and removes all frames which are too old to be used; I.e., the end of |
+ // their display interval is further than |max_acceptable_drift_| from the |
+ // given |deadline_min|. |
+ int frames_to_expire = 0; |
+ const base::TimeTicks mininum_frame_time = |
+ deadline_min - max_acceptable_drift_ - frame_duration_; |
+ for (; static_cast<size_t>(frames_to_expire) < frame_queue_.size() - 1; |
+ ++frames_to_expire) { |
+ if (frame_queue_[frames_to_expire].wall_clock_time >= mininum_frame_time) |
+ break; |
+ } |
+ |
+ if (!frames_to_expire) |
+ return 0; |
+ |
+ frame_queue_.erase(frame_queue_.begin(), |
+ frame_queue_.begin() + frames_to_expire); |
+ |
+ last_frame_index_ = std::max(0, last_frame_index_ - frames_to_expire); |
+ return frames_to_expire; |
+} |
+ |
+void VideoRendererAlgorithm::OnLastFrameDropped() { |
+ DCHECK(!frame_queue_.empty()); |
+ |
+ // We only care if the frame was never rendered at all; otherwise we assume |
+ // that the frame was redisplayed even if the renderer was told otherwise. |
+ if (frame_queue_[last_frame_index_].render_count == 1) |
+ frame_queue_[last_frame_index_].render_count = 0; |
+} |
+ |
+void VideoRendererAlgorithm::Reset() { |
+ last_frame_index_ = ideal_cadence_ = fractional_cadence_ = 0; |
+ last_detected_cadence_ = render_intervals_cadence_held_ = 0; |
+ have_rendered_frames_ = last_render_had_glitch_ = false; |
+ cadence_hysteresis_enabled_ = true; |
+ last_deadline_max_ = base::TimeTicks(); |
+ frame_duration_ = render_interval_ = base::TimeDelta(); |
+ frame_queue_.clear(); |
+ |
+ // Default to ATSC IS/191 recommendations for maximum acceptable drift before |
+ // we have enough frames to base the the maximum on frame duration. |
+ max_acceptable_drift_ = base::TimeDelta::FromMilliseconds(15); |
+} |
+ |
+size_t VideoRendererAlgorithm::EffectiveFramesQueued() const { |
+ if (!have_rendered_frames_ || !ideal_cadence_) |
+ return frame_queue_.size(); |
+ |
+ size_t renderable_frame_count = 0; |
+ for (size_t i = last_frame_index_; i < frame_queue_.size(); ++i) { |
+ if (frame_queue_[i].ideal_render_count > 0) |
+ ++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); |
+ |
+ // If a frame was inserted before the first frame, update the index. On the |
+ // next call to Render() it will be dropped. |
+ if (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 (ideal_cadence_ && !fractional_cadence_) |
+ it->ideal_render_count = ideal_cadence_; |
+ else if (fractional_cadence_) |
+ UpdateFractionalCadenceForFrames(fractional_cadence_); |
+ |
+ // 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()); |
+ } |
+} |
+ |
+void VideoRendererAlgorithm::AccountForMissedIntervals( |
+ base::TimeTicks deadline_min, |
+ base::TimeTicks deadline_max) { |
+ const base::TimeTicks previous_deadline_max = last_deadline_max_; |
+ last_deadline_max_ = deadline_max; |
+ |
+ if (previous_deadline_max.is_null()) |
+ return; |
+ |
+ const int64 render_cycle_count = |
+ std::abs((deadline_min - previous_deadline_max) / render_interval_); |
brianderson
2015/04/16 22:57:56
Is the abs here to round render_cycle_count down t
DaleCurtis
2015/04/17 02:38:12
Yes, fractional values should be zero. I see your
DaleCurtis
2015/04/18 01:29:21
Done.
