"Buttery Smooth" Tab Capture.

content/browser/media/capture/video_capture_oracle.h

Index: content/browser/media/capture/video_capture_oracle.h
diff --git a/content/browser/media/capture/video_capture_oracle.h b/content/browser/media/capture/video_capture_oracle.h
index 6242b6a29a8e5d24338c1c44b7fcf7f61f815402..4d5b94878deeb0f537fd1de2788cb3ec665e8f92 100644
--- a/content/browser/media/capture/video_capture_oracle.h
+++ b/content/browser/media/capture/video_capture_oracle.h
@@ -5,19 +5,22 @@
 #ifndef CONTENT_BROWSER_MEDIA_CAPTURE_VIDEO_CAPTURE_ORACLE_H_
 #define CONTENT_BROWSER_MEDIA_CAPTURE_VIDEO_CAPTURE_ORACLE_H_
 
+#include <deque>
+
 #include "base/callback_forward.h"
 #include "base/memory/scoped_ptr.h"
 #include "base/time/time.h"
 #include "content/common/content_export.h"
+#include "ui/gfx/geometry/rect.h"
 
 namespace content {
 
 // Filters a sequence of events to achieve a target frequency.
 class CONTENT_EXPORT SmoothEventSampler {
  public:
-  explicit SmoothEventSampler(base::TimeDelta capture_period,
-                              bool events_are_reliable,
-                              int redundant_capture_goal);
+  SmoothEventSampler(base::TimeDelta capture_period,
+                     bool events_are_reliable,
+                     int redundant_capture_goal);
 
   // Add a new event to the event history, and return whether it ought to be
   // sampled based on the desired |capture_period|. The event is not recorded as
@@ -51,6 +54,97 @@ class CONTENT_EXPORT SmoothEventSampler {
   DISALLOW_COPY_AND_ASSIGN(SmoothEventSampler);
 };
 
+// Analyzes a sequence of events to determine whether a subset of the events
+// looks like constant frame rate animated content.  Events are grouped into
+// subsets by their reported damage Rect.  Then, the largest of the subsets, if
+// it is a majority of the events, is examined to estimate the content's frame
+// rate.

[ncarter (slow), 2014/07/26 00:57:57]

This is really clever.

+class CONTENT_EXPORT AnimatedContentSampler {
+ public:
+  explicit AnimatedContentSampler(base::TimeDelta min_capture_period);
+  ~AnimatedContentSampler();
+
+  // Examines the given presentation event metadata, along with recent history,
+  // to detect animated content, updating the state of this sampler.  Returns
+  // true if animated content is detected.  |damage_rect| is the region of a
+  // frame about to be drawn, while |event_time| refers to the frame's estimated
+  // presentation time.
+  bool ConsiderPresentationEvent(const gfx::Rect& damage_rect,
+                                 base::TimeTicks event_time);
+
+  // Returns a "null" TimeTicks if the last event considered should not be
+  // sampled.  Otherwise, returns a frame timestamp to provide to consumers of
+  // the sampled frame.
+  base::TimeTicks next_frame_timestamp() const {
+    return next_frame_timestamp_;
+  }
+
+  // Accessors to currently-detected animating region/period, for logging.
+  const gfx::Rect& detected_region() const { return detected_region_; }
+  base::TimeDelta detected_period() const { return detected_period_; }
+
+  // Records that a frame with the given |frame_timestamp| was sampled.  This
+  // method should be called when *any* sampling is taken, even if it was not
+  // proposed by AnimatedContentSampler.
+  void RecordSample(base::TimeTicks frame_timestamp);
+
+ private:
+  // Data structure for efficient online analysis of recent event history.
+  typedef std::pair<gfx::Rect, base::TimeTicks> Observation;

[ncarter (slow), 2014/07/26 00:57:57]

I'd consider making this a struct so you can use .damage_rect and .timestamp
instead of .first and .second.

[miu, 2014/07/31 01:25:32]

Done.

