video_capture_oracle.h

// Copyright (c) 2013 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_CAPTURE_VIDEO_CAPTURE_ORACLE_H_
#define MEDIA_CAPTURE_VIDEO_CAPTURE_ORACLE_H_

#include "base/callback_forward.h"
#include "base/memory/scoped_ptr.h"
#include "base/time/time.h"
#include "media/base/media_export.h"
#include "media/capture/animated_content_sampler.h"
#include "media/capture/capture_resolution_chooser.h"
#include "media/capture/feedback_signal_accumulator.h"
#include "media/capture/smooth_event_sampler.h"
#include "ui/gfx/geometry/rect.h"

namespace media {

// VideoCaptureOracle manages the producer-side throttling of captured frames
// from a video capture device.  It is informed of every update by the device;
// this empowers it to look into the future and decide if a particular frame
// ought to be captured in order to achieve its target frame rate.
class MEDIA_EXPORT VideoCaptureOracle {
 public:
  enum Event {
    kTimerPoll,
    kCompositorUpdate,
    kNumEvents,
  };

  enum {
    // The recommended minimum amount of time between calls to
    // ObserveEventAndDecideCapture() for the kTimerPoll Event type.  Anything
    // lower than this isn't harmful, just wasteful.
    kMinTimerPollPeriodMillis = 125,  // 8 FPS
  };

  VideoCaptureOracle(base::TimeDelta min_capture_period,
                     const gfx::Size& max_frame_size,
                     media::ResolutionChangePolicy resolution_change_policy,
                     bool enable_auto_throttling);

  virtual ~VideoCaptureOracle();

  // Sets the source content size.  This may not have an immediate effect on the
  // proposed capture size, as the oracle will prevent too-frequent changes from
  // occurring.
  void SetSourceSize(const gfx::Size& source_size);

  // Record a event of type |event|, and decide whether the caller should do a
  // frame capture.  |damage_rect| is the region of a frame about to be drawn,
  // and may be an empty Rect, if this is not known.  If the caller accepts the
  // oracle's proposal, it should call RecordCapture() to indicate this.
  bool ObserveEventAndDecideCapture(Event event,
                                    const gfx::Rect& damage_rect,
                                    base::TimeTicks event_time);

  // Record and update internal state based on whether the frame capture will be
  // started.  |pool_utilization| is a value in the range 0.0 to 1.0 to indicate
  // the current buffer pool utilization relative to a sustainable maximum (not
  // the absolute maximum).  This method should only be called if the last call
  // to ObserveEventAndDecideCapture() returned true.  The first method returns
  // the |frame_number| to be used with CompleteCapture().
  int RecordCapture(double pool_utilization);
  void RecordWillNotCapture(double pool_utilization);

  // Notify of the completion of a capture, and whether it was successful.
  // Returns true iff the captured frame should be delivered.  |frame_timestamp|
  // is set to the timestamp that should be provided to the consumer of the
  // frame.
  bool CompleteCapture(int frame_number,
                       bool capture_was_successful,
                       base::TimeTicks* frame_timestamp);

  // Record the resource utilization feedback for a frame that was processed by
  // the consumer.  This allows the oracle to reduce/increase future data volume
  // if the consumer is overloaded/under-utilized.  |resource_utilization| is a
  // value in the range 0.0 to 1.0 to indicate the current consumer resource
  // utilization relative to a sustainable maximum (not the absolute maximum).
  // This method should only be called for frames where CompleteCapture()
  // returned true.
  void RecordConsumerFeedback(int frame_number, double resource_utilization);

  base::TimeDelta min_capture_period() const {
    return smoothing_sampler_.min_capture_period();
  }

  // Returns the oracle's estimate of the duration of the next frame.  This
  // should be called just after ObserveEventAndDecideCapture(), and will only
  // be non-zero if the call returned true.
  base::TimeDelta estimated_frame_duration() const {
    return duration_of_next_frame_;
  }

  // Returns the capture frame size the client should use.  This is updated by
  // calls to ObserveEventAndDecideCapture().  The oracle prevents too-frequent
  // changes to the capture size, to avoid stressing the end-to-end pipeline.
  gfx::Size capture_size() const { return capture_size_; }

 private:
  // Retrieve/Assign a frame timestamp by capture |frame_number|.  Only valid
  // when IsFrameInRecentHistory(frame_number) returns true.
  base::TimeTicks GetFrameTimestamp(int frame_number) const;
  void SetFrameTimestamp(int frame_number, base::TimeTicks timestamp);

