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 <algorithm>
   8
   9 #include "base/format_macros.h"
  10 #include "base/strings/stringprintf.h"
  11 #include "base/trace_event/trace_event.h"
  12
  13 namespace content {
  14
  15 namespace {
  16
  17 // This value controls how many redundant, timer-base captures occur when the
  18 // content is static. Redundantly capturing the same frame allows iterative
  19 // quality enhancement, and also allows the buffer to fill in "buffered mode".
  20 //
  21 // TODO(nick): Controlling this here is a hack and a layering violation, since
  22 // it's a strategy specific to the WebRTC consumer, and probably just papers
  23 // over some frame dropping and quality bugs. It should either be controlled at
  24 // a higher level, or else redundant frame generation should be pushed down
  25 // further into the WebRTC encoding stack.
  26 const int kNumRedundantCapturesOfStaticContent = 200;
  27
  28 // These specify the minimum/maximum amount of recent event history to examine
  29 // to detect animated content.  If the values are too low, there is a greater
  30 // risk of false-positive detections and low accuracy.  If they are too high,
  31 // the the implementation will be slow to lock-in/out, and also will not react
  32 // well to mildly-variable frame rate content (e.g., 25 +/- 1 FPS).
  33 //
  34 // These values were established by experimenting with a wide variety of
  35 // scenarios, including 24/25/30 FPS videos, 60 FPS WebGL demos, and the
  36 // transitions between static and animated content.
  37 const int kMinObservationWindowMillis = 1000;
  38 const int kMaxObservationWindowMillis = 2000;
  39
  40 // The maximum amount of time that can elapse before declaring two subsequent
  41 // events as "not animating."  This is the same value found in
  42 // cc::FrameRateCounter.
  43 const int kNonAnimatingThresholdMillis = 250;  // 4 FPS
  44
  45 // The slowest that content can be animating in order for AnimatedContentSampler
  46 // to lock-in.  This is the threshold at which the "smoothness" problem is no
  47 // longer relevant.
  48 const int kMaxLockInPeriodMicros = 83333;  // 12 FPS
  49
  50 // The amount of time over which to fully correct the drift of the rewritten
  51 // frame timestamps from the presentation event timestamps.  The lower the
  52 // value, the higher the variance in frame timestamps.
  53 const int kDriftCorrectionMillis = 2000;
  54
  55 // Given the amount of time between frames, compare to the expected amount of
  56 // time between frames at |frame_rate| and return the fractional difference.
  57 double FractionFromExpectedFrameRate(base::TimeDelta delta, int frame_rate) {
  58   DCHECK_GT(frame_rate, 0);
  59   const base::TimeDelta expected_delta =
  60       base::TimeDelta::FromSeconds(1) / frame_rate;
  61   return (delta - expected_delta).InMillisecondsF() /
  62       expected_delta.InMillisecondsF();
  63 }
  64
  65 }  // anonymous namespace
  66
  67 VideoCaptureOracle::VideoCaptureOracle(base::TimeDelta min_capture_period)
  68     : frame_number_(0),
  69       last_delivered_frame_number_(-1),
  70       smoothing_sampler_(min_capture_period,
  71                          kNumRedundantCapturesOfStaticContent),
  72       content_sampler_(min_capture_period) {
  73 }
  74
  75 VideoCaptureOracle::~VideoCaptureOracle() {}
  76
  77 bool VideoCaptureOracle::ObserveEventAndDecideCapture(
  78     Event event,
  79     const gfx::Rect& damage_rect,
  80     base::TimeTicks event_time) {
  81   DCHECK_GE(event, 0);
  82   DCHECK_LT(event, kNumEvents);
  83   if (event_time < last_event_time_[event]) {
  84     LOG(WARNING) << "Event time is not monotonically non-decreasing.  "
  85                  << "Deciding not to capture this frame.";
  86     return false;
  87   }
  88   last_event_time_[event] = event_time;
  89
  90   bool should_sample;
  91   switch (event) {
  92     case kCompositorUpdate:
  93     case kSoftwarePaint:
  94       smoothing_sampler_.ConsiderPresentationEvent(event_time);
  95       content_sampler_.ConsiderPresentationEvent(damage_rect, event_time);
  96       if (content_sampler_.HasProposal()) {
  97         should_sample = content_sampler_.ShouldSample();
