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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 <stddef.h>
#include <stdint.h>
#include "base/callback.h"
#include "base/containers/circular_deque.h"
#include "base/macros.h"
#include "base/memory/ref_counted.h"
#include "base/time/time.h"
#include "media/base/media_export.h"
#include "media/base/moving_average.h"
#include "media/base/video_frame.h"
#include "media/base/video_renderer.h"
#include "media/filters/video_cadence_estimator.h"
namespace media {
class MediaLog;
// VideoRendererAlgorithm manages a queue of VideoFrames from which it chooses
// frames with the goal of providing a smooth playback experience. I.e., the
// selection process results in the best possible uniformity for displayed frame
// durations over time.
// Clients will provide frames to VRA via EnqueueFrame() and then VRA will yield
// one of those frames in response to a future Render() call. Each Render()
// call takes a render interval which is used to compute the best frame for
// display during that interval.
// Render() calls are expected to happen on a regular basis. Failure to do so
// will result in suboptimal rendering experiences. If a client knows that
// Render() callbacks are stalled for any reason, it should tell VRA to expire
// frames which are unusable via RemoveExpiredFrames(); this prevents useless
// accumulation of stale VideoFrame objects (which are frequently quite large).
// The primary means of smooth frame selection is via forced integer cadence,
// see VideoCadenceEstimator for details on this process. In cases of non-
// integer cadence, the algorithm will fall back to choosing the frame which
// covers the most of the current render interval. If no frame covers the
// current interval, the least bad frame will be chosen based on its drift from
// the start of the interval.
// Combined these three approaches enforce optimal smoothness in many cases.
class MEDIA_EXPORT VideoRendererAlgorithm {
VideoRendererAlgorithm(const TimeSource::WallClockTimeCB& wall_clock_time_cb,
MediaLog* media_log);
VideoRendererAlgorithm(const VideoRendererAlgorithm&) = delete;
VideoRendererAlgorithm& operator=(const VideoRendererAlgorithm&) = delete;
// Chooses the best frame for the interval [deadline_min, deadline_max] based
// on available and previously rendered frames.
// Under ideal circumstances the deadline interval provided to a Render() call
// should be directly adjacent to the deadline given to the previous Render()
// call with no overlap or gaps. In practice, |deadline_max| is an estimated
// value, which means the next |deadline_min| may overlap it slightly or have
// a slight gap. Gaps which exceed the length of the deadline interval are
// assumed to be repeated frames for the purposes of cadence detection.
// If provided, |frames_dropped| will be set to the number of frames which
// were removed from |frame_queue_|, during this call, which were never
// returned during a previous Render() call and are no longer suitable for
// rendering since their wall clock time is too far in the past.
scoped_refptr<VideoFrame> Render(base::TimeTicks deadline_min,
base::TimeTicks deadline_max,
size_t* frames_dropped);
// Removes all video frames which are unusable since their ideal render
// interval [timestamp, timestamp + duration] is too far away from
// |deadline_min| than is allowed by drift constraints.
// At least one frame will always remain after this call so that subsequent
// Render() calls have a frame to return if no new frames are enqueued before
// then. Returns the number of frames removed that were never rendered.
// Note: In cases where there is no known frame duration (i.e. perhaps a video
// with only a single frame), the last frame can not be expired, regardless of
// the given deadline. Clients must handle this case externally.
size_t RemoveExpiredFrames(base::TimeTicks deadline);
// Clients should call this if the last frame provided by Render() was never
// rendered; it ensures the presented cadence matches internal models. This
// must be called before the next Render() call.
void OnLastFrameDropped();
// Adds a frame to |frame_queue_| for consideration by Render(). Out of order
// timestamps will be sorted into appropriate order. Do not enqueue end of
// stream frames. Frames inserted prior to the last rendered frame will not
// be used. They will be discarded on the next call to Render(), counting as
// dropped frames, or by RemoveExpiredFrames(), counting as expired frames.
// Attempting to enqueue a frame with the same timestamp as a previous frame
// will result in the previous frame being replaced if it has not been
// rendered yet. If it has been rendered, the new frame will be dropped.
// EnqueueFrame() will compute the current start time and an estimated end
// time of the frame based on previous frames or the value of
// VideoFrameMetadata::FRAME_DURATION if no previous frames, so that
// EffectiveFramesQueued() is relatively accurate immediately after this call.
void EnqueueFrame(scoped_refptr<VideoFrame> frame);
// Removes all frames from the |frame_queue_| and clears predictors. The
// algorithm will be as if freshly constructed after this call. By default
// everything is reset, but if kPreserveNextFrameEstimates is specified, then
// predictors for the start time of the next frame given to EnqueueFrame()
// will be kept; allowing EffectiveFramesQueued() accuracy with one frame.
enum class ResetFlag { kEverything, kPreserveNextFrameEstimates };
void Reset(ResetFlag reset_flag = ResetFlag::kEverything);
// Returns the number of frames currently buffered which could be rendered
// assuming current Render() interval trends.
