content/browser/media/capture/video_capture_oracle.cc

   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/media/capture/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::TimeTicks 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::TimeTicks 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(
  92     base::TimeTicks event_time) {
  93   DCHECK(!event_time.is_null());
  94
  95   // Add tokens to the bucket based on advancement in time.  Then, re-bound the
  96   // number of tokens in the bucket.  Overflow occurs when there is too much
  97   // time between events (a common case), or when RecordSample() is not being
  98   // called often enough (a bug).  On the other hand, if RecordSample() is being
  99   // called too often (e.g., as a reaction to IsOverdueForSamplingAt()), the
 100   // bucket will underflow.
 101   if (!current_event_.is_null()) {
 102     if (current_event_ < event_time) {
 103       token_bucket_ += event_time - current_event_;
 104       if (token_bucket_ > token_bucket_capacity_)
 105         token_bucket_ = token_bucket_capacity_;
 106     }
 107     // Side note: If the system clock is reset, causing |current_event_| to be
 108     // greater than |event_time|, everything here will simply gracefully adjust.
 109     if (token_bucket_ < base::TimeDelta())
 110       token_bucket_ = base::TimeDelta();
 111     TRACE_COUNTER1("mirroring",
 112                    "MirroringTokenBucketUsec",
 113                    std::max<int64>(0, token_bucket_.InMicroseconds()));
 114   }
 115   current_event_ = event_time;
 116
 117   // Return true if one capture period's worth of tokens are in the bucket.
 118   return token_bucket_ >= capture_period_;
 119 }
 120
 121 void SmoothEventSampler::RecordSample() {
 122   token_bucket_ -= capture_period_;
 123   TRACE_COUNTER1("mirroring",
 124                  "MirroringTokenBucketUsec",
 125                  std::max<int64>(0, token_bucket_.InMicroseconds()));
 126
 127   bool was_paused = overdue_sample_count_ == redundant_capture_goal_;
 128   if (HasUnrecordedEvent()) {
 129     last_sample_ = current_event_;
 130     overdue_sample_count_ = 0;
 131   } else {
 132     ++overdue_sample_count_;
 133   }
 134   bool is_paused = overdue_sample_count_ == redundant_capture_goal_;
 135
 136   VLOG_IF(0, !was_paused && is_paused)
 137       << "Tab content unchanged for " << redundant_capture_goal_
 138       << " frames; capture will halt until content changes.";
 139   VLOG_IF(0, was_paused && !is_paused)
 140       << "Content changed; capture will resume.";
 141 }
 142
 143 bool SmoothEventSampler::IsOverdueForSamplingAt(base::TimeTicks event_time)
 144     const {
 145   DCHECK(!event_time.is_null());
 146
 147   // If we don't get events on compositor updates on this platform, then we
 148   // don't reliably know whether we're dirty.
 149   if (events_are_reliable_) {
 150     if (!HasUnrecordedEvent() &&
 151         overdue_sample_count_ >= redundant_capture_goal_) {
 152       return false;  // Not dirty.
 153     }
 154   }
 155
 156   if (last_sample_.is_null())
 157     return true;
 158
 159   // If we're dirty but not yet old, then we've recently gotten updates, so we
 160   // won't request a sample just yet.
 161   base::TimeDelta dirty_interval = event_time - last_sample_;
 162   if (dirty_interval < capture_period_ * 4)
 163     return false;
 164   else
 165     return true;
 166 }
 167
 168 bool SmoothEventSampler::HasUnrecordedEvent() const {
 169   return !current_event_.is_null() && current_event_ != last_sample_;
 170 }
 171
 172 }  // namespace content