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      1 // Copyright (c) 2013 The Chromium Authors. All rights reserved.
      2 // Use of this source code is governed by a BSD-style license that can be
      3 // found in the LICENSE file.
      4 
      5 #include "content/browser/renderer_host/media/video_capture_oracle.h"
      6 
      7 #include "base/debug/trace_event.h"
      8 
      9 namespace content {
     10 
     11 namespace {
     12 
     13 // This value controls how many redundant, timer-base captures occur when the
     14 // content is static. Redundantly capturing the same frame allows iterative
     15 // quality enhancement, and also allows the buffer to fill in "buffered mode".
     16 //
     17 // TODO(nick): Controlling this here is a hack and a layering violation, since
     18 // it's a strategy specific to the WebRTC consumer, and probably just papers
     19 // over some frame dropping and quality bugs. It should either be controlled at
     20 // a higher level, or else redundant frame generation should be pushed down
     21 // further into the WebRTC encoding stack.
     22 const int kNumRedundantCapturesOfStaticContent = 200;
     23 
     24 }  // anonymous namespace
     25 
     26 VideoCaptureOracle::VideoCaptureOracle(base::TimeDelta capture_period,
     27                                        bool events_are_reliable)
     28     : capture_period_(capture_period),
     29       frame_number_(0),
     30       last_delivered_frame_number_(0),
     31       sampler_(capture_period_,
     32                events_are_reliable,
     33                kNumRedundantCapturesOfStaticContent) {}
     34 
     35 bool VideoCaptureOracle::ObserveEventAndDecideCapture(
     36       Event event,
     37       base::Time event_time) {
     38   // Record |event| and decide whether it's a good time to capture.
     39   const bool content_is_dirty = (event == kCompositorUpdate ||
     40                                  event == kSoftwarePaint);
     41   bool should_sample;
     42   if (content_is_dirty) {
     43     frame_number_++;
     44     should_sample = sampler_.AddEventAndConsiderSampling(event_time);
     45   } else {
     46     should_sample = sampler_.IsOverdueForSamplingAt(event_time);
     47   }
     48   return should_sample;
     49 }
     50 
     51 int VideoCaptureOracle::RecordCapture() {
     52   sampler_.RecordSample();
     53   return frame_number_;
     54 }
     55 
     56 bool VideoCaptureOracle::CompleteCapture(int frame_number,
     57                                          base::Time timestamp) {
     58   // Drop frame if previous frame number is higher or we're trying to deliver
     59   // a frame with the same timestamp.
     60   if (last_delivered_frame_number_ > frame_number ||
     61       last_delivered_frame_timestamp_ == timestamp) {
     62     LOG(ERROR) << "Frame with same timestamp or out of order delivery. "
     63                << "Dropping frame.";
     64     return false;
     65   }
     66 
     67   if (last_delivered_frame_timestamp_ > timestamp) {
     68     // We should not get here unless time was adjusted backwards.
     69     LOG(ERROR) << "Frame with past timestamp (" << timestamp.ToInternalValue()
     70                << ") was delivered";
     71   }
     72 
     73   last_delivered_frame_number_ = frame_number;
     74   last_delivered_frame_timestamp_ = timestamp;
     75 
     76   return true;
     77 }
     78 
     79 SmoothEventSampler::SmoothEventSampler(base::TimeDelta capture_period,
     80                                        bool events_are_reliable,
     81                                        int redundant_capture_goal)
     82     :  events_are_reliable_(events_are_reliable),
     83        capture_period_(capture_period),
     84        redundant_capture_goal_(redundant_capture_goal),
     85        token_bucket_capacity_(capture_period + capture_period / 2),
     86        overdue_sample_count_(0),
     87        token_bucket_(token_bucket_capacity_) {
     88   DCHECK_GT(capture_period_.InMicroseconds(), 0);
     89 }
     90 
     91 bool SmoothEventSampler::AddEventAndConsiderSampling(base::Time event_time) {
     92   DCHECK(!event_time.is_null());
     93 
     94   // Add tokens to the bucket based on advancement in time.  Then, re-bound the
     95   // number of tokens in the bucket.  Overflow occurs when there is too much
     96   // time between events (a common case), or when RecordSample() is not being
     97   // called often enough (a bug).  On the other hand, if RecordSample() is being
     98   // called too often (e.g., as a reaction to IsOverdueForSamplingAt()), the
     99   // bucket will underflow.
    100   if (!current_event_.is_null()) {
    101     if (current_event_ < event_time) {
    102       token_bucket_ += event_time - current_event_;
    103       if (token_bucket_ > token_bucket_capacity_)
    104         token_bucket_ = token_bucket_capacity_;
    105     }
    106     // Side note: If the system clock is reset, causing |current_event_| to be
    107     // greater than |event_time|, everything here will simply gracefully adjust.
    108     if (token_bucket_ < base::TimeDelta())
    109       token_bucket_ = base::TimeDelta();
    110     TRACE_COUNTER1("mirroring",
    111                    "MirroringTokenBucketUsec", token_bucket_.InMicroseconds());
    112   }
    113   current_event_ = event_time;
    114 
    115   // Return true if one capture period's worth of tokens are in the bucket.
    116   return token_bucket_ >= capture_period_;
    117 }
    118 
    119 void SmoothEventSampler::RecordSample() {
    120   token_bucket_ -= capture_period_;
    121   TRACE_COUNTER1("mirroring",
    122                  "MirroringTokenBucketUsec", token_bucket_.InMicroseconds());
    123 
    124   bool was_paused = overdue_sample_count_ == redundant_capture_goal_;
    125   if (HasUnrecordedEvent()) {
    126     last_sample_ = current_event_;
    127     overdue_sample_count_ = 0;
    128   } else {
    129     ++overdue_sample_count_;
    130   }
    131   bool is_paused = overdue_sample_count_ == redundant_capture_goal_;
    132 
    133   VLOG_IF(0, !was_paused && is_paused)
    134       << "Tab content unchanged for " << redundant_capture_goal_
    135       << " frames; capture will halt until content changes.";
    136   VLOG_IF(0, was_paused && !is_paused)
    137       << "Content changed; capture will resume.";
    138 }
    139 
    140 bool SmoothEventSampler::IsOverdueForSamplingAt(base::Time event_time) const {
    141   DCHECK(!event_time.is_null());
    142 
    143   // If we don't get events on compositor updates on this platform, then we
    144   // don't reliably know whether we're dirty.
    145   if (events_are_reliable_) {
    146     if (!HasUnrecordedEvent() &&
    147         overdue_sample_count_ >= redundant_capture_goal_) {
    148       return false;  // Not dirty.
    149     }
    150   }
    151 
    152   // If we're dirty but not yet old, then we've recently gotten updates, so we
    153   // won't request a sample just yet.
    154   base::TimeDelta dirty_interval = event_time - last_sample_;
    155   if (dirty_interval < capture_period_ * 4)
    156     return false;
    157   else
    158     return true;
    159 }
    160 
    161 bool SmoothEventSampler::HasUnrecordedEvent() const {
    162   return !current_event_.is_null() && current_event_ != last_sample_;
    163 }
    164 
    165 }  // namespace content
    166