|
+ |
+ // In the ideal case this value will be zero. |
+ if (!render_cycle_count) |
+ return; |
+ |
+ DVLOG(2) << "Missed " << render_cycle_count << " Render() intervals."; |
+ |
+ // Only update display count if the frame was displayed at all; it may not |
+ // have been if OnFrameDropped() was called. |
+ if (frame_queue_[last_frame_index_].render_count) |
brianderson
2015/04/16 22:57:56
Does this need to be a loop for the case were rend
DaleCurtis
2015/04/17 02:38:12
No, the FindBestFrameByCadence() will handle overa
|
+ frame_queue_[last_frame_index_].render_count += render_cycle_count; |
+} |
+ |
+base::TimeDelta VideoRendererAlgorithm::CalculateTimeUntilGlitch( |
+ double perfect_cadence, |
+ double clamped_cadence, |
+ bool fractional) { |
+ if (clamped_cadence == 0.0) |
+ return base::TimeDelta(); |
+ |
+ // Calculate the drift in microseconds for each frame we render at cadence |
+ // instead of for its real duration. |
+ const double rendered_frame_duration = |
+ fractional ? render_interval_.InMicroseconds() |
+ : clamped_cadence * render_interval_.InMicroseconds(); |
+ |
+ // When computing a fractional drift, we render the first of |clamped_cadence| |
+ // frames and drop |clamped_cadence| - 1 frames. To make the calculations |
+ // below work we need to project out the timestamp of the frame which would be |
+ // displayed after accounting for those |clamped_cadence| frames. |
+ const double actual_frame_duration = |
+ fractional ? clamped_cadence * frame_duration_.InMicroseconds() |
+ : frame_duration_.InMicroseconds(); |
+ |
+ const double rendered_vs_actual_duration_delta = |
+ std::abs(rendered_frame_duration - actual_frame_duration); |
+ if (rendered_vs_actual_duration_delta < |
+ std::numeric_limits<double>::epsilon()) { |
+ return kInfiniteDuration(); |
+ } |
+ |
+ const double frames_rendered_before_drift_exhausted = |
+ std::ceil(max_acceptable_drift_.InMicroseconds() / |
+ rendered_vs_actual_duration_delta); |
+ |
+ const base::TimeDelta time_until_glitch = base::TimeDelta::FromMicroseconds( |
+ rendered_frame_duration * frames_rendered_before_drift_exhausted); |
+ |
+ return time_until_glitch; |
+} |
+ |
+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 (auto& frame_info : frame_queue_) { |
+ frame_info.wall_clock_time = |
+ time_converter_cb_.Run(frame_info.media_timestamp); |
+ |
+ // If time stops or never started, exit immediately. |
+ if (frame_info.wall_clock_time.is_null()) |
+ return false; |
+ } |
+ |
+ // Do we have enough frames to compute statistics? |
+ const bool have_frame_duration = frame_duration_ != base::TimeDelta(); |
+ if (frame_queue_.size() < 2 && !have_frame_duration) |
+ return true; |
+ |
+ // Update |frame_duration_| estimate weighted towards the existing duration. |
+ const base::TimeDelta wall_clock_delta = frame_queue_.back().wall_clock_time - |
+ frame_queue_.front().wall_clock_time; |
+ if (!have_frame_duration) { |
+ frame_duration_ = wall_clock_delta / (frame_queue_.size() - 1); |
+ } else { |
+ frame_duration_ = |
+ (frame_duration_ + wall_clock_delta) / frame_queue_.size(); |
+ } |
+ |
+ // 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(frame_duration_ / 2, base::TimeDelta::FromSecondsD(1.0 / 120)); |
+ |
+ // The perfect cadence is the number of render intervals per frame, while the |
+ // clamped cadence is the nearest matching integer cadence. |
+ const double perfect_cadence = |
+ frame_duration_.InSecondsF() / render_interval_.InSecondsF(); |
+ const int clamped_cadence = perfect_cadence + 0.5; |
+ |
+ // Inverse cadence is checked to see if we have a fractional cadence which |
+ // would look best if we consistently drop the same frames. A fractional |
+ // cadence is something like 120fps content on a 60Hz display. |
+ const double inverse_perfect_cadence = 1.0 / perfect_cadence; |
+ const int clamped_inverse_cadence = inverse_perfect_cadence + 0.5; |
+ |
+ const base::TimeDelta minimum_glitch_time = |
+ base::TimeDelta::FromSeconds(kMinimumAcceptableTimeBetweenGlitchesSecs); |
+ |
+ // See if the clamped cadence fits acceptable thresholds for exhausting drift. |
brianderson
2015/04/16 22:57:56
Ok. I understand now how this block makes sure 29.