  98         if (should_sample)
  99           event_time = content_sampler_.frame_timestamp();
 100       } else {
 101         should_sample = smoothing_sampler_.ShouldSample();
 102       }
 103       break;
 104     default:
 105       should_sample = smoothing_sampler_.IsOverdueForSamplingAt(event_time);
 106       break;
 107   }
 108
 109   SetFrameTimestamp(frame_number_, event_time);
 110   return should_sample;
 111 }
 112
 113 int VideoCaptureOracle::RecordCapture() {
 114   smoothing_sampler_.RecordSample();
 115   content_sampler_.RecordSample(GetFrameTimestamp(frame_number_));
 116   return frame_number_++;
 117 }
 118
 119 bool VideoCaptureOracle::CompleteCapture(int frame_number,
 120                                          base::TimeTicks* frame_timestamp) {
 121   // Drop frame if previous frame number is higher.
 122   if (last_delivered_frame_number_ > frame_number) {
 123     LOG(WARNING) << "Out of order frame delivery detected.  Dropping frame.";
 124     return false;
 125   }
 126   last_delivered_frame_number_ = frame_number;
 127
 128   *frame_timestamp = GetFrameTimestamp(frame_number);
 129
 130   // If enabled, log a measurement of how this frame timestamp has incremented
 131   // in relation to an ideal increment.
 132   if (VLOG_IS_ON(2) && frame_number > 0) {
 133     const base::TimeDelta delta =
 134         *frame_timestamp - GetFrameTimestamp(frame_number - 1);
 135     if (content_sampler_.HasProposal()) {
 136       const double estimated_frame_rate =
 137           1000000.0 / content_sampler_.detected_period().InMicroseconds();
 138       const int rounded_frame_rate =
 139           static_cast<int>(estimated_frame_rate + 0.5);
 140       VLOG(2) << base::StringPrintf(
 141           "Captured #%d: delta=%" PRId64 " usec"
 142           ", now locked into {%s}, %+0.1f%% slower than %d FPS",
 143           frame_number,
 144           delta.InMicroseconds(),
 145           content_sampler_.detected_region().ToString().c_str(),
 146           100.0 * FractionFromExpectedFrameRate(delta, rounded_frame_rate),
 147           rounded_frame_rate);
 148     } else {
 149       VLOG(2) << base::StringPrintf(
 150           "Captured #%d: delta=%" PRId64 " usec"
 151           ", d/30fps=%+0.1f%%, d/25fps=%+0.1f%%, d/24fps=%+0.1f%%",
 152           frame_number,
 153           delta.InMicroseconds(),
 154           100.0 * FractionFromExpectedFrameRate(delta, 30),
 155           100.0 * FractionFromExpectedFrameRate(delta, 25),
 156           100.0 * FractionFromExpectedFrameRate(delta, 24));
 157     }
 158   }
 159
 160   return !frame_timestamp->is_null();
 161 }
 162
 163 base::TimeTicks VideoCaptureOracle::GetFrameTimestamp(int frame_number) const {
 164   DCHECK_LE(frame_number, frame_number_);
 165   DCHECK_LT(frame_number_ - frame_number, kMaxFrameTimestamps);
 166   return frame_timestamps_[frame_number % kMaxFrameTimestamps];
 167 }
 168
 169 void VideoCaptureOracle::SetFrameTimestamp(int frame_number,
 170                                            base::TimeTicks timestamp) {
 171   frame_timestamps_[frame_number % kMaxFrameTimestamps] = timestamp;
 172 }
 173
 174 SmoothEventSampler::SmoothEventSampler(base::TimeDelta min_capture_period,
 175                                        int redundant_capture_goal)
 176     :  min_capture_period_(min_capture_period),
 177        redundant_capture_goal_(redundant_capture_goal),
 178        token_bucket_capacity_(min_capture_period + min_capture_period / 2),
 179        overdue_sample_count_(0),
 180        token_bucket_(token_bucket_capacity_) {
 181   DCHECK_GT(min_capture_period_.InMicroseconds(), 0);
 182 }
 183
 184 void SmoothEventSampler::ConsiderPresentationEvent(base::TimeTicks event_time) {
 185   DCHECK(!event_time.is_null());
 186
 187   // Add tokens to the bucket based on advancement in time.  Then, re-bound the
 188   // number of tokens in the bucket.  Overflow occurs when there is too much
 189   // time between events (a common case), or when RecordSample() is not being
 190   // called often enough (a bug).  On the other hand, if RecordSample() is being
 191   // called too often (e.g., as a reaction to IsOverdueForSamplingAt()), the
 192   // bucket will underflow.
