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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.
- #ifndef MEDIA_FILTERS_VIDEO_RENDERER_ALGORITHM_H_
- #define MEDIA_FILTERS_VIDEO_RENDERER_ALGORITHM_H_
- #include <stddef.h>
- #include <stdint.h>
- #include "base/callback.h"
- #include "base/containers/circular_deque.h"
- #include "base/memory/raw_ptr.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 {
- public:
- VideoRendererAlgorithm(const TimeSource::WallClockTimeCB& wall_clock_time_cb,
- MediaLog* media_log);
- VideoRendererAlgorithm(const VideoRendererAlgorithm&) = delete;
- VideoRendererAlgorithm& operator=(const VideoRendererAlgorithm&) = delete;
- ~VideoRendererAlgorithm();
- // 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
- };
- private:
- 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);
- ~ReadyFrame();
- // 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;
- raw_ptr<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
- #endif // MEDIA_FILTERS_VIDEO_RENDERER_ALGORITHM_H_
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