|
+ int new_cadence = 0, new_fractional_cadence = 0; |
+ if (CalculateTimeUntilGlitch(perfect_cadence, clamped_cadence, false) >= |
+ minimum_glitch_time) { |
+ new_cadence = clamped_cadence; |
+ } else if (CalculateTimeUntilGlitch(inverse_perfect_cadence, |
+ clamped_inverse_cadence, |
+ true) >= minimum_glitch_time) { |
+ new_cadence = 1; |
+ new_fractional_cadence = clamped_inverse_cadence; |
+ } |
+ |
+ // If hysteresis is enabled, require cadence to hold for some time before |
+ // switching in or out of cadence based rendering mode. |
+ if (cadence_hysteresis_enabled_) { |
+ if (new_fractional_cadence) { |
+ if (last_detected_cadence_ == new_fractional_cadence) { |
+ ++render_intervals_cadence_held_; |
+ } else { |
+ last_detected_cadence_ = new_fractional_cadence; |
+ render_intervals_cadence_held_ = 0; |
+ } |
+ } else { |
+ if (last_detected_cadence_ == new_cadence) { |
+ ++render_intervals_cadence_held_; |
+ } else { |
+ last_detected_cadence_ = new_cadence; |
+ render_intervals_cadence_held_ = 0; |
+ } |
+ } |
+ |
+ // To prevent oscillation of cadence selection, ensure cadence selections |
+ // are held for some time before applying them. Value chosen arbitrarily. |
+ const base::TimeDelta cadence_hysteresis = |
+ base::TimeDelta::FromMilliseconds(100); |
+ if (render_intervals_cadence_held_ * render_interval_ < cadence_hysteresis) |
+ return true; |
+ } |
+ |
+ // No need to update cadence if there's been no change; cadence will be set |
+ // as frames are added to the queue. |
+ if (ideal_cadence_ == new_cadence && |
+ new_fractional_cadence == fractional_cadence_) { |
+ return true; |
+ } |
+ |
+ if (new_fractional_cadence) { |
+ UpdateFractionalCadenceForFrames(new_fractional_cadence); |
+ } else { |
+ // |new_cadence| may be zero at this point, which clears previous cadences. |
+ for (auto& frame_info : frame_queue_) |
+ frame_info.ideal_render_count = new_cadence; |
+ } |
+ |
+ // 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. |
+ |
+ if (ideal_cadence_ != new_cadence) { |
+ DVLOG(1) << "Cadence switch from " << ideal_cadence_ << " to " |
+ << new_cadence << "; perfect_cadence: " << perfect_cadence; |
+ } |
+ if (fractional_cadence_ != new_fractional_cadence) { |
+ DVLOG(1) << "Fractional cadence switch from " << fractional_cadence_ |
+ << " to " << new_fractional_cadence |
+ << "; inverse_perfect_cadence: " << inverse_perfect_cadence; |
+ } |
+ |
+ ideal_cadence_ = new_cadence; |
+ fractional_cadence_ = new_fractional_cadence; |
+ return true; |
+} |
+ |
+void VideoRendererAlgorithm::UpdateFractionalCadenceForFrames( |
+ int fractional_cadence) { |
+ // Cadence is 1 for the first of every |fractional_cadence| frames and zero |
+ // elsewhere. |
+ for (size_t i = last_frame_index_; i < frame_queue_.size(); ++i) { |
+ frame_queue_[i].ideal_render_count = |
+ static_cast<int>((i - last_frame_index_) % fractional_cadence == 0); |
+ } |
+} |
+ |
+int VideoRendererAlgorithm::FindBestFrameByCadence() { |
+ DCHECK(!frame_queue_.empty()); |
+ if (!ideal_cadence_) |
+ return -1; |
+ |
+ 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) |
+ return last_frame_index_; |
+ |
+ // If the current frame is on cadence, find the next displayable frame. |
+ if (current_frame.render_count == current_frame.ideal_render_count) { |
+ for (size_t i = last_frame_index_ + 1; i < frame_queue_.size(); ++i) { |