 193   if (!current_event_.is_null()) {
 194     if (current_event_ < event_time) {
 195       token_bucket_ += event_time - current_event_;
 196       if (token_bucket_ > token_bucket_capacity_)
 197         token_bucket_ = token_bucket_capacity_;
 198     }
 199     TRACE_COUNTER1("mirroring",
 200                    "MirroringTokenBucketUsec",
 201                    std::max<int64>(0, token_bucket_.InMicroseconds()));
 202   }
 203   current_event_ = event_time;
 204 }
 205
 206 bool SmoothEventSampler::ShouldSample() const {
 207   return token_bucket_ >= min_capture_period_;
 208 }
 209
 210 void SmoothEventSampler::RecordSample() {
 211   token_bucket_ -= min_capture_period_;
 212   if (token_bucket_ < base::TimeDelta())
 213     token_bucket_ = base::TimeDelta();
 214   TRACE_COUNTER1("mirroring",
 215                  "MirroringTokenBucketUsec",
 216                  std::max<int64>(0, token_bucket_.InMicroseconds()));
 217
 218   if (HasUnrecordedEvent()) {
 219     last_sample_ = current_event_;
 220     overdue_sample_count_ = 0;
 221   } else {
 222     ++overdue_sample_count_;
 223   }
 224 }
 225
 226 bool SmoothEventSampler::IsOverdueForSamplingAt(base::TimeTicks event_time)
 227     const {
 228   DCHECK(!event_time.is_null());
 229
 230   if (!HasUnrecordedEvent() && overdue_sample_count_ >= redundant_capture_goal_)
 231     return false;  // Not dirty.
 232
 233   if (last_sample_.is_null())
 234     return true;
 235
 236   // If we're dirty but not yet old, then we've recently gotten updates, so we
 237   // won't request a sample just yet.