+  typedef std::deque<Observation> ObservationFifo;
+
+  // Adds an observation to |observations_|, and prunes-out the old ones.
+  void AddObservation(const gfx::Rect& damage_rect, base::TimeTicks event_time);
+
+  // Analyzes the observations relative to the current |event_time| to detect
+  // stable animating content.  If detected, returns true and sets the output
+  // arguments to the region of the animating content and its mean frame
+  // duration.
+  bool AnalyzeObservations(base::TimeTicks event_time,
+                           gfx::Rect* rect,
+                           base::TimeDelta* period) const;
+
+  // Updates |next_frame_timestamp_|.  This is called by
+  // ConsiderPresentationEvent() when the current event is part of a detected
+  // animation.
+  void UpdateNextFrameTimestamp(base::TimeTicks event_time);
+
+  // The client expects frame timestamps to be at least this far apart.
+  const base::TimeDelta min_capture_period_;
+
+  // A recent history of observations in chronological order, maintained by
+  // AddObservation().
+  ObservationFifo observations_;
+
+  // The region of currently-detected animated content.  If empty, that means
+  // "not detected."
+  gfx::Rect detected_region_;
+
+  // The mean frame duration of currently-detected animated content.  If zero,
+  // that means "not detected."
+  base::TimeDelta detected_period_;
+
+  // The timestamp to use for the next frame, updated as each event is
+  // considered.  If the last event should not be sampled, this is set to a
+  // "null" value.
+  base::TimeTicks next_frame_timestamp_;
+
+  // The frame timestamp provided in the last call to RecordSample().  This
+  // timestamp may or may not have been one proposed by AnimatedContentSampler.
+  base::TimeTicks recorded_frame_timestamp_;
+
+  // Accumulates all the time advancements since the last call to
+  // RecordSample().  When this is greater than zero, there have been one or
+  // more events proposed for sampling, but not yet recorded.  This accounts for
+  // the cases where AnimatedContentSampler indicates a frame should be sampled,
+  // but the client chooses not to do so.
+  base::TimeDelta sequence_offset_;
+
+  // A token bucket that is used to decide which frames to drop whenever
+  // |detected_period_| is less than |min_capture_period_|.
+  base::TimeDelta borrowed_time_;
+};
+
 // 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
@@ -61,43 +155,62 @@ class CONTENT_EXPORT VideoCaptureOracle {
     kTimerPoll,
     kCompositorUpdate,
     kSoftwarePaint,
+    kNumEvents,
   };
 
-  VideoCaptureOracle(base::TimeDelta capture_period,
+  VideoCaptureOracle(base::TimeDelta min_capture_period,
                      bool events_are_reliable);
-  virtual ~VideoCaptureOracle() {}
+  virtual ~VideoCaptureOracle();
 
-  // Record an event of type |event|, and decide whether the caller should do a
-  // frame capture immediately. Decisions of the oracle are final: the caller
-  // must do what it is told.
-  bool ObserveEventAndDecideCapture(Event event, base::TimeTicks event_time);
+  // 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 the start of a capture.  Returns a frame_number to be used with
   // CompleteCapture().
   int RecordCapture();
 
-  // Record the completion of a capture.  Returns true iff the captured frame
-  // should be delivered.
-  bool CompleteCapture(int frame_number, base::TimeTicks timestamp);
+  // Notify of the completion of a capture.  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, base::TimeTicks* frame_timestamp);
 
-  base::TimeDelta capture_period() const { return capture_period_; }
+  base::TimeDelta min_capture_period() const { return min_capture_period_; }
 
  private:
+  // Retrieve/Assign a frame timestamp by capture |frame_number|.
+  base::TimeTicks GetFrameTimestamp(int frame_number) const;
+  void SetFrameTimestamp(int frame_number, base::TimeTicks timestamp);
 
-  // Time between frames.
-  const base::TimeDelta capture_period_;
+  // Hard boundary: Minimum time between frames.
+  const base::TimeDelta min_capture_period_;
 
   // Incremented every time a paint or update event occurs.
   int 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];
+
   // Stores the frame number from the last delivered frame.
   int last_delivered_frame_number_;
 
-  // Stores the timestamp of the last delivered frame.
-  base::TimeTicks last_delivered_frame_timestamp_;
-
-  // Tracks present/paint history.
-  SmoothEventSampler sampler_;
+  // 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_;
+
+  // 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];
 };
 
 }  // namespace content