+ if (frame_queue_[i].ideal_render_count > 0) |
+ return i; |
+ } |
+ } |
+ |
+ // The frame is overdisplayed or we don't have enough frames to find a better |
+ // once by cadence, so return nothing. |
+ return -1; |
+} |
+ |
+int VideoRendererAlgorithm::FindBestFrameByCoverage( |
+ base::TimeTicks deadline_min, |
+ base::TimeTicks deadline_max, |
+ int* second_best) { |
+ 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 0% |
+ // coverage, while those which completely overlap the interval have 100%. |
+ double best_coverage = 0.0; |
+ int best_frame_by_coverage = -1; |
+ std::vector<double> coverage(frame_queue_.size(), 0.0); |
+ 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) |
+ continue; |
+ |
+ // Clamp frame times to a maximum of |deadline_max|. |
+ const base::TimeTicks next_frame_time = std::min( |
+ deadline_max, i + 1 < frame_queue_.size() |
+ ? frame_queue_[i + 1].wall_clock_time |
+ : frame_queue_[i].wall_clock_time + frame_duration_); |
+ |
+ // Frames entirely before the deadline interval have zero coverage. |
+ if (next_frame_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 = |
+ next_frame_time - |
+ std::max(deadline_min, frame_queue_[i].wall_clock_time); |
+ |
+ coverage[i] = duration.InSecondsF() / render_interval_.InSecondsF(); |
+ 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] = 0.0; |
+ auto it = std::max_element(coverage.begin(), coverage.end()); |
+ if (*it > 0) |
+ *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 double kAllowableJitter = 500.0 / render_interval_.InMicroseconds(); |
+ if (*second_best >= 0 && best_frame_by_coverage > *second_best && |
+ std::abs(best_coverage - coverage[*second_best]) <= kAllowableJitter) { |
+ std::swap(best_frame_by_coverage, *second_best); |
+ } |
+ |
+ // TODO(dalecurtis): We may want to make a better decision about what to do |
brianderson
2015/04/16 22:57:56
Thanks for adding this TODO. Fixing this shouldn't
|
+ // 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. |
+ // |
+ // 23.974 fps and 24 fps 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) { |
+ DCHECK(!frame_queue_.empty()); |
+ |
+ int best_frame_by_drift = -1; |
+ base::TimeDelta min_drift = base::TimeDelta::Max(); |
+ |
+ for (size_t i = last_frame_index_; i < frame_queue_.size(); ++i) { |
+ const base::TimeDelta drift = CalculateDriftForFrame(deadline_min, i); |
+ if (drift < min_drift) { |
+ min_drift = drift; |
+ best_frame_by_drift = i; |
+ } |
+ } |
+ |
+ return best_frame_by_drift; |
+} |
+ |
+base::TimeDelta VideoRendererAlgorithm::CalculateDriftForFrame( |
+ base::TimeTicks deadline_min, |
+ int frame_index) { |
+ const ReadyFrame& frame = frame_queue_[frame_index]; |
+ |
+ // If the frame lies before the deadline, compute the delta against the end |
+ // of the frame's correct display duration. |
+ if (frame.wall_clock_time < deadline_min && |
+ frame.wall_clock_time + frame_duration_ < deadline_min) { |
+ return deadline_min - (frame.wall_clock_time + frame_duration_); |
+ } |
+ |
+ // If the frame lies after the deadline, compute the delta against the start |
+ // of the frame's correct display time. |
+ 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.wall_clock_time + frame_duration_, deadline_min); |
+ return base::TimeDelta(); |
+} |
+ |
+} // namespace media |