  // Returns true if the frame timestamp ring-buffer currently includes a
  // slot for the given |frame_number|.
  bool IsFrameInRecentHistory(int frame_number) const;

  // Queries the ResolutionChooser to update |capture_size_|, and resets all the
  // FeedbackSignalAccumulator members to stable-state starting values.  The
  // accumulators are reset such that they can only apply feedback updates for
  // future frames (those with a timestamp after |last_frame_time|).
  void CommitCaptureSizeAndReset(base::TimeTicks last_frame_time);

  // Called after a capture or no-capture decision was recorded.  This analyzes
  // current state and may result in a future change to the capture frame size.
  void AnalyzeAndAdjust(base::TimeTicks analyze_time);

  // Analyzes current feedback signal accumulators for an indication that the
  // capture size should be decreased.  Returns either a decreased area, or -1
  // if no decrease should be made.
  int AnalyzeForDecreasedArea(base::TimeTicks analyze_time);

  // Analyzes current feedback signal accumulators for an indication that the
  // the system has had excellent long-term performance and the capture size
  // should be increased to improve quality.  Returns either an increased area,
  // or -1 if no increase should be made.
  int AnalyzeForIncreasedArea(base::TimeTicks analyze_time);

  // Set to false to prevent the oracle from automatically adjusting the capture
  // size in response to end-to-end utilization.
  const bool auto_throttling_enabled_;

  // Incremented every time a paint or update event occurs.
  int next_frame_number_;

  // Stores the last |event_time| from the last observation/decision.  Used to
  // sanity-check that event times are monotonically non-decreasing.
  base::TimeTicks last_event_time_[kNumEvents];

  // Updated by the last call to ObserveEventAndDecideCapture() with the
  // estimated duration of the next frame to sample.  This is zero if the method
  // returned false.
  base::TimeDelta duration_of_next_frame_;

  // Stores the frame number from the last successfully delivered frame.
  int last_successfully_delivered_frame_number_;

  // Stores the number of pending frame captures.
  int num_frames_pending_;

  // These track present/paint history and propose whether to sample each event
  // for capture.  |smoothing_sampler_| uses a "works for all" heuristic, while
  // |content_sampler_| specifically detects animated content (e.g., video
  // playback) and decides which events to sample to "lock into" that content.
  SmoothEventSampler smoothing_sampler_;
  AnimatedContentSampler content_sampler_;

  // Determines video capture frame sizes.
  CaptureResolutionChooser resolution_chooser_;

  // The current capture size.  |resolution_chooser_| may hold an updated value
  // because the oracle prevents this size from changing too frequently.  This
  // avoids over-stressing consumers (e.g., when a window is being activly
  // drag-resized) and allowing the end-to-end system time to stabilize.
  gfx::Size capture_size_;

  // Recent history of frame timestamps proposed by VideoCaptureOracle.  This is
  // a ring-buffer, and should only be accessed by the Get/SetFrameTimestamp()
  // methods.
  enum { kMaxFrameTimestamps = 16 };
  base::TimeTicks frame_timestamps_[kMaxFrameTimestamps];

  // Recent average buffer pool utilization for capture.
  FeedbackSignalAccumulator buffer_pool_utilization_;

  // Estimated maximum frame area that currently can be handled by the consumer,
  // in number of pixels per frame.  This is used to adjust the capture size up
  // or down to a data volume the consumer can handle.  Note that some consumers
  // do not provide feedback, and the analysis logic should account for that.
  FeedbackSignalAccumulator estimated_capable_area_;

  // The time of the first analysis which concluded the end-to-end system was
  // under-utilized.  If null, the system is not currently under-utilized.  This
  // is used to determine when a proving period has elapsed that justifies an
  // increase in capture size.
  base::TimeTicks start_time_of_underutilization_;

  // The timestamp of the frame where |content_sampler_| last detected
  // animation.  This determines whether capture size increases will be
  // aggressive (because content is not animating).
  base::TimeTicks last_time_animation_was_detected_;
};

}  // namespace media

#endif  // MEDIA_CAPTURE_VIDEO_CAPTURE_ORACLE_H_