 238   base::TimeDelta dirty_interval = event_time - last_sample_;
 239   return dirty_interval >=
 240       base::TimeDelta::FromMilliseconds(kNonAnimatingThresholdMillis);
 241 }
 242
 243 bool SmoothEventSampler::HasUnrecordedEvent() const {
 244   return !current_event_.is_null() && current_event_ != last_sample_;
 245 }
 246
 247 AnimatedContentSampler::AnimatedContentSampler(
 248     base::TimeDelta min_capture_period)
 249     : min_capture_period_(min_capture_period) {}
 250
 251 AnimatedContentSampler::~AnimatedContentSampler() {}
 252
 253 void AnimatedContentSampler::ConsiderPresentationEvent(
 254     const gfx::Rect& damage_rect, base::TimeTicks event_time) {
 255   AddObservation(damage_rect, event_time);
 256
 257   if (AnalyzeObservations(event_time, &detected_region_, &detected_period_) &&
 258       detected_period_ > base::TimeDelta() &&
 259       detected_period_ <=
 260           base::TimeDelta::FromMicroseconds(kMaxLockInPeriodMicros)) {
 261     if (damage_rect == detected_region_)
 262       UpdateFrameTimestamp(event_time);
 263     else
 264       frame_timestamp_ = base::TimeTicks();
 265   } else {
 266     detected_region_ = gfx::Rect();
 267     detected_period_ = base::TimeDelta();
 268     frame_timestamp_ = base::TimeTicks();
 269   }
 270 }
 271
 272 bool AnimatedContentSampler::HasProposal() const {
 273   return detected_period_ > base::TimeDelta();
 274 }
 275
 276 bool AnimatedContentSampler::ShouldSample() const {
 277   return !frame_timestamp_.is_null();
 278 }
 279
 280 void AnimatedContentSampler::RecordSample(base::TimeTicks frame_timestamp) {
 281   recorded_frame_timestamp_ =
 282       HasProposal() ? frame_timestamp : base::TimeTicks();
 283   sequence_offset_ = base::TimeDelta();
 284 }
 285
 286 void AnimatedContentSampler::AddObservation(const gfx::Rect& damage_rect,
 287                                             base::TimeTicks event_time) {
 288   if (damage_rect.IsEmpty())
 289     return;  // Useless observation.
 290
 291   // Add the observation to the FIFO queue.
 292   if (!observations_.empty() && observations_.back().event_time > event_time)
 293     return;  // The implementation assumes chronological order.
 294   observations_.push_back(Observation(damage_rect, event_time));
 295
 296   // Prune-out old observations.
 297   const base::TimeDelta threshold =
 298       base::TimeDelta::FromMilliseconds(kMaxObservationWindowMillis);
 299   while ((event_time - observations_.front().event_time) > threshold)
 300     observations_.pop_front();
 301 }
 302
 303 gfx::Rect AnimatedContentSampler::ElectMajorityDamageRect() const {
 304   // This is an derivative of the Boyer-Moore Majority Vote Algorithm where each
 305   // pixel in a candidate gets one vote, as opposed to each candidate getting
 306   // one vote.
 307   const gfx::Rect* candidate = NULL;
 308   int64 votes = 0;
 309   for (ObservationFifo::const_iterator i = observations_.begin();
 310        i != observations_.end(); ++i) {
 311     DCHECK_GT(i->damage_rect.size().GetArea(), 0);
 312     if (votes == 0) {
 313       candidate = &(i->damage_rect);
 314       votes = candidate->size().GetArea();
 315     } else if (i->damage_rect == *candidate) {
 316       votes += i->damage_rect.size().GetArea();
 317     } else {
 318       votes -= i->damage_rect.size().GetArea();
 319       if (votes < 0) {
 320         candidate = &(i->damage_rect);
 321         votes = -votes;
 322       }
 323     }
 324   }
 325   return (votes > 0) ? *candidate : gfx::Rect();
 326 }
 327
 328 bool AnimatedContentSampler::AnalyzeObservations(
 329     base::TimeTicks event_time,
 330     gfx::Rect* rect,
 331     base::TimeDelta* period) const {
 332   const gfx::Rect elected_rect = ElectMajorityDamageRect();
 333   if (elected_rect.IsEmpty())
 334     return false;  // There is no regular animation present.
 335
 336   // Scan |observations_|, gathering metrics about the ones having a damage Rect
 337   // equivalent to the |elected_rect|.  Along the way, break early whenever the
 338   // event times reveal a non-animating period.
 339   int64 num_pixels_damaged_in_all = 0;
 340   int64 num_pixels_damaged_in_chosen = 0;
 341   base::TimeDelta sum_frame_durations;
 342   size_t count_frame_durations = 0;
 343   base::TimeTicks first_event_time;
 344   base::TimeTicks last_event_time;
 345   for (ObservationFifo::const_reverse_iterator i = observations_.rbegin();
 346        i != observations_.rend(); ++i) {
 347     const int area = i->damage_rect.size().GetArea();
 348     num_pixels_damaged_in_all += area;
 349     if (i->damage_rect != elected_rect)
 350       continue;
 351     num_pixels_damaged_in_chosen += area;
 352     if (last_event_time.is_null()) {
 353       last_event_time = i->event_time;
 354       if ((event_time - last_event_time) >=
 355               base::TimeDelta::FromMilliseconds(kNonAnimatingThresholdMillis)) {
 356         return false;  // Content animation has recently ended.