// If a cadence has been identified, this will return the number of frames
// which have a non-zero ideal render count.
// If cadence has not been identified, this will return the number of frames
// which have a frame end time greater than the end of the last render
// interval passed to Render(). Note: If Render() callbacks become suspended
// and the duration is unknown the last frame may never be stop counting as
// effective. Clients must handle this case externally.
// In either case, frames enqueued before the last displayed frame will not
// be counted as effective.
size_t effective_frames_queued() const { return effective_frames_queued_; }
// Returns an estimate of the amount of memory (in bytes) used for frames.
int64_t GetMemoryUsage() const;
// Tells the algorithm that Render() callbacks have been suspended for a known
// reason and such stoppage shouldn't be counted against future frames.
void set_time_stopped() { was_time_moving_ = false; }
size_t frames_queued() const { return frame_queue_.size(); }
// Returns the average of the duration of all frames in |frame_queue_|
// as measured in wall clock (not media) time.
base::TimeDelta average_frame_duration() const {
return average_frame_duration_;
// End time of the last frame.
base::TimeTicks last_frame_end_time() const {
return frame_queue_.back().end_time;
const VideoFrame& last_frame() const { return *frame_queue_.back().frame; }
// Current render interval.
base::TimeDelta render_interval() const { return render_interval_; }
// Method used for testing which disables frame dropping, in this mode the
// algorithm will never drop frames and instead always return every frame
// for display at least once.
void disable_frame_dropping() { frame_dropping_disabled_ = true; }
enum : int {
// The number of frames to store for moving average calculations. Value
// picked after experimenting with playback of various local media and
// YouTube clips.
kMovingAverageSamples = 32
friend class VideoRendererAlgorithmTest;
// The determination of whether to clamp to a given cadence is based on the
// number of seconds before a frame would have to be dropped or repeated to
// compensate for reaching the maximum acceptable drift.
// We've chosen 8 seconds based on practical observations and the fact that it
// allows 29.9fps and 59.94fps in 60Hz and vice versa.
// Most users will not be able to see a single frame repeated or dropped every
// 8 seconds and certainly should notice it less than the randomly variable
// frame durations.
static const int kMinimumAcceptableTimeBetweenGlitchesSecs = 8;
// Metadata container for enqueued frames. See |frame_queue_| below.
struct ReadyFrame {
ReadyFrame(scoped_refptr<VideoFrame> frame);
ReadyFrame(const ReadyFrame& other);
// For use with std::lower_bound.
bool operator<(const ReadyFrame& other) const;
scoped_refptr<VideoFrame> frame;
// |start_time| is only available after UpdateFrameStatistics() has been
// called and |end_time| only after we have more than one frame.
base::TimeTicks start_time;
base::TimeTicks end_time;
// True if this frame's end time is based on the average frame duration and
// not the time of the next frame.
bool has_estimated_end_time;
int ideal_render_count;
int render_count;
int drop_count;
// Updates the render count for the last rendered frame based on the number
// of missing intervals between Render() calls.
void AccountForMissedIntervals(base::TimeTicks deadline_min,
base::TimeTicks deadline_max);
// Updates the render count and wall clock timestamps for all frames in
// |frame_queue_|. Updates |was_time_stopped_|, |cadence_estimator_| and
// |frame_duration_calculator_|.
// Note: Wall clock time is recomputed each Render() call because it's
// expected that the TimeSource powering TimeSource::WallClockTimeCB will skew
// slightly based on the audio clock.
// TODO(dalecurtis): Investigate how accurate we need the wall clock times to
// be, so we can avoid recomputing every time (we would need to recompute when
// playback rate changes occur though).
void UpdateFrameStatistics();
// Updates the ideal render count for all frames in |frame_queue_| based on
// the cadence returned by |cadence_estimator_|. Cadence is assigned based
// on |frame_counter_|.
void UpdateCadenceForFrames();
// If |cadence_estimator_| has detected a valid cadence, attempts to find the
// next frame which should be rendered. Returns -1 if not enough frames are
// available for cadence selection or there is no cadence.
int FindBestFrameByCadence() const;
// Iterates over |frame_queue_| and finds the frame which covers the most of
// the deadline interval. If multiple frames have coverage of the interval,
// |second_best| will be set to the index of the frame with the next highest
// coverage. Returns -1 if no frame has any coverage of the current interval.
// Prefers the earliest frame if multiple frames have similar coverage (within
// a few percent of each other).
int FindBestFrameByCoverage(base::TimeTicks deadline_min,
base::TimeTicks deadline_max,
int* second_best) const;
// Iterates over |frame_queue_| and find the frame which drifts the least from
// |deadline_min|. There's always a best frame by drift, so the return value
// is always a valid frame index. |selected_frame_drift| will be set to the
// drift of the chosen frame.
// Note: Drift calculations assume contiguous frames in the time domain, so
// it's not possible to have a case where a frame is -10ms from |deadline_min|
// and another frame which is at some time after |deadline_min|. The second
// frame would be considered to start at -10ms before |deadline_min| and would
// overlap |deadline_min|, so its drift would be zero.
int FindBestFrameByDrift(base::TimeTicks deadline_min,
base::TimeDelta* selected_frame_drift) const;
// Calculates the drift from |deadline_min| for the given |frame_index|. If
// the [start_time, end_time] lies before |deadline_min| the drift is
// the delta between |deadline_min| and |end_time|. If the frame
// overlaps |deadline_min| the drift is zero. If the frame lies after
// |deadline_min| the drift is the delta between |deadline_min| and
// |start_time|.
base::TimeDelta CalculateAbsoluteDriftForFrame(base::TimeTicks deadline_min,
int frame_index) const;
// Returns the index of the first usable frame or -1 if no usable frames.
int FindFirstGoodFrame() const;
// Updates |effective_frames_queued_| which is typically called far more
// frequently (~4x) than the value changes. This must be called whenever
// frames are added or removed from the queue or when any property of a
// ReadyFrame within the queue changes.
void UpdateEffectiveFramesQueued();
// Computes the unclamped count of effective frames. Used by
// UpdateEffectiveFramesQueued().
size_t CountEffectiveFramesQueued() const;
MediaLog* media_log_;
int out_of_order_frame_logs_ = 0;
// Queue of incoming frames waiting for rendering.
using VideoFrameQueue = base::circular_deque<ReadyFrame>;
VideoFrameQueue frame_queue_;
// Handles cadence detection and frame cadence assignments.
VideoCadenceEstimator cadence_estimator_;
// Indicates if any calls to Render() have successfully yielded a frame yet.
bool have_rendered_frames_;
// Callback used to convert media timestamps into wall clock timestamps.
const TimeSource::WallClockTimeCB wall_clock_time_cb_;
// The last |deadline_max| provided to Render(), used to predict whether
// frames were rendered over cadence between Render() calls.
base::TimeTicks last_deadline_max_;
// The average of the duration of all frames in |frame_queue_| as measured in
// wall clock (not media) time at the time of the last Render().
MovingAverage frame_duration_calculator_;
base::TimeDelta average_frame_duration_;
// The length of the last deadline interval given to Render(), updated at the
// start of Render().
base::TimeDelta render_interval_;
// The maximum acceptable drift before a frame can no longer be considered for
// rendering within a given interval.
base::TimeDelta max_acceptable_drift_;
// Indicates that the last call to Render() experienced a rendering glitch; it
// may have: under-rendered a frame, over-rendered a frame, dropped one or
// more frames, or chosen a frame which exceeded acceptable drift.
bool last_render_had_glitch_;
// For testing functionality which enables clockless playback of all frames,
// does not prevent frame dropping due to equivalent timestamps.
bool frame_dropping_disabled_;
// Tracks frames dropped during enqueue when identical timestamps are added
// to the queue. Callers are told about these frames during Render().
size_t frames_dropped_during_enqueue_;
// When cadence is present, we don't want to start counting against cadence
// until the first frame has reached its presentation time.
bool first_frame_;
// The frame number of the last rendered frame; incremented for every frame
// rendered and every frame dropped or expired since the last rendered frame.
// Given to |cadence_estimator_| when assigning cadence values to the
// ReadyFrameQueue. Cleared when a new cadence is detected.
uint64_t cadence_frame_counter_;
// Indicates if time was moving, set to the return value from
// UpdateFrameStatistics() during Render() or externally by
// set_time_stopped().
bool was_time_moving_;
// Current number of effective frames in the |frame_queue_|. Updated by calls
// to UpdateEffectiveFramesQueued() whenever the |frame_queue_| is changed.
size_t effective_frames_queued_;
} // namespace media