 357       }
 358     } else {
 359       const base::TimeDelta frame_duration = first_event_time - i->event_time;
 360       if (frame_duration >=
 361               base::TimeDelta::FromMilliseconds(kNonAnimatingThresholdMillis)) {
 362         break;  // Content not animating before this point.
 363       }
 364       sum_frame_durations += frame_duration;
 365       ++count_frame_durations;
 366     }
 367     first_event_time = i->event_time;
 368   }
 369
 370   if ((last_event_time - first_event_time) <
 371           base::TimeDelta::FromMilliseconds(kMinObservationWindowMillis)) {
 372     return false;  // Content has not animated for long enough for accuracy.
 373   }
 374   if (num_pixels_damaged_in_chosen <= (num_pixels_damaged_in_all * 2 / 3))
 375     return false;  // Animation is not damaging a supermajority of pixels.
 376
 377   *rect = elected_rect;
 378   DCHECK_GT(count_frame_durations, 0u);
 379   *period = sum_frame_durations / count_frame_durations;
 380   return true;
 381 }
 382
 383 void AnimatedContentSampler::UpdateFrameTimestamp(base::TimeTicks event_time) {
 384   // This is how much time to advance from the last frame timestamp.  Never
 385   // advance by less than |min_capture_period_| because the downstream consumer
 386   // cannot handle the higher frame rate.  If |detected_period_| is less than
 387   // |min_capture_period_|, excess frames should be dropped.
 388   const base::TimeDelta advancement =
 389       std::max(detected_period_, min_capture_period_);
 390
 391   // Compute the |timebase| upon which to determine the |frame_timestamp_|.
 392   // Ideally, this would always equal the timestamp of the last recorded frame
 393   // sampling.  Determine how much drift from the ideal is present, then adjust
 394   // the timebase by a small amount to spread out the entire correction over
 395   // many frame timestamps.
 396   //
 397   // This accounts for two main sources of drift: 1) The clock drift of the
 398   // system clock relative to the video hardware, which affects the event times;
 399   // and 2) The small error introduced by this frame timestamp rewriting, as it
 400   // is based on averaging over recent events.
 401   base::TimeTicks timebase = event_time - sequence_offset_ - advancement;
 402   if (!recorded_frame_timestamp_.is_null()) {
 403     const base::TimeDelta drift = recorded_frame_timestamp_ - timebase;
 404     const int64 correct_over_num_frames =
 405         base::TimeDelta::FromMilliseconds(kDriftCorrectionMillis) /
 406             detected_period_;
 407     DCHECK_GT(correct_over_num_frames, 0);
 408     timebase = recorded_frame_timestamp_ - (drift / correct_over_num_frames);
 409   }
 410
 411   // Compute |frame_timestamp_|.  Whenever |detected_period_| is less than
 412   // |min_capture_period_|, some extra time is "borrowed" to be able to advance
 413   // by the full |min_capture_period_|.  Then, whenever the total amount of
 414   // borrowed time reaches a full |min_capture_period_|, drop a frame.  Note
 415   // that when |detected_period_| is greater or equal to |min_capture_period_|,
 416   // this logic is effectively disabled.
 417   borrowed_time_ += advancement - detected_period_;
 418   if (borrowed_time_ >= min_capture_period_) {
 419     borrowed_time_ -= min_capture_period_;
 420     frame_timestamp_ = base::TimeTicks();
 421   } else {
 422     sequence_offset_ += advancement;
 423     frame_timestamp_ = timebase + sequence_offset_;
 424   }
 425 }
 426
 427 }  // namespace content